View Full Version : Crown IC 150 Preamp popping noise - source of problem?
Have a Crown IC 150 preamp, 14,000 series serial number so last revision made, that has a odd popping sound under very specific circumstances. Initially thought it was just dirty controls but after cleaning convinced it's a part failure, just not sure which one.
The problem occurs on the RIGHT channel only (no source audio connected to rule out source issues), and only when one of the two phono preamps are selected (either) and only under these 3 conditions:
1) Engaging or disengaging the loudness control and the volume is set to approx 75% or less (if volume is higher, the engage/disengage does not cause a problem)
2) If the volume control is moved in the 80-90% range (approximately), no other controls touched
3) If the volume is approx 75% or higher and the phono input is DE-selected over to another non phono source input type. Watching the pot as I turn the control, it occurs the moment both the Phono input and non phono input are simultaneously connected by the control which is just briefly as you turn the control and the fin moves from the phono terminal to the non phono terminal.
The popping noise does not seem to occur with any other input type selected, only the phonos.
Visually, C8 on the main board (100pf brown mica) appears darkened, much moreso than it's counter part on the other channel, almost like it has been burnt. Given it's place in the circuit (see pic - between the loudness and the volume control) does it make sense that this might be the culprit? Should like ONLY be change out with another mica, or is a different cap type acceptable?
Or should I be looking at a different spot for the problem? I have already changed the 220uf caps all out, both on the main board and the phono board, as they were old and some out of spec. No change. Thanks is advance for any helpful suggestions!
Main board excerpt
19920
Phono board
19921
100pf cap
19922
Light Dependant Resistor
07-04-2017, 02:29
Hi Jim
There are no significant currents or voltages on C8 which would cause it to fail
it is just a low pass filter ( rolling off high frequencies ) I think it has always
been like that, and this is the first time you have noticed.
A popping noise is most likely to be the volume control and its mechanical =dreadful
relationship with the audio being passed. The op amp to the right of the main schematic
has quite a bit of gain on it, hence will amplify audio as well as any noise present. If you can install
a socket for that op amp - you could keep the original but trial other more modern
types such as LM6171 http://www.ti.com/lit/ds/symlink/lm6171.pdf
or LME49710 http://www.ti.com/lit/ds/symlink/lme49710.pdf
In the circumstance I would turn off your Crown and unplug it, purchase some
proper electronics contact spray cleaner and clean the pot and any other mechanical point
where contacts touch- such as audio sockets like the phono input - but
do them all including switches. leave to dry for a half hour, and should work well again
without noise - other than what is naturally caused by mechanical volume controls,
and in so much as you wish to tolerate them.
Cheers / Chris
I had already cleaned the controls, buttons, and the control pot (multiple times) prior to this post. It did not resolve the issue with either the volume control, nor the loudness button nor the input selector switch. I need to make the point that there are 3 independent control cases that make the problem occur. It is not likely, especially after repeated cleaning, all 3 controls are bad. Also, this is not typical "static in the control" type of sound that I am very familiar with and a good electronic cleaner usually resolves. this is a mid to low freq pop, unlike anything I've heard before on any piece of equipment that I have encountered.
Given the scenarios i described, it's as if something is getting to a limit and exceeding that limit. the switching in of the loudness circuit, or the movement of the volume past a certain range, or the deslection on the phono input as the circuit flips over to another input (whiole both are momentarily active) is causing some threshold to be exceeded - but only on the right channel and only when the phono circuit is the common denominator.
Light Dependant Resistor
07-04-2017, 03:34
Hi Jim
Well then remove C27 and C28 using solderwick,noting polarity and replace with new solid tantalum equivalents.
These are coupling audio to the high gain op amp - see if that resolves. It is a bit of DC getting through
causing the noise, and the symptoms sound a lot like a capacitor or two passed its use by date.
I would also review the date code marked on power supply caps , as they eventually reach the end of their life
and cause burbles and pops, and inexplicable moments.
Also check solder joints on circuit boards and for continuity 0 ohms
from applicable points on pcb's and input socket mountings to earth chassis.
Cheers / Chris
Thanks. I will get some tantalums and replace those two caps. I had mentioned in the original post I did replace all the 220uf's (which were the power supply caps) but possibly there was another value there in the power supply I didn't replace. If I skipped it, it was because it checked out on esr and capacitance, whereas the 220ufs definitely were getting out of spec (or were)...
So no concerns on the overly dark c8 100pf mica ?
Light Dependant Resistor
07-04-2017, 05:45
Hi Jim
No, because C8 does very little just rolls off high frequencies, it looks like a mica type cap,
but if you are losing sleep over it a polystyrene 100pf would be a good replacement.
This site has some very comprehensive info http://kenrockwell.com/audio/crown/ic-150.htm
Note the integrated circuit to the left of the image showing the volume control - if you have
similarly sockets installed, cleaning them might be advantageous. The best socket type
is a turned pin one, because it better grips, but better still is to solder integrated circuits in,
and replace with modern equivalents. Op amps today are much better in spec. If you see parts
like 741's these are ancient. You will be bowled over with how good a decent op amp can sound
like the ones i linked to.
Cheers / Chris
RothwellAudio
07-04-2017, 08:37
Visually, C8 on the main board (100pf brown mica) appears darkened, much moreso than it's counter part on the other channel, almost like it has been burnt. Given it's place in the circuit (see pic - between the loudness and the volume control) does it make sense that this might be the culprit? Should like ONLY be change out with another mica, or is a different cap type acceptable?
The volume control appears to have two taps on it which are part of the loudness control. The fact that the problem occurs only when the volume control is in a certain range suggests it is linked to this. Yes, I think C8 might be faulty and as the volume control passes the tapped point on its travel the pop occurs. It would be a fairly simple job to replace the cap (if access to the underside of the PCB is feasible) and cheap to do so it's worth a try. Any 100pF capacitor would be ok.
The fact that the problem only occurs when phono is selected might be due to the phono circuit sending DC to the line stage, which causes the problem with C8.
Or I could be completely wrong. :scratch:
OK. Well I just ordered some 1uf tantalums so maybe I didn't need them, but that will be a phase 2 attempt if the replacement c8 doesn't work. i don't want to change too many things at once. The board is partially out but connected by many wires, so I do have some access to the underside. Should be enough to unsolder and solder a new part in though.
Question - why is that 100pf such a high voltage rating? (like 600v or 630v I can't remember) There's no way that circuit is seeing that kind of voltage right? Can i use a lower value? and if so , what is the safest lowest value to use?
There are two ics only on this revision. They are marked S 430 3532 but the parts list calls it a AN 301 Op Amp. I'm not clear from the kenrockwell link, or comments here, if those should be replaced and if so, is it a direct drop in with no circuit modification (I don't want to change any circuit design). I guess one thing I could try is swapping those two to see if the problem moves channels too.
Light Dependant Resistor
09-04-2017, 09:47
OK. Well I just ordered some 1uf tantalums so maybe I didn't need them, but that will be a phase 2 attempt if the replacement c8 doesn't work. i don't want to change too many things at once. The board is partially out but connected by many wires, so I do have some access to the underside. Should be enough to unsolder and solder a new part in though.
Question - why is that 100pf such a high voltage rating? (like 600v or 630v I can't remember) There's no way that circuit is seeing that kind of voltage right? Can i use a lower value? and if so , what is the safest lowest value to use?
There are two ics only on this revision. They are marked S 430 3532 but the parts list calls it a AN 301 Op Amp. I'm not clear from the kenrockwell link, or comments here, if those should be replaced and if so, is it a direct drop in with no circuit modification (I don't want to change any circuit design). I guess one thing I could try is swapping those two to see if the problem moves channels too.
The rated voltage is so high because the capacitance is so low. 100pf is basically so low in capacitance - particularly used in a
filter, its as though Crown had nothing better to do You could entirely remove C8 and not notice any difference - but Crown wanted some extremely mild
high frequency roll off. A oscilloscope likely showed them slightly better stability with the 100pf installed, and that is likely, why it went in.
The very best 100pf caps are polystyrene, so as I said if you are losing unnecessary sleep over C8 change it for a polystyrene type.
Quite to the contrary C27 and C28 really deserve replacing- they are connected directly to the op amp high gain stage and any fault from
ageing is highly likely to be those two. 1UF tatalums are the best type of cap for coupling - or for that matter anywhere where stability is needed.
If you are working on the boards make sure you have insulation sufficient so pcb does not touch chassis. Or better not have power connected whilst boards are dangling.
Op amps there are hundreds of different single types. The 301 used as an option a compensation capacitor- you can see this on
a Quad 405 Mk 1 schematic. I would start with a TL071 as it is a generally good sounding device, A LME49710 or a LM6171
would be a fair bit better though.
Cheers / Chris
RothwellAudio
10-04-2017, 09:20
C8 appears to be part of the loudness filter. Without a resistance value for the volume control and without knowing exactly where the taps on the volume control are it's difficult to say exactly what C8 is doing but it appears to be the part of the circuit which boosts the HF. It does this by being in parallel with the top part of the volume control. It does not look to me like it's there to roll-off high frequencies - it's there to boost them when the loudness control is selected. It is a high voltage capacitor simply because small value capacitors commonly come as high voltage devices. It doesn't need to rated at high voltage.
Your description of the symptoms suggests to me that the problem occurs when the volume control passes the tap points which feed into the loudness circuit. If the problem was related to C27 or C28 it would occur regardless of which input was selected, so I don't think they have anything to do with it.
I think the problem is DC from the phono boards. You can check it be looking for any DC at the input to the volume control - ie the point where the volume control and C8 meet. If there is DC on the right channel but not the left, that's the problem. If DC is the problem it's most likely because C12 on the phono board has failed, or R20 has failed. They are the components which are doing the DC blocking.
C8 appears to be part of the loudness filter. Without a resistance value for the volume control and without knowing exactly where the taps on the volume control are it's difficult to say exactly what C8 is doing but it appears to be the part of the circuit which boosts the HF. It does this by being in parallel with the top part of the volume control. It does not look to me like it's there to roll-off high frequencies - it's there to boost them when the loudness control is selected. It is a high voltage capacitor simply because small value capacitors commonly come as high voltage devices. It doesn't need to rated at high voltage.
Your description of the symptoms suggests to me that the problem occurs when the volume control passes the tap points which feed into the loudness circuit. If the problem was related to C27 or C28 it would occur regardless of which input was selected, so I don't think they have anything to do with it.
I think the problem is DC from the phono boards. You can check it be looking for any DC at the input to the volume control - ie the point where the volume control and C8 meet. If there is DC on the right channel but not the left, that's the problem. If DC is the problem it's most likely because C12 on the phono board has failed, or R20 has failed. They are the components which are doing the DC blocking.
You might have nailed it. -5V at the volume control on that channel. Cap on phono board won't even register on my esr/cap meter. Other channel's cap is also out of spec a bit (reading 6uf) so I might change it as well. Will have to order parts aas I don't have any tantalums.
RothwellAudio
11-04-2017, 09:25
You might have nailed it. -5V at the volume control on that channel. Cap on phono board won't even register on my esr/cap meter. Other channel's cap is also out of spec a bit (reading 6uf) so I might change it as well. Will have to order parts as I don't have any tantalums.
The caps don't have to be tantalums. There was a time when tantalum capacitors were considered superior to aluminium electrolytics but they gained a reputation for being unreliable and are now mostly shunned by audiophiles. I'm no expert on tantalums but I've read that they're very reliable as long as their voltage rating isn't exceeded and they aren't subject to any reverse voltage, and the newer ones are constructed differently from the older ones anyway, so maybe the bad reputation is undeserved. But personally, I would replace the caps in your phonostage with large electrolytics, as large as will fit in the space. 25V rating will be enough. I'd use something much bigger than 4.7uF.
Ok small step forward today, larger step back.
Got the 4.7uf tants. Installed. Also replaced the 500ufs on that phono board as well since I tested them as out of spec too.
It did not resolve the noise or the voltage on the right channel volume but is is *different*. previously I was seeing -0.5vdc on the left channel of the volume and slightly over -5vdc on the right side, where the major of volume of that noise was occur. the voltage there now is between -3 and -3.7 vdc, so it is reduced, but not eliminated. No change to the left side.
I did figure out that by engaging the low pass filter control that eliminates that DC voltages completely (on both sides) so that might be a helpful clue as to the remaining cause.
I did also get that 1uf tants in today as well but haven't yet swapped those out. I thought I would post here first with the new info to see if any light bulbs came on. I also swapped the two IC chips. No change so they are not a source of the issue.
A new problem was also identified during my testing. Likely unrelated and I just hadn't got far enough along in testing to realize it but I'll mention now just in case it's related.
The LEFT channel (one with only -0.5vdc on it and not the bigger headache of noise) is not responding to any Bass or Treble controls. There is a tone defeat switch and when engaged (all tone controls out) there is an audible change in the sound on that channel so there is a difference between engaged and not, however when not engaged, those bass & treble controls are having zero effect on the sound and should. being on the "non problematic noise channel" my gut says unrelated and just another issue to fix but using some reverse logic perhaps the fact the right channel tone controls ARE working correctly is a clue to the voltage it is producing at the volume control (and should due to some other yet to find part issue along that path)?
One last item of note - now that all the boards are pulled out, when I touch the shaft of the controls, particularly the volume control, and either phono is selected, there is a huge grounding hum. It's a much deeper/louder hum on the volume control vs the balance or stereo reverse shafts, though they generate some as well when touched. Is that simply because the pots are no longer in contact with the case?
RothwellAudio
15-04-2017, 10:18
Got the 4.7uf tants. Installed. Also replaced the 500ufs on that phono board as well since I tested them as out of spec too.
It did not resolve the noise or the voltage on the right channel volume but is is *different*. previously I was seeing -0.5vdc on the left channel of the volume and slightly over -5vdc on the right side, where the major of volume of that noise was occur. the voltage there now is between -3 and -3.7 vdc, so it is reduced, but not eliminated. No change to the left side.
I did figure out that by engaging the low pass filter control that eliminates that DC voltages completely (on both sides) so that might be a helpful clue as to the remaining cause.
Yes, I thought that engaging the Lo Filter would cure it if DC was indeed the problem - it simply puts another capacitor in the signal path to cut the low frequencies and therefore also blocks DC. I should have mentioned that before. Anyway, that seems to confirm that DC on the outputs of the phono section is the problem. Can you remove R13 to isolate the phono section from the rest? If so, does the DC at the junction of C12/R20 remain? Where does R13 go? Is the DC coming from the other end of R13?
Have you put the tantalum capacitors in the right way round? Have you got any electrolytics you could try?
I also swapped the two IC chips. No change so they are not a source of the issue.
No, I didn't think they would be.
A new problem was also identified during my testing. Likely unrelated and I just hadn't got far enough along in testing to realize it but I'll mention now just in case it's related.
The LEFT channel (one with only -0.5vdc on it and not the bigger headache of noise) is not responding to any Bass or Treble controls. There is a tone defeat switch and when engaged (all tone controls out) there is an audible change in the sound on that channel so there is a difference between engaged and not, however when not engaged, those bass & treble controls are having zero effect on the sound and should. being on the "non problematic noise channel" my gut says unrelated and just another issue to fix but using some reverse logic perhaps the fact the right channel tone controls ARE working correctly is a clue to the voltage it is producing at the volume control (and should due to some other yet to find part issue along that path)?
Yes, the non-functioning tone controls might be an unrelated problem.
One last item of note - now that all the boards are pulled out, when I touch the shaft of the controls, particularly the volume control, and either phono is selected, there is a huge grounding hum. It's a much deeper/louder hum on the volume control vs the balance or stereo reverse shafts, though they generate some as well when touched. Is that simply because the pots are no longer in contact with the case?
Yes, not being in contact with the case will be the cause of that.
Light Dependant Resistor
15-04-2017, 10:47
Hi
The ground connection at C5 and C40 and C7 to a degree, make the filter a low pass type
it is removing high frequencies, being a complex series capacitive and resistive filter
ie, a tone control. Tone controls are usually bad for accurate audio - but a feature of that design for reasons
the manufacturer took interest in.
The phono schematic fails to show what is on the other end of R13 ?
but a 4.7uf cap suggests DC blocking and AC audio coupling at that point
meaning there should be no DC
You need to ground the boards to the chassis per the schematic, to make
valid measurements.
Cheers / Chris
RothwellAudio
15-04-2017, 11:05
The ground connection at C5 and C40 and C7 to a degree, make the filter a low pass type
it is removing high frequencies, being a complex series capacitive and resistive filter
ie, a tone control.
I can't see C40 but C5 and C7 are part of the loudness control, ie they're part of the circuit which boosts bass and treble to compensate for the ear's loss of sensitivity when listening at low volume.
Light Dependant Resistor
15-04-2017, 11:17
I can't see C40 but C5 and C7 are part of the loudness control, ie they're part of the circuit which boosts bass and treble to compensate for the ear's loss of sensitivity when listening at low volume.
C40 is yet another series cap from circuit board ground to chassis gnd proper. so
suggesting all filtering routing through it ie formula C1xC2 /C1+C2 . The designer
seemed to love series caps and their phase changing ability.
In the upper section more low pass filtering using 22NF C3 and C4 and resistors.
I think we could be brave here and bypass filtering altogether - but that is up to the owner.
Cheers / Chris
RothwellAudio
15-04-2017, 16:36
In the upper section more low pass filtering using 22NF C3 and C4 and resistors.
C3 and C4 are the left channel equivalents of C7 and C5 which are used in the right channel. They aren't really low pass filters at all. The whole filter network is there to boost bass and treble - it's a loudness contour filter and is even marked on the circuit diagram as such. Yes, the filtering could be bypassed but there's no need to make any modifications, switch SW4 bypasses it.
Light Dependant Resistor
15-04-2017, 18:28
C3 and C4 are the left channel equivalents of C7 and C5 which are used in the right channel. They aren't really low pass filters at all. The whole filter network is there to boost bass and treble - it's a loudness contour filter and is even marked on the circuit diagram as such. Yes, the filtering could be bypassed but there's no need to make any modifications, switch SW4 bypasses it.
They are fixed low pass filters because of the ground connection at the end of R2
that is in the upper gang a C R filter , and the lower gang R3 assuming the wiper remains in the middle
a R C filter a reversal - but complicated by the resistance of the potentiometer erring toward
R3 and C4 being a constant dominant pole, rather than cancellation by the resistance of the potentiometer.
C1 and C8 via the 10 megaohm are emphasis circuits depending on the position of the switch
but also low pass filters but well outside audio frequencies, where the switch remains as positioned on the schematic.
But then the lower half, as though informing a design layout error, invites variable resistance to not even match
the other channels fixed resistance. Why on earth would you not present Left and right channels and their filtering evenly for
subsequent amplification - but NO there it is- plain to see. Which is why I suggest bypassing the lot to hurdle
what is just a distortion circuit and point of failure, and I refer to the use of switches, including the potentiometer
wiper that most people dismiss as being beyond reproach.
We then see further mutilation of audio on the right hand side of the schematic with more filters in this case
hoping like a deckchair on the Titanic, that midway setting of the pots will enable a flat frequency response.
Cheers / Chris
Alligator clips from control pots to ground - no effect on previous measurements just fyi. The board is/was already grounded via other methods, at least as far as my measurements have been concerned so far.
R13 (560 ohm resistor, measures 600 ohm) runs to the input selector switch, on to the tape monitor switches then to the main board (left side of schematic in original post). There are no electronics between the phono and main board (other than switches).
Disconnected phono board r13 line - voltage is present on Phono board (no longer present on main board volume pot) so voltage is coming from phono board. Voltage is present at C12/R20 junction, which I don't understand how that could be, given a brand new 4.7 tant installed.
4.7 tantalums are in correctly, installed as previous caps were and reconfirmed now per schematic.
RothwellAudio
17-04-2017, 21:38
Disconnected phono board r13 line - voltage is present on Phono board (no longer present on main board volume pot) so voltage is coming from phono board. Voltage is present at C12/R20 junction, which I don't understand how that could be, given a brand new 4.7 tant installed.
You're right - with a good capacitor and a good resistor for C12/R20 there should be no DC. That means either a dodgy cap or the 100k resistor has gone open circuit. Measuring the resistor should be easy enough.
Actually, there's one other possibility - a DC path in parallel with the capacitor caused by dirt/dampness or some other contaminant on the circuit board in the vicinity of the capacitor.
Light Dependant Resistor
17-04-2017, 21:53
You're right - with a good capacitor and a good resistor for C12/R20 there should be no DC. That means either a dodgy cap or the 100k resistor has gone open circuit. Measuring the resistor should be easy enough.
Actually, there's one other possibility - a DC path in parallel with the capacitor caused by dirt/dampness or some other contaminant on the circuit board in the vicinity of the capacitor.
And one more, earth is not earth at all !
RothwellAudio
17-04-2017, 21:59
And one more, earth is not earth at all !
Sorry, I don't understand that - could you elaborate?
Light Dependant Resistor
17-04-2017, 22:10
Where earth we presume to be zero volts, is not at all.
A measurement from the chassis to the 0V point should confirm.
American equipment fails today on a number of fronts concerning isolation
and best practice.
We really need to see the full schematic, or provided a link to assist further.
Cheers / Chris
Ive seen tvs that had chassis at half mains potential too or something similar. Had to pull earth from scope or blow the damn thing up. Those were a pain in rectum.
Where earth we presume to be zero volts, is not at all.
A measurement from the chassis to the 0V point should confirm.
American equipment fails today on a number of fronts concerning isolation
and best practice.
We really need to see the full schematic, or provided a link to assist further.
Cheers / Chris
Full schematic included, though the source PDF quality is quite poor. The board layout diagrams are almost completely unreadable....
20029
You're right - with a good capacitor and a good resistor for C12/R20 there should be no DC. That means either a dodgy cap or the 100k resistor has gone open circuit. Measuring the resistor should be easy enough.
Actually, there's one other possibility - a DC path in parallel with the capacitor caused by dirt/dampness or some other contaminant on the circuit board in the vicinity of the capacitor.
Pulled and checked resistor - measures correct. Over -2V at that junction, over -4V on the other side of C12, so it is doing SOME DC reduction. Mesured that new cap both for capacitance and esr. good. Checked every (i think) resistor on that board - all good. Checked the diode. Good. Put my magnifying glasses on and don't see any bridging that would cause the leakage. My soldering skills are not the best, so I could easily attributed it to that, except it's been doing this since before I ever even touched the board whern it was completely stock.
Light Dependant Resistor
18-04-2017, 03:19
Pulled and checked resistor - measures correct. Over -2V at that junction, over -4V on the other side of C12, so it is doing SOME DC reduction. Mesured that new cap both for capacitance and esr. good. Checked every (i think) resistor on that board - all good. Checked the diode. Good. Put my magnifying glasses on and don't see any bridging that would cause the leakage. My soldering skills are not the best, so I could easily attributed it to that, except it's been doing this since before I ever even touched the board whern it was completely stock.
Hi
A few checks on your measurements. You are measuring with a multimeter DC -not AC, and DC relative to ? - usually earth , and you state you have negative 2 volts( - not mv ? ) at the upper side of
R13 leading to the switch. If so can you check the same voltage is measured across the tape out RCA sockets ?
The design is extensively AC capacitor coupled somewhat negating any possibility of DC being coupled
Check the op amp IC1 is not inserted around the wrong way, there should be a dot meaning pin 1
The IC150 has a voltage regulator check for steady 12v + and steady 12v neg at the emitters of the transistors
located near the bridge rectifier ( sorry cannot read the part numbers )
Hear from you soon.
Cheers / Chris
Definitely DC measurement, definitely full volts, not mv. With the R13 line connected back, yes it is present at the tape output to (direct line from the switch there so it should be)
Note that when connected, that -2V does grow the longer I leave the unit on. That's why you see me state 4v 5v etc in previous posts. It starts at about 2v though on power up.
Measurement taken relative to the chassis. Same connect point the main amp board connects to via .1uf cap. (also have done same connect point as phono board connect via .1uf cap too, since it's the same ground (chassis) no difference.
IC is inserted correct. keep in mind, i didn't even swap them until recently and the voltage occurs at the phono board, with R13 line disconnected to switch/main board so not clear how an incorrect IC on the amp board would have any impact there anyway.
I forgot to check the 12V source, but that would be for the main amp board anyway. I can check that tomorrow. I did check the 18v source for the phono board though - reading about 19v. Again with chassis as the ground reference.
Another thing I might try tomorrow is to take that 4.7uf tant out of the circuit on the phono board, just to confirm there is no voltage present on the R13 side of it. If there is, I guess that would confirm a voltage leak onto that part of the circuit as RothwellAudio mentioned as a possible trace hop, though I couldn't find any today and I looked really close.
Light Dependant Resistor
18-04-2017, 06:37
Definitely DC measurement, definitely full volts, not mv. With the R13 line connected back, yes it is present at the tape output to (direct line from the switch there so it should be)
Note that when connected, that -2V does grow the longer I leave the unit on. That's why you see me state 4v 5v etc in previous posts. It starts at about 2v though on power up.
Measurement taken relative to the chassis. Same connect point the main amp board connects to via .1uf cap. (also have done same connect point as phono board connect via .1uf cap too, since it's the same ground (chassis) no difference.
IC is inserted correct. keep in mind, i didn't even swap them until recently and the voltage occurs at the phono board, with R13 line disconnected to switch/main board so not clear how an incorrect IC on the amp board would have any impact there anyway.
I forgot to check the 12V source, but that would be for the main amp board anyway. I can check that tomorrow. I did check the 18v source for the phono board though - reading about 19v. Again with chassis as the ground reference.
Another thing I might try tomorrow is to take that 4.7uf tant out of the circuit on the phono board, just to confirm there is no voltage present on the R13 side of it. If there is, I guess that would confirm a voltage leak onto that part of the circuit as RothwellAudio mentioned as a possible trace hop, though I couldn't find any today and I looked really close.
So what measurement do you get when measuring R13 upper and lower when switched to AC ?
It could be there is a very low frequency oscillation from the phono board and the duty cycle nature of
multimeters is translating an average DC voltage like a RMS ( Root Mean Square )
because it is averaging then resident within a much larger AC voltage.
The typical cause of low frequency oscillation is positive feedback
As I see it Q4 causes emitter degeneration at Q3 which couples to the base of Q1 and then Q2 giving amplification
R16 and R11 are feedback and signal related with a lot of reliance on Q4 collector to present opposite phase, as the emitter of
Q4 is also a feedback path having emitter degeneration. So its not great at keeping positive feedback from starting.
At C14 we see ability for Q4 to not be steady as its emitter is AC coupled. And emitter of Q4 taking shorter path
through C14 and C16 for delivering amplified audio. Q6 is a cap multiplier
I would check R29 variable as there is a lot of reliance on establishing a form of frequency summing point via C16
In preference using a mosfet with a tiny bit of resistance prior to ground establishes a much better ability for Q4 to
be predictable.
I really need to breadboard the phono circuit to see what its characteristics are.
Cheers / Chris
RothwellAudio
18-04-2017, 09:09
Measurement taken relative to the chassis.
I would not rely on the chassis as a good reference point. Use a known 0V point such as the end of R20 or the end of R17. 0V and earth are not necessarily the same thing. In fact, "earth" is a very ambiguous word.
I would not rely on the chassis as a good reference point. Use a known 0V point such as the end of R20 or the end of R17. 0V and earth are not necessarily the same thing. In fact, "earth" is a very ambiguous word.
I tried using the end of R20 - same readings from that point as well.
I also took that 4.7uf tant out of circuit and confirmed about -4V on the plus side were C12 had been, NO voltage on the other side on R13. Circuit is definitely broken with it out so that confirms no board trace leak. Put (another) fresh 4.7uf back in, just in the one in a million chance the new one was bad somehow, same readings once back in. About -2V on the R13 side. How is that possible?? It just doesn't make any sense. With R13 no going anywhere (disconnected from the selector switch) the ONLY other is through R20 to ground. the 4.7uf removal established there is absolutely no voltage present on that side. Perhaps I should replace R20 anyway, even though it measures fine? But I thought the Cap did all the DC blocking anyway?
So what measurement do you get when measuring R13 upper and lower when switched to AC ?
It could be there is a very low frequency oscillation from the phono board and the duty cycle nature of
multimeters is translating an average DC voltage like a RMS ( Root Mean Square )
because it is averaging then resident within a much larger AC voltage.
The typical cause of low frequency oscillation is positive feedback
As I see it Q4 causes emitter degeneration at Q3 which couples to the base of Q1 and then Q2 giving amplification
R16 and R11 are feedback and signal related with a lot of reliance on Q4 collector to present opposite phase, as the emitter of
Q4 is also a feedback path having emitter degeneration. So its not great at keeping positive feedback from starting.
At C14 we see ability for Q4 to not be steady as its emitter is AC coupled. And emitter of Q4 taking shorter path
through C14 and C16 for delivering amplified audio. Q6 is a cap multiplier
I would check R29 variable as there is a lot of reliance on establishing a form of frequency summing point via C16
In preference using a mosfet with a tiny bit of resistance prior to ground establishes a much better ability for Q4 to
be predictable.
I really need to breadboard the phono circuit to see what its characteristics are.
Cheers / Chris
On AC reading, that Phono board R13 yields about 0.06vac coming off it. About the same on either side of it.
Check R29 on the main board. 5.6K as it should. Broke it trying to take out to confirm so now there's a brand new one there ugh...)
Light Dependant Resistor
19-04-2017, 02:24
So it looks like there is just some residue DC voltage contained in the capacitor
Using a parallel 1k resistor across resistor R20 which is 100k, dissipate that voltage
from C 12 it should take 30 seconds and not return once the 1k resistor is removed.
You could load the phono board output slightly by changing R20 for 22k, the effect will
be to achieve lower noise
Did you replace the capacitors at the high gain op amp - which I advised earlier ?.
Cheers / Chris
So it looks like there is just some residue DC voltage contained in the capacitor
Using a parallel 1k resistor across resistor R20 which is 100k, dissipate that voltage
from C 12 it should take 30 seconds and not return once the 1k resistor is removed.
You could load the phono board output slightly by changing R20 for 22k, the effect will
be to achieve lower noise
Did you replace the capacitors at the high gain op amp - which I advised earlier ?.
Cheers / Chris
i don't understand how there could be residue voltage across (now) 3 C12 capacitors that were put there. The original, and now two new ones. How can there be residue voltage on a brand new capacitor? Also - why isn't that same level of residue voltage present on the other channel?
I'm uncomfortable with changing values, even if to improve performance, because it's not yet clear to me that the source of the problem. My goal is to first and formost get it back to operating - as spec'd. then to talk about possible improvements.
As for the 1uf tants on the main board, they came in Sat but I have not put them in yet since I have been chasing down this phono board voltage problem. Are you saying that they are in any way responsible with the voltage problem we are experiencing on the phono board? If so i don't see how. Or are you just saying that because of the voltage problem on the phono board, those main board caps aren't handling things well. If so that's a symptom, good to fix, but shouldn't the priority be to fix the source of the problem and not just mask it? Maybe I'm just not understand your intent clearly.
Light Dependant Resistor
19-04-2017, 03:48
i don't understand how there could be residue voltage across (now) 3 C12 capacitors that were put there. The original, and now two new ones. How can there be residue voltage on a brand new capacitor? Also - why isn't that same level of residue voltage present on the other channel?
I'm uncomfortable with changing values, even if to improve performance, because it's not yet clear to me that the source of the problem. My goal is to first and formost get it back to operating - as spec'd. then to talk about possible improvements.
As for the 1uf tants on the main board, they came in Sat but I have not put them in yet since I have been chasing down this phono board voltage problem. Are you saying that they are in any way responsible with the voltage problem we are experiencing on the phono board? If so i don't see how. Or are you just saying that because of the voltage problem on the phono board, those main board caps aren't handling things well. If so that's a symptom, good to fix, but shouldn't the priority be to fix the source of the problem and not just mask it? Maybe I'm just not understand your intent clearly.
The original issue was popping noise, now you are chasing DC voltages from coupling capacitors, they are related,
so let the coupling caps do their job. The Crown has intent to load extremely lightly hence it may be intentional to have
what are called offset voltages present. Most circuits today do the opposite - they load circuits down to reduce noise.
A capacitor blocks DC from passing, but in the Crown we see the ground plane is itself capacitively coupled
so you are unlikely to ever rid the circuit of offsets unless measures are taken to bleed DC voltages continuously
The best method at lower safe voltages that the Crown uses is to use back to back diodes, these afford connection and also
a defined ground lift from earth much along the same reasoning why Crown use a capacitor placed to the chassis,
I would try a parallel diodes back to back around C40 which is a capacitive connection to ground - the capacitor is useless
on its own ,as it is then frequency dependent passing AC but not DC.
You can use 1N4007 or UF 4007 types. This should instantly remove the popping noise or any DC on the board
sitting around latently on coupling capacitors causing the problems.
You should notice the Crown is then extremely quiet, and will sound much better too.
So one diode is placed anode ( from the roman word meaning way in ) to the ground plane and cathode ( way out ) to
the chassis. The other diode is placed anode to the chassis and cathode to the ground plane..... hence back to back
You could strap these across C40 if there is room.
This will afford exactly the same isolation that Crown desired, but allow proper DC connection as well
A measurement will show a semiconductor lift of approximately 0.6v.
The more complete analysis of this circuit also involves a resistor which sadly will not give the isolation
Crown wanted, but adding 2 diodes will.
Hope this helps
Cheers / Chris
RothwellAudio
19-04-2017, 08:09
i don't understand how there could be residue voltage across (now) 3 C12 capacitors that were put there.
Neither do I.
I'm uncomfortable with changing values, even if to improve performance, because it's not yet clear to me that the source of the problem. My goal is to first and formost get it back to operating - as spec'd. then to talk about possible improvements.
Seems sensible to me.
I suggest you measure the voltage on the other side of C12 where it meets R19/R16 and the emitter of the transistor. Then do a little mock-up of that bit of the circuit by having a capacitor and 100k resistor (like C12 and R20) and putting DC on one side of the capacitor, then measure the voltage on the other. This should at least verify that your meter is making sensible readings.
Light Dependant Resistor
19-04-2017, 08:34
Neither do I..
Put two capacitors in series such as C12 + C40 and it makes a lot of sense why
there are residual voltages. Even if one side is chassis grounded. Such is the nature
of capacitance to store voltage . Back to back diodes will instantly cure.
Cheers / Chris
RothwellAudio
19-04-2017, 09:22
Put two capacitors in series such as C12 + C40 and it makes a lot of sense why
there are residual voltages. Even if one side is chassis grounded. Such is the nature
of capacitance to store voltage . Back to back diodes will instantly cure.
Cheers / Chris
C40 has nothing to do with anything. The OP is measuring voltages with respect to 0V, not the chassis or the mains earth.
Light Dependant Resistor
19-04-2017, 10:17
C40 has nothing to do with anything. The OP is measuring voltages with respect to 0V, not the chassis or the mains earth.
How wrong you are, 0v in the Crown iC150 is not zero volts as stated, as 0v is always in series
with C40. Connect 2 capacitors in series you get voltage at their junction... no mystery to that.
C40 is on the schematic is a capacitive connection to chassis ground,
hence a zero volt reference up to the currents involved with a preamp.
Crown's mistake was to take the floating ground concept a bit too far by relying on capacitors
alone. Two diodes should retain Crowns concept and allow connection at a semiconductor junction
at the same time, allowing residual voltages to properly dissipate.
Cheers / Chris
RothwellAudio
19-04-2017, 10:53
How wrong you are, 0v in the Crown iC150 is not zero volts as stated, as 0v is always in series
with C40. Connect 2 capacitors in series you get voltage at their junction... no mystery to that.
C40 is on the schematic is a capacitive connection to chassis ground,
hence a zero volt reference up to the currents involved with a preamp.
Crown's mistake was to take the floating ground concept a bit too far by relying on capacitors
alone. Two diodes should retain Crowns concept and allow connection at a semiconductor junction
at the same time, allowing residual voltages to properly dissipate.
Cheers / Chris
0 volts is whatever you choose as the reference voltage - there's nothing absolute about it. All voltages are measured with respect to something else, ie a voltage difference between two points is what is measured. It might be useful to the OP and to anyone who cares to contribute to this thread to establish what is being used as the reference point for voltage measurements. I suggest the negative side of the power supply should be taken as 0V, and that should be the same voltage as the negative end of R17 and the negative end of R20 and the negative side of C17 etc.
The design of this pre-amp is not really the issue here, it's how to rectify the fault so the pre-amp functions as it did when new.
Light Dependant Resistor
19-04-2017, 12:00
It is a dual rail design having positive voltages , earth in the middle- and negative voltages
Some of the voltages are regulated with zeners and good use is made of capacitor multipliers as well.
The transistor amplifier stages borrow some single ended techniques, rather than split rail
so has the appearance of being single ended. Negative voltages though are put to good effect.
Earth then can exist either with sufficient capability of its own, usually termed a virtual earth
and this is probably what you are referring to. A multimeter will flash 00.00 when encountering
a virtual earth. But I see no deliberate virtual earth here.
Rather capacitance via numerous branches of resistance are negotiating a common point
that sadly contains other currents, they all are trying to settle to be a zero volt point we think
is zero volts. The circuit board traces may have more resistance now, than when new hence
the very fine see saw balance they achieved way back then, might not be as good
Crown have gone a step further trying to separate chassis ground which
they have designed as signal ground by using C40 so all those 0v resistance smallish capacitances
see C40 and are desperately trying to get to ground but cannot quite get there.
The sensible option is a pair of back to back diodes, it is a very professional way
of accessing chassis earth by semiconductor lifting,
If voltages are still residual after that, I would head to replacing the bridge rectifier and
carefully examining the power supply components including the transformer.
We are being very nice to each other trying to help, and I am proud to
be part of a forum that goes about discussions this way.
Cheers / Chris
Arkless Electronics
19-04-2017, 12:16
The 100pF cap is fine and supposed to look like that. The 0V is 0V as in any other unit. If only phono effected try R10, 10M resistor.
RothwellAudio
19-04-2017, 13:09
BTW, this page might be of interest to the OP
http://kenrockwell.com/audio/crown/ic-150.htm
Apologies if you've seen it already.
Arkless Electronics
19-04-2017, 13:18
Getting off topic but the DC300A power amp actually measures very well indeed in spite of it's crude (but clever) OPS which runs pretty much in class B! I recall measuring around 0.003% THD from one of mine! Their bass performance is superb and in fact I can't name an amp with better bass... pity about the top end!
(Some) Progress!
First some old business:
Ground - According to the phono schematic, Earth ground and Schematic ground are tied to together. The phono board has a direct line from it's (schematic) ground right to the chassis. It also has another line to the chassis off via C9 (0.1uf 200v filmatic) on the phono board. And of course C40 on the main board etc etc.. My chassis point of ground reference is valid. My meter is working (tried another fluke today- same readings).
I later today actually broke that C9 cap with all my mucking around on the phono board and I don't have a direct replacement for it. ** What is a simple (readily available) equivalent that i can get for that? It broke off after the majority of my testing so did not impact results, but may now be causing other issues.
R20 100K resistor on phono board - replaced with fresh just to confirm no effect on voltage present.
1ufs by the ICs - These were replaced. They did not correct the voltage on the phono board nor at the volume control. They did not correct the popping when switching from phone to tuner or back. They did not correct popping when moving the volume control though MAY of corrected popping when leaving the volume control at a static 85% worth phono selected. I say "may" because it's been a while since testing this specific artifact/ A number of components have been changed since the original post in effort to reduce the post C12 voltage present, including general power supply cap replacements throughout.
Back to back diodes on C40 main baord - I did not have 007's, only 005's to try this. No effect or change on post C12 voltage. I did not expect any. You may have missed the post where it was confirmed this voltage is present on the phono board with the line from R13 (on the phono board) completely disconnect from the main board (via selector switch) C12/C40 are not in series when the line from R13 has been disconnected. There is no audio feed nor connectivity between board on that channel, other than the power line.
Which brings me to my partial solution. I stated earlier the Phono board had good voltage. What I failed to realize (then) was there were actually TWO voltage feeds to that board. I had measured one a 19 volts (schematic says 18) I did not realize to check the other side until just now. It was -15V (schematic says -18v) too low!! traced that back to the power supply section and found a trimmer R28 and adjusted that via the main board -18v test point and WHAMO Problem solved! NO popping on right channel when using controls, when selecting between phono/tuner. None. Clean and sounds great. this is the channel with the fully working EQ and as far as I can tell, this channel is golden now. No voltage (mv's) on R13 anymore (connected or not to the main board). No popping. Nothing to do with anything except for the fact the voltage was incorrect (though I don't understand the "why" here, just that it was the source of the problem.).
That would be it and onto the Left channel tone problem except.....
Now there is voltage on the left channel, even more than before on that channel (now 1.5v), and of course popping. My immediate assumption was I just need to adjust that 19 volts down to 18v, which i did with it's trimmer, but it did not change anything. I went through the same test processes I had done on the other channel. Disconnected R13 (equivalent other channel's) from selector switch/main board. Confirmed a 1.5V dc still there so not induced by main board, must be sourced by phono board. Took C12 (equivalent) cap out, confirmed R13 voltage gone so no board leaks are happening. Back in and updated the R20 (Equivalent) 100K resistor. No change. Back to the same problem to resolve - unexpected DC voltage post C12 - just on a different channel now.
So how did swing up the left channel voltage 3 volts move this problem to the right channel? Or is the Lack of C9 now playing some role?
Another item of note - when Powering up or down and phono selected by default there is a substantial POP that occurs. Again manybe related to C9 missing. Not sure I noticed this happening before C9 broke or not.
RothwellAudio
20-04-2017, 08:46
I later today actually broke that C9 cap with all my mucking around on the phono board and I don't have a direct replacement for it. ** What is a simple (readily available) equivalent that i can get for that?
Any 100nF cap should be fine - there's nothing special about it.
Which brings me to my partial solution. I stated earlier the Phono board had good voltage. What I failed to realize (then) was there were actually TWO voltage feeds to that board. I had measured one a 19 volts (schematic says 18) I did not realize to check the other side until just now. It was -15V (schematic says -18v) too low!!
I didn't see a -18V supply marked on the phono diagram though Q5 and C17 make a lot more sense if the supply to Q5 is -18V. It looks like you have tuned out the DC offset by balancing the positive and negative supply voltages. However, they shouldn't need to be balanced because the DC blocking capacitor (C12) should take care of any DC offset. It remains a mystery to me.
One clue is that adjusting the supply voltage has affected a cure on one channel but created a problem on the other. I'm suspecting an issue with 0V and whether both channels are connected to it properly. I recommend not using the chassis as your voltage reference - it's really only connected to the circuit for safety and for screening and the metalwork should not be used as a conductor within the audio circuit. It might be convenient to stick your meter probe onto the chassis when making readings but it might also be misleading you.
RothwellAudio
20-04-2017, 10:50
BTW, there is a way to check if the problem does have something to do with various bits of circuit not having the same 0V. Look for any voltage between various points that should be at 0V. For example, check for any voltage between the 0V end of R20 (phono section) and the 0V end of R27 (line section). Check between other points where you would expect there to be no voltage.
Any 100nF cap should be fine - there's nothing special about it.
I didn't see a -18V supply marked on the phono diagram though Q5 and C17 make a lot more sense if the supply to Q5 is -18V. It looks like you have tuned out the DC offset by balancing the positive and negative supply voltages. However, they shouldn't need to be balanced because the DC blocking capacitor (C12) should take care of any DC offset. It remains a mystery to me.
One clue is that adjusting the supply voltage has affected a cure on one channel but created a problem on the other. I'm suspecting an issue with 0V and whether both channels are connected to it properly. I recommend not using the chassis as your voltage reference - it's really only connected to the circuit for safety and for screening and the metalwork should not be used as a conductor within the audio circuit. It might be convenient to stick your meter probe onto the chassis when making readings but it might also be misleading you.
18 v vs -18v - The line off of R56 on the phono board that says 18V is a typo (or a PDF anomaly) All other board revisions pdf show this as -18v. That's what threw me and why I never checked it.
Board ground vs chassis ground - If you look at the bottom of R17 where the ground symbol is, on the phono board at that location, there is a wire that comes off the board direct to the chassis. there is no difference on my meter whether I measure using my probe direct on the board at that point, or on the chassis. The meters readings are identical. I understand that in some designs, the chassis is not the same as the circuit ground and may have a different potential but it appears in this case (no pun intended) they are one and the same.
BTW, there is a way to check if the problem does have something to do with various bits of circuit not having the same 0V. Look for any voltage between various points that should be at 0V. For example, check for any voltage between the 0V end of R20 (phono section) and the 0V end of R27 (line section). Check between other points where you would expect there to be no voltage.
Checked all the 0V (ground) points on the phono board. All good. they are common trace points on both channels. I'll have to map out the Main board as it's hard to read the layout schematic, but keep in mind the voltage occurs on the phono board with the input line disconnected from the main board.
Did add a (temporary) 0.1uf for C9 - no change.
What's interesting about the voltage is it starts at about .5v (same as before when i was troubleshooting the other channel) I/knew it was always wrong but it the was lesser of two evil channels, so to speak. After turn on, it climbs fairly rapidly up to the roughly 1.2-1.5v, maybe about 0.1th volt every 15-30 seconds. I don't recall it climbing originally (on that channel). This happens with the R13 (equivalent) line disconnected from the main board , so it's building up a charge on the phono board.
Arkless Electronics
20-04-2017, 17:23
Yes it's -18V
I thought I woudl take a different approach to solving this. Since we can't figure out why C12 isn't filtering DC properly, I thought may we should figure out why it getting so much voltage in the first place, so i took some measurements (see pic). The first value is the problem left channel, second value is the now working right channel. If there is only one value, it is the same on both channels. You can see there is roughly 3 volts floating around on the left channel where there is not on on the right. Q1 was in a socket so I pulled and tested - tested good no leaks. Q2 was soldered in, but I pulled and tested too, again, tested good no leaks. R56 and R19 slightly out of tolerance so I pulled and replaced. R12 measures fine so left in place. C12/R20 of course were replaced previously. The only other item I suspected was C10 that 27pf cap. I pulled but not sure my meter reads correctly that low. Seeing about 70pf, but then again, my meter shows some 30+pf with no leads connected to anything, so I'm not trusting any readings there. i don't have a replacement to test with. Does it make sense that if C10 was bad that could be the cause of the 3 volts there? Or possibly C15 that 8.8nf cap? Or Q2 really is bad even though tested good? (Since Q1 was socketed I swapped channels - no change) I don't see any other avenues for that "extra" voltage to get created.
20058
it's also coincidental that the right channel was 3v too low, raising it up 3 volts and now 3 volts show up on the left where unexpected....
RothwellAudio
21-04-2017, 09:43
Since we can't figure out why C12 isn't filtering DC properly, I thought may we should figure out why it getting so much voltage in the first place, so i took some measurements (see pic).
It shouldn't really matter what the voltage is on the left hand side of C12 (as we look at the diagram) because it's the voltage on the right hand side that is being sent to the line stage, and that voltage should be zero. Measurements of that voltage might be useful.
I suspect a DC path in parallel with C12 is causing the problem, as if a large value resistor was in parallel with it. You could remove the cap and try to measure resistance across the PCB pads. Is the PCB fiberglass or paper? Old compressed paper boards are more likely to become conductive.
BTW, there are two components in the phono circuit which look a bit odd to me - R21 and R23. Why is R21 a preset? It looks like it's for fine-tuning the gain and should have any effect at DC, but I'm not sure about that. Also, why is R23 "selected"? And what is that arrow pointing to? I can't even work out what Q5 is doing. Jez might have a better idea than me.
Arkless Electronics
21-04-2017, 12:01
It shouldn't really matter what the voltage is on the left hand side of C12 (as we look at the diagram) because it's the voltage on the right hand side that is being sent to the line stage, and that voltage should be zero. Measurements of that voltage might be useful.
I suspect a DC path in parallel with C12 is causing the problem, as if a large value resistor was in parallel with it. You could remove the cap and try to measure resistance across the PCB pads. Is the PCB fiberglass or paper? Old compressed paper boards are more likely to become conductive.
BTW, there are two components in the phono circuit which look a bit odd to me - R21 and R23. Why is R21 a preset? It looks like it's for fine-tuning the gain and should have any effect at DC, but I'm not sure about that. Also, why is R23 "selected"? And what is that arrow pointing to? I can't even work out what Q5 is doing. Jez might have a better idea than me.
It's certainly a very weird design! It also shows use of basically a Hawksford cascode long before Hawksford "discovered" it! Q5 appears to be further filtering of the -18V rail in conjunction with c17. This is probably shared between the channels hence the arrow from there to the other channel. It has separate AC and DC feedback loops and it appears to me that r23 is selected to set up the correct DC conditions but will also affect RIAA accuracy and gain, hence the preset to adjust out any inaccuracy due to this.
I've only skimmed through this thread but if the problem is only when phono is selected I still suggest R10 as a likely culprit with R11, Q3 and C11 worth looking at.
RothwellAudio
21-04-2017, 12:37
... it appears to me that r23 is selected to set up the correct DC conditions but will also affect RIAA accuracy and gain, hence the preset to adjust out any inaccuracy due to this.
Having investigated further it appears that this pre-amp has adjustable gain for the phonostage, as can be seen in this picture
http://www.soundsgoodtomehouston.com/crown/ic150/ic150_rightrear.jpg
I presume the adjustment pots seen in the picture are R21 on the diagram.
BTW, I wish I know how to embed images into a thread :scratch:
Arkless Electronics
21-04-2017, 13:10
Having investigated further it appears that this pre-amp has adjustable gain for the phonostage, as can be seen in this picture
http://www.soundsgoodtomehouston.com/crown/ic150/ic150_rightrear.jpg
I presume the adjustment pots seen in the picture are R21 on the diagram.
BTW, I wish I know how to embed images into a thread :scratch:
Ah yes, it's on the next page down in the manual if I'd looked.... 50-70dB variable gain. That's going to have an effect on RIAA accuracy I reckon.
BTW, I wish I know how to embed images into a thread :scratch:
http://theartofsound.net/forum/showthread.php?2-How-to-host-and-embed-images-in-your-posts
RothwellAudio
21-04-2017, 15:36
http://theartofsound.net/forum/showthread.php?2-How-to-host-and-embed-images-in-your-posts
I tried that and failed :doh:
I'll try again...
http://www.soundsgoodtomehouston.com/crown/ic150/ic150_rightrear.jpg
RothwellAudio
21-04-2017, 15:37
Flippin'eck - it worked :eek:
:yesbruv:
Flippin'eck - it worked :eek:
It shouldn't really matter what the voltage is on the left hand side of C12 (as we look at the diagram) because it's the voltage on the right hand side that is being sent to the line stage, and that voltage should be zero. Measurements of that voltage might be useful.
I suspect a DC path in parallel with C12 is causing the problem, as if a large value resistor was in parallel with it. You could remove the cap and try to measure resistance across the PCB pads. Is the PCB fiberglass or paper? Old compressed paper boards are more likely to become conductive.
BTW, there are two components in the phono circuit which look a bit odd to me - R21 and R23. Why is R21 a preset? It looks like it's for fine-tuning the gain and should have any effect at DC, but I'm not sure about that. Also, why is R23 "selected"? And what is that arrow pointing to? I can't even work out what Q5 is doing. Jez might have a better idea than me.
I mentioned a few posts back that after I correct the power supply voltage feed and the problem flipped channels to voltage on the right side of C12 was about 1.5v (after a couple minutes. It starts immediately about 0.5v at power on and climbs steadily up.) This occurs whether or not the upper side of R13 is connected onto the main board or not, so the voltage is sourced from the phono board itself. So C12 is doing SOME DC reduction (3-3.5 volts down to 1.5) but not all. I also mentioned taking C12 out of circuit brings that voltage to zero so if that board had leaks between traces, there should still be voltage on the the post side of C12 where it was removed, but there is none. I did not measure the resistance though but can take it out again and get that measurement. But as for C12 DC removal, i figured there is not supposed to be any (substantial) voltage on the left side of C12 to begin with that needs to be reduced. there is none on the right channel side, only this channel. Maybe we could find out where that is coming from and stop it?? If the voltage source has come from the RIGHT side of C12 (e.g from a trace leak somewhere) how could it get bigger on the left side? That doesn't makes sense to me.
I'm sorry that I can't say what kind of board or material is used. I really don't know.
The schematic is a little misleading in that they show everything that is for one channel side and you have a duplicate for the other channel EXCEPT the section around Q5/C17/D1. Those components are shared between the channels and there are only one of them on the board. I had confirmed D1 is good a while back, and C17 was replaced with a 220uf (spec called for 250uf - I could put a 330uf there if it makes a difference). In any case, any issue with that portion of the circuit would have an impact on both channels. That Arrow shown simply goes to the other channel at the same spot in the schematic (to the other channel's selected R value)
FYI - With C12 out of circuit it measures both 0v and 0 ohms across those traces where C12 was.
It's certainly a very weird design! It also shows use of basically a Hawksford cascode long before Hawksford "discovered" it! Q5 appears to be further filtering of the -18V rail in conjunction with c17. This is probably shared between the channels hence the arrow from there to the other channel. It has separate AC and DC feedback loops and it appears to me that r23 is selected to set up the correct DC conditions but will also affect RIAA accuracy and gain, hence the preset to adjust out any inaccuracy due to this.
I've only skimmed through this thread but if the problem is only when phono is selected I still suggest R10 as a likely culprit with R11, Q3 and C11 worth looking at.
I guess I missed you previous suggestion to check these. I can check them but if any one of them were bad, wouldn't that change the DC voltage at the base/emitter of Q1 and/or left side of R15? Currently, those readings are exactly the same on both channels. I would think a part failure, or even just a value drift, where you describe would affect those readings. Am I incorrect?
Arkless Electronics
21-04-2017, 18:46
I guess I missed you previous suggestion to check these. I can check them but if any one of them were bad, wouldn't that change the DC voltage at the base/emitter of Q1 and/or left side of R15? Currently, those readings are exactly the same on both channels. I would think a part failure, or even just a value drift, where you describe would affect those readings. Am I incorrect?
It probably would yes but large value carbon composition resistors (1M and above) are liable to go open after many years and are always a good suspect...
Pulled and broke the 10M resistor. Had done a comparative reading before hand to the other channel - both measuring about 6M in circuit but figured I had to lift it to get accurate on it and my clumsiness broke it so there's a brand new one there now. Other resistor measures fine. Cap fine.
As for Q3, labeled 2962 on the schematic but the service manual has it listed as "TZ81 selected" - once desoldered that one is failed in a strange way on my old transistor meter. It registers as NPN on two different pairs, but one pair, which which shows full leakage, is regular intermittent showing it as NPN. Never seen this before. Usually I see two pairs register as the same thing 100% of the time when the transistor is failed. Regardless, looks like it needs to be replaced. Just to confirm nothing strange with my meter going on, pulled Q1 (which is socketed and exact same transistor) and tested fine. Pulled Q1 on the other channel and tested - registers as NPN on two different legs pairs (though no leakage on either) so I assume that one is going too (yet that channel is working fine??) Did not pull Q2 the soldered one on that channel yet, but suspect I need to check it as well.
***Anyway - suggestions for replacement for this transistor?
Also, assuming it was failed all along and not my desoldering effort that hurt it, How does that failed Q3 explain DC voltage on the other side of C12? This make no sense to me.
RothwellAudio
22-04-2017, 10:18
So C12 is doing SOME DC reduction (3-3.5 volts down to 1.5) but not all. I also mentioned taking C12 out of circuit brings that voltage to zero so if that board had leaks between traces, there should still be voltage on the the post side of C12 where it was removed, but there is none.
Yes, if removing C12 causes the DC to disappear it looks like there isn't a DC path between the PCB pads.
i figured there is not supposed to be any (substantial) voltage on the left side of C12 to begin with that needs to be reduced. there is none on the right channel side, only this channel. Maybe we could find out where that is coming from and stop it?? If the voltage source has come from the RIGHT side of C12 (e.g from a trace leak somewhere) how could it get bigger on the left side? That doesn't makes sense to me.
I'm sorry that I can't say what kind of board or material is used. I really don't know.
Finding DC on the 0V side of C12 is baffling to me and I can't figure why it's there. Yes, if you could get rid of the DC offset on the other side of C12 you would cure the popping - but then there wouldn't be any need for C12 to be there at all. It still wouldn't explain why a DC blocking cap isn't blocking DC.
Personally, I would do what I suggested in post #37.
BTW, I've since learned the circuit board is fiberglass. I asked because old paper circuit boards can absorb moisture and dirt and become (a bit) conductive.
FYI - With C12 out of circuit it measures both 0v and 0 ohms across those traces where C12 was.
0 ohms? Did you mean open circuit? 0 ohms would explain the problem entirely.
Arkless Electronics
22-04-2017, 12:51
Pulled and broke the 10M resistor. Had done a comparative reading before hand to the other channel - both measuring about 6M in circuit but figured I had to lift it to get accurate on it and my clumsiness broke it so there's a brand new one there now. Other resistor measures fine. Cap fine.
As for Q3, labeled 2962 on the schematic but the service manual has it listed as "TZ81 selected" - once desoldered that one is failed in a strange way on my old transistor meter. It registers as NPN on two different pairs, but one pair, which which shows full leakage, is regular intermittent showing it as NPN. Never seen this before. Usually I see two pairs register as the same thing 100% of the time when the transistor is failed. Regardless, looks like it needs to be replaced. Just to confirm nothing strange with my meter going on, pulled Q1 (which is socketed and exact same transistor) and tested fine. Pulled Q1 on the other channel and tested - registers as NPN on two different legs pairs (though no leakage on either) so I assume that one is going too (yet that channel is working fine??) Did not pull Q2 the soldered one on that channel yet, but suspect I need to check it as well.
***Anyway - suggestions for replacement for this transistor?
Also, assuming it was failed all along and not my desoldering effort that hurt it, How does that failed Q3 explain DC voltage on the other side of C12? This make no sense to me.
BC550C or similar should work fine
Yes, if removing C12 causes the DC to disappear it looks like there isn't a DC path between the PCB pads.
Finding DC on the 0V side of C12 is baffling to me and I can't figure why it's there. Yes, if you could get rid of the DC offset on the other side of C12 you would cure the popping - but then there wouldn't be any need for C12 to be there at all. It still wouldn't explain why a DC blocking cap isn't blocking DC.
Personally, I would do what I suggested in post #37.
BTW, I've since learned the circuit board is fiberglass. I asked because old paper circuit boards can absorb moisture and dirt and become (a bit) conductive.
0 ohms? Did you mean open circuit? 0 ohms would explain the problem entirely.
I misstated - open circuit, not zero ohms. Confirmed on two meters and a continuity tester as well. To clarify, that only reads open circuit if I lift the ground leg off R20 (R13 leg is already lifted from main board). If I leave R20 in place I get about 280-300K+ and climbing resistance between traces where C12 was (100K from R20 through ground, and the rest from someplace else in circuit that ultimate goes to the same ground that R20 does. I assume one the the caps since the resistance is climbing as I measure. Again, this is reading between the two points that C12 was. I have 100% confidence there is not a dead short between those points.
I don't have a separate power supply to rig up the suggestion in post 37, but I have confirmed readings on two meters, one is a fluke (which although very accurate I don't regularly use because it doesn't have auto range and that's a pain!). There is nothing wrong with my meter.
BC550C or similar should work fine
Don't have any of those but can order them. I have BC549B's and BC547's.
Can't find hardly any info on a TZ81. Only thing I saw on a data sheet was hfe value of 60. Google search on alternatives yields only 2 or 3 mentions of alternatives. One said 2n9249 was the alternative but now defunct. Another said people have used 2n3904 with success. Those seem to be readily available too. Thoughts on that vs the BC550c (or my 549 or 547) given the limited info on the TZ81 spec? I confess I'm a novice with respect to understanding transistor specs (and a lot of other electronic related things too lol but i like to learn.)
Arkless Electronics
22-04-2017, 20:41
Don't have any of those but can order them. I have BC549B's and BC547's.
Can't find hardly any info on a TZ81. Only thing I saw on a data sheet was hfe value of 60. Google search on alternatives yields only 2 or 3 mentions of alternatives. One said 2n9249 was the alternative but now defunct. Another said people have used 2n3904 with success. Those seem to be readily available too. Thoughts on that vs the BC550c (or my 549 or 547) given the limited info on the TZ81 spec? I confess I'm a novice with respect to understanding transistor specs (and a lot of other electronic related things too lol but i like to learn.)
Pretty much any NPN transistor should work. The BC550C is a low noise very high gain device. The BC549B is of the same family and should work great. Any similar spec USA part should work just the same as well.
RothwellAudio
23-04-2017, 10:02
I misstated - open circuit, not zero ohms. Confirmed on two meters and a continuity tester as well. To clarify, that only reads open circuit if I lift the ground leg off R20 (R13 leg is already lifted from main board). If I leave R20 in place I get about 280-300K+ and climbing resistance between traces where C12 was (100K from R20 through ground, and the rest from someplace else in circuit that ultimate goes to the same ground that R20 does. I assume one the the caps since the resistance is climbing as I measure. Again, this is reading between the two points that C12 was. I have 100% confidence there is not a dead short between those points.
I don't have a separate power supply to rig up the suggestion in post 37, but I have confirmed readings on two meters, one is a fluke (which although very accurate I don't regularly use because it doesn't have auto range and that's a pain!). There is nothing wrong with my meter.
I wasn't suggesting there was a dead short between those points - there doesn't have to be to cause a problem. A high resistance of a few hundred ohms would create a DC offset.
Could you post a few photographs so we can see the physical layout of the boards etc? That might be useful.
I wasn't suggesting there was a dead short between those points - there doesn't have to be to cause a problem. A high resistance of a few hundred ohms would create a DC offset.
Could you post a few photographs so we can see the physical layout of the boards etc? That might be useful.
To clarify my comment If I leave R20 in place I get about 280-300K+ and climbing resistance between traces where C12 was... That was really 280K* - 300K.
Board pics attached though it's messy to look at. Keep in mind, it was doing this long before my soldering iron ever touched that board! Also attached the board layout from the service manual, but it's so poor not sure that will be much help. My pics are laid out in the same format to match.
I did replace that channel transistor pair with the new BC549B's I had, and those seem to work just fine. Seems even compare to the the other channel with no issues there. Didn't resolve the voltage post C12, but I didn't expect it to either. Really not sure how the channel worked at all given those previous transistor readings, but now there is at least a fresh pair there and those are verified good transistors
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RothwellAudio
24-04-2017, 08:33
Actually, in my previous post when I said "a high resistance of a few hundred ohms would create a DC offset" I meant to say "a high resistance of a few hundred kilohms would create a DC offset". Sorry for the typo.
Imagine 200k in parallel with C12: in conjunction with the 100k of R20 you would have a DC potential divider leaving you with one third of the DC offset on C12.
Thanks for the pictures but what I had in mind was more of an overall shot. I was wondering if it's all laid out on one PCB or if there are lots of individual PCBs and whether or not the controls are on PCBs etc. I still think there's something odd about the way the sections are linked.
Arkless Electronics
24-04-2017, 11:07
Jim. Can you be more specific as to the nature of the problem please... Is it a continuous repetitive popping? a single pop?
Also what test gear do you have available?
Just two pcbs - the phono board (pics I posted) and the main board. Main board has all the controls mounted. Pic attached for that board. You might have something about that resistance though. With everything back in circuit, all connected etc. as well as to the main board. I'm seeing about a 110K resistance across C12 on that problem side, whereas it's about 185K across C12 on the other channel.
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Jim. Can you be more specific as to the nature of the problem please... Is it a continuous repetitive popping? a single pop?
Also what test gear do you have available?
Initially, it was popping heavily on the RIGHT channel when switching to or from the phono input, as well as continuous popping when phono was selected. It was identified there was a large unexpected DC voltage on the other side of C12 (4 - 5 volts, C12 on the left channel also had an issue of about 0.5 volts but just not as bad). Also confirmed this voltage is being sourced by the phono board itself, as it is measured with the main board disconnected from it on that channel. A number of components were replaced (per spec) on both boards in effort to reduce the popping and voltage. Ultimately, I found the negative source voltage to the phono board was far too low (should have been -18v, was -15v). That was adjusted via main board trimmer and eliminated the problem on that channel (post C12 DC voltage now ok on that channel)
So that brings us to the LEFT channel with it's unexpected DC voltage post C12. This time, the source voltage (+18v) was correct and did not need to be adjusted. A number of components checked/replaced to spec, found a couple leaky transistors as well which have been replaced but nothing has resolved it yet. However, the popping is not as bad as it was on the left channel. Neither as much in volume when switch to/from phono (but is present and obvious), nor is it continuous anymore once selected. That may simply be because the Post C12 DC voltage is substantially lower than the other case.
As far as test equipment, I have a voltmeter (two actually), capacitance meter, ESR meter (which also does capacitance). I just picked up an oscilloscope last week at an auction but I have to find some leads for it, confirm it even really works, and relearn how use one, since I haven't used one since I was a kid in high school (decades ago LOL)
To clarify the above - It's a single pop now on select or deselect of phono (or power on/off with phono preselected) with a Post C12 DC voltage that starts initially about 0.5v and ramps to about 1.5v quickly
RothwellAudio
25-04-2017, 12:31
After an exchange yesterday on another thread I think Jez may be reluctant to offer any more help and advice on the forum. Shame, because his input would have been valuable here. I'm afraid I'm running out of ideas.
That is a shame because his input has been helpful and I've learned something new from both his posts and yours. I may just have to live with the pop. Perhaps you could help me move forward with figuring out why the tone section doesn't seem to be working on that channel? I failed to mentioned that in lieu of me getting an oscilloscope up and running, I've always had an audio tester that has served me well for tracing audio circuit - just not always sure where to put it to check LOL.
So I get audio all the way through on that channel whether tone controls are engaged or not but when they are engaged the audio is "muffled" and the bass/treble controls don't have any effect. Disengaged, audio is nice and clear, just like the other channel's when disengegaged.
RothwellAudio
25-04-2017, 20:11
Resistors and capacitors tend not to fail easily unless they're dissipating enough power to get hot or have lots of voltage across them so they wouldn't be the first thing I'd check. On the other hand, switches and pots do get dirty and can be problematic so I would give them a squirt of lubricating cleaner if you haven't done so already. I've known horrible sounding amps to come back to sounding like new after a simple switch/pot clean-up.
Worth a try before tackling anything else.
That was the first thing I had done before starting the whole thread. I'll put some more in through that tone defeat swicth again though Guess I'm looking for advice for some spots to touch down on the audio tester to see where the signal might be getting lost (though it never really gets "lost" it still gets through, just isn't being "processed" correctly...
Arkless Electronics
01-05-2017, 14:47
After having had an apology from Marco over misconstrued comments in another thread (thanks Marco), I am once again helping out technically so lets get this mother sorted then Jim!
It will be helpful if you get that 'scope fired up....
Glad to hear you're back! I have to start by ordering some leads for the oscilloscope. I do have an audio tester in the meantime
RothwellAudio
01-05-2017, 17:45
I have to start by ordering some leads for the oscilloscope.
For audio work you don't really need anything fancy - you could cobble together almost anything with whatever bits of wire you have available if you have a suitable connector for the scope (BNC?). It would be good enough to get you up and running at least.
Arkless Electronics
01-05-2017, 17:57
For audio work you don't really need anything fancy - you could cobble together almost anything with whatever bits of wire you have available if you have a suitable connector for the scope (BNC?). It would be good enough to get you up and running at least.
+1 for basic audio stuff.
Jim. What scope do you have and what is this "audio tester" of which you speak?
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