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
crown150-phono board part.jpg
Crown IC150 Capacitor.jpg
Last edited by jimkarl; 07-04-2017 at 02:35.
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.
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 ?
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
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
Originally Posted by jimkarl
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
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.