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Thread: Crown IC 150 Preamp popping noise - source of problem?

  1. #11
    Join Date: Apr 2017

    Location: maryland

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    I'm jim.

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    Quote Originally Posted by RothwellAudio View Post
    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.

  2. #12
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 701
    I'm Andrew.

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    Quote Originally Posted by jimkarl View Post
    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.

  3. #13
    Join Date: Apr 2017

    Location: maryland

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    I'm jim.

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    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?

  4. #14
    Join Date: Oct 2016

    Location: Bolton, England

    Posts: 701
    I'm Andrew.

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    Quote Originally Posted by jimkarl View Post
    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?


    Quote Originally Posted by jimkarl View Post
    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.
    Quote Originally Posted by jimkarl View Post
    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.
    Quote Originally Posted by jimkarl View Post
    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.

  5. #15
    Join Date: Sep 2013

    Location: North Island New Zealand

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    I'm Chris.

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    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

  6. #16
    Join Date: Oct 2016

    Location: Bolton, England

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    I'm Andrew.

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    Quote Originally Posted by Light Dependant Resistor View Post
    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.

  7. #17
    Join Date: Sep 2013

    Location: North Island New Zealand

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    I'm Chris.

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    Quote Originally Posted by RothwellAudio View Post
    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

  8. #18
    Join Date: Oct 2016

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    Quote Originally Posted by Light Dependant Resistor View Post
    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.

  9. #19
    Join Date: Sep 2013

    Location: North Island New Zealand

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    I'm Chris.

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    Quote Originally Posted by RothwellAudio View Post
    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

  10. #20
    Join Date: Apr 2017

    Location: maryland

    Posts: 32
    I'm jim.

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    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.

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