Faulty audio pre-amp circuit troubleshooting Monacro PA-12040

[joellinn]

Hi folks,

I am troubleshooting a faulty Monacor PA-12040 PA Mixing Amplifier.
I already fixed some things:

• Bulging high-pass filter caps (per-channel, C25 1000u in the pic)
• Bad connector/switch contacts (isopropanol cleaning)

However, the output level of channel 3 is too low.
Compared with the other two channels, the inputs of the first stage (U5) are equal but the outputs of the op-amps differ. The output levels and phases are in fact equal to the inputs (using a test-tone, pins 1==2==3 and 7==5==6) as if they were configured in unity gain.
I checked most of the resistors that configure the gain in circuit (R42, R44, R45, R46, R47, R49) but they all seem to be in tolerance.
U6 is putting some gain on the signal.

I am leaning towards a broken op-amp U5 but I am unsure as I have basically zero experience troubleshooting stuff like this.
I am very grateful for any tips 🙂

[Audiorepair]

It is very common on these kind of circuits that the protection diodes go faulty.  (D13, 14, 15, 16)

Measure each one on diode mode (forward voltage) on a multimeter, you may find one or more reading different to the others, or to adjacent channels.


You can remove them altogether for testing purposes, it won't affect the preamp.

[floobydust]

Normally C25 does not see much voltage or current, certainly not enough to make gas and bulge out. It's connected only to op-amp inputs.
So I would say U5 failed to cause that.

[joellinn]

Measure each one on diode mode (forward voltage) on a multimeter, you may find one or more reading different to the others, or to adjacent channels.

I tested them already and they all seemed fine (0.6V +- ~10% IIRC).
Also the inputs of the op-omp U5 are the same level as on another channel for the same test signal. So I ruled out the diodes. Am I wrong?

[joellinn]

Normally C25 does not see much voltage or current, certainly not enough to make gas and bulge out. It's connected only to op-amp inputs.
So I would say U5 failed to cause that.

C25 had a bulged top on all 3 channels so it simply might have been a cheap brand (other caps are from another brand) and the years passing by

[Audiorepair]

Faulty op-amps nearly always go DC output, they rarely just stop working.

And yes, you appear to have ruled out the diodes.

[elecdonia]

Is it possible for you to post photos of PC board?
Did you replace C25 (1000uF)?
What effect does gain selector switch SW3 have? (if any)

I believe SW3 has 3 positions:
1)   Low gain for line-level signals. Only R44 (15k ohms) is in feedback circuit. For this case the gain at output pins of U5A/B will be < 2x
2)   High gain for microphone. R42 (47 ohms) is in parallel with R44. This raises gain much higher, to ~220x
3)   High gain for microphone with phantom power (48V DC) supplied to microphone

If gain fails to increase when SW3 is in microphone position then check C25, R42, and SW3
C25 needs to have low ESR ( < 1 ohm ). A brand-new 1000uF capacitor typically has < 0.1 ohm ESR.

For high-gain microphone input, the contact resistance of SW3 must be < 1 ohm and stable. Sometimes I find it necessary to remove switch from PC board, disassemble it, and clean/polish the contact surfaces until they are bright and shiny. Do not use sandpaper or abrasives. These contacts have thin metal plating. I use strips of “business card” paper saturated with contact cleaner. My preferred contact cleaner is “Deoxit”

[elecdonia]

Normally C25 does not see much voltage or current

Voltage is in millivolt (mV) range. Current is microamps (uA).

However, for microphone input (high gain) C25 is in series with R42  (47 ohm) gain setting resistor. Therefore ESR of C25 must be < 1 ohm.

Also SW3 contact resistance must be < 1 ohm.

Oxidation layer on SW3 contact surfaces would only need to be 100 molecules thick to become open circuit for millivolt signal amplitude. Spraying contact cleaner into assembled switch may fail to sufficiently clean the contacts. Sometimes it is necessary to remove switch from PC board, disassemble it, and hand-polish contact surfaces until bright and shiny.

[floobydust]

I think you need more power to generate gas in C25, like from backfeed through U5 input diodes or a shorted input stage in it. This mic reamp has high value resistors R93, R94 to protect the diodes and IC if a phantom power wallop hits it.
But no matter OP says he replaced C25. If high ESR it would cause low gain.

At rest there should be around 0V at the op-amp's outputs U5A pin 1, 7 and U6A pin 1. Not a lot of offset unless something is amiss.
This is with phantom power on or off, C22/C23 I usually outright replace and they are only 50V parts here. They could be leaky as well.

edit: server is acting up, had to reenter message

[joellinn]

Thanks for the suggestions. I was able to fix it.
I was testing in low gain configuration so the switch wasn't the issue.
The 1000u caps are Panasonic low ESR (20 mOhm) and were working fine for the other channels.
I also remember measuring the capacity between U5 pin 6 and C25 + successfully.
Nonetheless I tried bridging C25 with a soldered wire but the issue remained. When un-soldering the test-bridge the negativ solder joint of the cap looked "wrong".
Turns out there is some issue with the pad, now it was even open to the multi-meter and I couldn't fix it with the iron and plenty of flux. I ended up scratching the trace and putting in a wire (see pictures).
I can't say for sure if the pad was damaged even before I replaced the Cap but the width of the copper ring around the hole looks comparably small to me for a heavy cap.

Thanks for your help!

[joellinn]

front side for the interested