Equalizer Troubleshooting

[user181]

I have an AudioControl C-22 equalizer which has lower volume in one channel only when the EQ is engaged.

I have not yet opened it to begin troubleshooting, but I'm trying to get an idea of what kinds of things could be possible contributors to this. The unit is essentially mechanical switches and several op-amp circuits. Since there is still audio present when the EQ is engaged, I'm not sure if the engage/disengage switch could be a factor, unless it needs cleaned.

Would it be possible for a bad/failing capacitor in one of the gain stages to cause this sort of problem? I have ordered an in-circuit ESR capacitor tester, and my initial plan of attack is to check all the electrolytics.

Beyond that, how likely is it for an op-amp to be bad?

The basic schematic (though tough to read) is on page 31 of this PDF:

https://www.audiocontrol.com/downloads/home/discontinued/c-101/C-22_C-25 Owners Manual.pdf


Lastly, this issue has surfaced 3 times over the last 20 years. Each time I took it to a shop (3 different ones) and got it back in working order, but they never seemed to find a definitive issue. They've re-soldered connections and cleaned the controls, and one time they said they replaced a capacitor.

Thanks!

 

[SaSi]

I have had dirty switch contacts cause some attenuation on devices (like preamp/integrated amps). I would first make sure switches are clean before attempting any further troubleshooting or adjustments. Like you say, you have had it for years.

 

[elnaldo]

A coupling capacitor can do what you describe, to block or tame the signal. I've seen it some times. I think you need to trace the signal path, not difficult to do, even without powering the unit, injecting a signal at each stage, perhaps from one OPamp output to the next amplifier stage , to see if the signal keeps the level. Sometimes you discover the signal is lost between active components. If those tests are OK, you need to trace the live circuit, following the signal from the input.

 

[user181]

Thanks for the great tips!

I don't have an audio signal injector/tracer, and am shopping for one. I've found a number of kits and DIY circuits online, as well as vintage test gear on eBay. What do you guys use for this? I tend to prefer readymade purpose-built test equipment, but I don't have any qualms about building something either.

 

[elnaldo]

signal generator: A cell phone, with some app called test tone generator or similar

signal tracer, something like this, connected to a second amplifier to listen to the signal, or connected to a computer sound card to see the signal with some software like Visual Analyzer (no affiliation), or similar (there is a thread at the DIY forum about PC based oscilloscopes and test apps). If connected to the sound card, be careful with the levels, you can BURN your sound card with a high voltage or some spark or discharge. The PC based oscilloscope probes use to have protections (zeners and voltage dividers) to avoid high voltages reaching the computer.

Anyway I was thinking to measure the signal measuring AC voltage with your meter, at some low freq (so the meter is more accurate)

Don't discard the simplest, cleaning the switches or just finding a cold solder can solve your problem, do that before spending a lot of time making-buying this signal tracers.