Intermittent scratches/distortion/level changes in preamp

[ilya]

I'm repairing the BAE preamp (that is a copy of Neve 1073 mic preamp section). It has very unpleasant and difficult to trace problem. Once in a while the sound becomes distorted and changes in level (although the change is not drastic, my estimate is 2-4dB), and after some short time everything gets back to normal. The device can work fine for several days, and then show the above defect.

My initial thought was contact problem. But the components are top notch, and I can't force the device into the fault by massaging switches and output pot.
My next thought is that maybe it's some failing capacitor. Maybe a transistor, but I'm thinking that the cap is more likely.

How would you approach the trouble shooting? Hot air/freeze spray? Any ideas?

The circuit attached shows two main amp stages, top being the first one, bottom being the second one.

[mediatechnology]

I would try freeze spray.

[ilya]

I would try freeze spray.

I don't have a lot of experience with freezer. Is it safe to use it on live circuit? Or should I power it down, apply freezer and then power up again?

[mediatechnology]

The product I use which is TechSpray is OK to use on a live circuit.

Sometimes condensation will form which will affect high impedance circuits and cause short term leakage but usually it will break whatever is intermittent.

You might also try a hair dryer.

[ilya]

Tried the freezer. No dice.
The next is a hair dryer. But I doubt it will show anything.

Another suspects are the push buttons, which are a type of multipole (ala dialistat, but modern) thingies. I tried to make them produce the fault that I described and I sort of succeeded. Sort of, because it’s rather difficult to put these buttons in such position.

[mediatechnology]

I would focus on the buttons.

[ilya]

As expected, hair dryer did not make anything break.

I've tried cleaning switches, but there's a ton of components and wires dangling on these buttons (at both sides!), so I didn't dare to unsolder it all and disassemble the buttons. I hope that all the chemistry I used managed to get inside the buttons and make its "magic" there.
Finger crossed...

[ilya]

I think I found the culprit. It's the buttons. Wayne, you were right.
The manufacturer solution is very questionable. Basically, this is a single input preamp that is pretending to be able to take line levels. But in fact, what "Line in" button does is insertion of the 36dB pad. This is not that bad, but with phantom engaged, the 48V passes right through to the input. Now, imagine pushing the button with 48V present. This is a guaranteed disaster and a click/pop factory.
I tried cleaning the buttons with contact cleaner, but no luck. I never liked pushbuttons for signal lines...

[mediatechnology]

Glad you found it. Too bad contact cleaner didn't resolve it.
Are the contacts silver?

[ilya]

I can’t tell because I don’t recognize the exact brand of the button. My bet is they are silver.