Repairing mic-pre popping

[object88]

I just landed a deal for $200 24-channel Soundcraft Studio Spirit (score!) with bad mic-preamps. Apparently, they are silent below 3/4ers gain, and are intermittent above that with popping as they come in and out. I haven't seen the board yet (outside a photo), so I don't the exact condition, and I haven't done any hands-on testing.

Does anyone know anything about making repairs of this sort? I'm not afraid of opening this sucker up and getting busy with a soldering iron-- I'm still high from a repair job I did to an old Ampeg tube amp. Any idea what such intermittent sound and popping means? And since it's on all channels (don't know if it's all preamps or all channels-- preamps and line-ins), should I look more at the bus first?

Alternately, I can ignore the preamps, since I mostly use synths, and go outboard for those rare "other" instances. That would be a good excuse to invest in a Hamptone kit...

Thanks for any advice! I'm excited...

[puffpastry]

I assume that only some of the channels are popping, correct?

I'm also guessing it uses a discrete input stage, followed by 5532's or TL074's

Off the top of my head, I can think of three things that would cause your mic preamp to pop.

The simplest thing is, of course, dirty or intermittent gain pots. Clean or replace the offenders, and you're fine.

The second is DC voltage caused by something leaking--an electrolytic cap, a weak junction in a transistor, that sort of thing.

The third is some type of low-frequency oscillation (motor-boating) caused by a faulty decoupling cap along a power supply rail.

[object88]

I assume that only some of the channels are popping, correct?

My understanding is that all channels are popping. The tape returns are good.

[object88]

bump...

If no-one here has an answer, any ideas where else I might ask around? Thanks!