So I replaced the FET and the capacitor in the compressor circuit, and it works like a dream now. Couple of other issues to work out still, but that was definitely the biggest one. Thanks for the help!

Every time... When I turn the compressor off, I can play as loud as I want and it never pops; when the compressor switch is on and the compressor threshold is set really high, beyond the achievable volume of the input signal, it never pops; when the compressor threshold is turned down, the pop happens right when the input signal crosses the threshold (if I play quietly, below the threshold, it's fine).

Schematics, if it helps at all:

http://www.netads.com/~meo/Guitar/Amps/Lab/schem.html

I am not sure if the original problem was solved... it usually took about 45 minutes for it to shut off, and I haven't kept it on that long since I replaced the transistors. The compressor was pretty much fine before I replaced the parts, though. It did act up once or twice over the course of a year, but in different ways, and has been stable for awhile now. So I think it was something I did, but not sure what...

FWIW, so far I've tried adjusting the compressor trim pot, and replacing the A112 4558 opamp with a new one. Still does the same thing.

Also, to add to the original post - if I play constantly above the threshold for the compressor, i.e. the LED stays on, it compresses normally and sounds fine. It's just when the compressor activates that it has these issues.

OK... so my Lab L5 has been giving me some trouble lately... it would cut out intermittently, so it was too unreliable to play live. Long story short, I narrowed it down to the power supply board causing the problems. My weekend project was to replace the six transistors (Q201-206) to see if that fixed it.

While I had it open, I decided to put sockets on all the opamps on the preamp board so I could experiment with different chips and swap them out easily. When I tried to put it back together, it just barely didn't fit in place because the sockets made the chips too high... so I had to take them all back out. Save yourself the trouble and don't ever try putting sockets on the preamp board

Now, the compressor pops every time it kicks on, and it quiets the sound and then gradually brings it back up, very similar to a Slow Gear. I'd be tempted to think something was wrong with the compressor opamp, but I switched it out with a replacement a couple of times and resoldered it a few more times and it always did the same thing.

My suspicion is that it's actually something in the power supply causing it, because the weird behavior is directly related to the compressor's LED turning on and off. I know LEDs draw a lot of current when they first turn on, and I'm wondering if something could be wrong in the power supply which would cause it to not be able to supply the necessary current.

The service manual has this to offer:

F.  Distortion or Popping Sound
1.  Out of adjustment.
2.  Compressor component failure problem of A112, 4558, A111, 3080 or CR101, 102, 1N4004 leaky & introducing ADC voltage on the input.

I have spares for each of the components it says to check, but the compressor has never given me a bit of trouble before I replaced the components in the power supply, so I'd like to get some opinions before I go switching out those components. Any ideas on other things I can check?

I have a 1980 Lab Series L5 that's been giving me a lot of trouble lately. After resoldering the preamp and power supply PCBs and replacing a couple of components that looked weak, I've brought it down to the following two problems:

1) A constant "crackle" sound. It's not a hum, hiss or popping sound, but I don't know how else to describe it. It fluctuates in volume – sometimes it's pretty loud and sometimes it's barely audible, but it's always there. The noise level stays the same regardless of the master volume, so I think this is coming from someplace in the power supply rather than the preamp. It still processes input signal just fine, but that crackling is very unnerving! Does this sound like a problem with a particular component? The next step is replacing components one by one which I'd rather not do if I can avoid it...

2) The compressor sounds like it's on its last legs. Whenever it kicks in and the compressor LED shows action, it cuts in and out very erratically until the signal decays below the compressor's threshold. This is a new problem. It sounds like it might be a bad solder joint or something, but it started happening about three weeks after I resoldered all of the joints on the preamp. It's fine if I turn the compressor switch off. Would this be fixed by replacing an opamp someplace, or is it more complicated than that?

I found a few older threads on this board about Lab Series troubleshooting, and it seems like there are a few people hanging around who know a lot about these amps. But just in case, here's a link to the schematic, board layout and service docs:

http://www.netads.com/~meo/Guitar/Amps/Lab/schem.html

Thanks everyone!