Lab Series L5 compressor (or power supply) problem...

[kvandekrol]

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?

[kvandekrol]

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.

[phatt]

From what I can gather,, I take it the original problem (which did not involve the compressor part) was solved?
And Now the compressor is acting up? 

I'm not familiar with this Amp though I have a fair idea how it works.
As far as the compressor issue,,, I would be looking at a dud cap,, leaky Electro maybe.

No schemo so that's all I can offer.
Phil.

[kvandekrol]

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...

[phatt]

Try in the area of *C139*
Don't forget Q101 (the FET)
Check you have all the DC voltages at the pins shown in the schematic.
Some of those comp circuits require a cap to charge which governs the speed of response,,, at a guess C139 might have something to do with time.
sorry can't be more helpfull,,, Teemu where are you?  He will be of more help:)
Cheers Phil.

[Enzo]

Let me ask you this - and this is really a pretty important question.

Are you REALLY sure the compressor pops every time it kicks in?   COuld it be that the amp pops, and every time it does, the compressor reacts to it?

Once triggered, it is normal for the compressor to slowly release.

[kvandekrol]

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).

[kvandekrol]

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!

[joecool85]

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!

That's great!  Always good to hear when things work out like that.  Enjoy the amp, and please - feel free to post your other issues so we can help you out with those as well.