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Topics - rockman627

#1
Amplifier Discussion / I want to fix my Lab Series L5
February 09, 2009, 04:31:33 PM
I just dug my Lab Series L5 out of the closet. I forgot exactly what the problem was that made me put it away (in the late 80's), but I do remember that somehow the speakers got disconnected, and it was running without a load for awhile.
So I took my chances, pulled the chassis out, connected an 8ohm speaker cab and fired it up. Hey! I got sound! Channel 2 worked, channel 1 did not (but as I turned the knobs on channel 1, I could hear crackling and hiss). Also, the compressor did not work. After about 10-15 minutes, a loud hum suddenly appeared, and you could no longer hear the guitar. Also, there was a smell of something hot, similar to a soldering iron. There was no smoke and I wouldn't really characterize it as a burning smell.If you turned the amp off and back on, you were good again for another 10 minutes.
The next day, I plugged a guitar into the power amp in section. It played, but the hum came around again in 10-15 minutes. I tried to see if the preamp out worked while it was humming, but I found out the preamp out doesn't work at all. I also found out the amp will hum after 10-15 minutes even if you don't have a guitar plugged in.
I bought a service manual from ebay (it should be here any day now), and I have a little electronics repair experience. I've been modding and fixing my tube amps the past couple of years.
The schematic looked daunting at first, but after constant staring, it seems somewhat comprehensible now.
So, I need as much advice as possible:
How do you drain the filter caps? It's very easy on a tube amp but I haven't found any info on how its' done on a ss amp. Once I do that I'll have enough courage to see what shape those caps are in. I'm assuming that, besides the 2 big caps you can see on the PS board (C207 and C208), the 2 metal cylinders sticking out of the bottom are covering some more caps.
And that's half of my troubleshooting plan (for now); after I check the power supply, the other half of my plan is checking the power amp section.
What do you guys out there say?