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[rockman627]

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?

[J M Fahey]

Hi rockman, beautiful amp !! One of the best SS amps ever made , probably the best built.
It was called "the Twin Killer" and used by BBKing, somewhat like getting a Nobel Prize.
You have a thermal problem (you already know that), your amp is sick but not dead, try to avoid killing it, it would be a pity. To begin with, ONLY plug it into a series lamp (Teemu posted the schematic, search for it) .
You'll have to make some measurements to guess what the actual fault is. You have also a non-sounding but not dead channel, thatīll be seen later. Post your schematic or link to a published one so we all talk about the same one, or weīll quickly become crazy.
Bye,
JM.

[rockman627]

I'm going to have to try to assemble that series lamp device this weekend. Gerald Weber says you can use this same device to slowly charge up brand new filter caps. Thanks for the tip.
And here's that schematic
http://schematicheaven.com/newamps/moog_labseries_l5.pdf
Wow! I hope this works out because for those few minutes the amp worked (before the hum) the tone was NIIICE....but, no more playing until I get the series lamp device.

[rockman627]

This might also be of use:
http://www.diyguitarist.com/GuitarAmps/LabSeriesL5.htm

[rockman627]

hey, I built my series lamp current limiter and managed to survive, I got my Lab Series service manual in the mail, and I only have a few chores to do around the house so I'm diggin in this weekend. Does anybody have any suggestions on how to proceed, given the symptoms I've outlined?

[J M Fahey]

Hi Rockman.
Not having the amp on my bench (duh), this will look a little like a long range Chess game ... by mail. Anyway, thatīs what we've got, so ... no complains here.
To begin with, set up some kind of bench (any corner of an old table will do), where it can lay undisturbed for at least a few days (be very kind with the Missus). We donīt want constant moving , unplugging, etc. , introducing variables in the middle of measurement.
1) Under a very good light, and using reading glasses if needed or a loupe, look around carefully trying to see some charred or heat-discolored parts, any "run-away" thermal grease, any plastic parts that look molten, cracked resistors or transistors, blown electrolytics, cracked-pin components in general, etc. In short, any physical suspect symptoms.
2) plug it into the series lamp (it will remain there practically to the end), for now *without* any load connected, all controls to "0", nothing plugged. Turn it on, the series lamp should glow brightly for a second (capacitors charging) and then almost turn off (dark orange or barely red)
Measure +B and -B at the supply. The regular value should be around +-60 something volts, (always relative to ground/chassis), but because of the series lamp, you should get around +-40 volts (even +-30 volts). Also measure the voltage at the "hot" speaker output jack , it should be around or less than 100 mV (DC).
Around 200 or 250 mV there means some imbalance, but not very dangerous yet.
2, 3 , or more volts already mean trouble, and + or - tens of volts speak of disaster. Let it on for an hour, checking every 10 minutes or so, to see if itīs stable or starts going down the road.
Look if some part starts smoking, overheats, starts smelling very funny, etc.
Post what you measure at the beginning and after an hour (or lesser time if you start seeing trouble, or the lamp starts to glow brighter and brighter every minute)
Bye.
J M Fahey.

[teemuk]

[rockman627]

Well, my grandson ended up staying over for the weekend, so I didn't get a chance to dig in like I thought I would; but, although I didn't yet try any of JM Fahey's recommendations, I did spend an hour before going to bed and 2 hours before going to work (my shift is noon to 8pm) trying a couple of things.
I hooked up the limiter and the amp, plugged a guitar into a looper and let the loop run, hoping to see what the light would do when the humming started.
Very interesting: the symptom does not appear when plugged into the limiter. Or at least, it did not show after 45 minutes. Then I plugged it back into the wall and the symptom re appeared after 10 minutes . I turned the amp off and checked for the smell. It wasn't too strong but I noticed something else: the heat seemed to be coming from around the Q308-Q310 area.
Tonight, I'm going to go through Mr Faheys recommendations and also check the soldering on Q308, as per teemuk's service manual contribution. I'll keep you all posted and thanks a lot for the help.