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.

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?

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

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.

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?