B-52 AT-100 Guitar head repair question

[drummer31337]

I bought a used B-52 AT-100 all tube head with matching cab used last year. Everything worked when i bought it [except the jewel light in the front], and was very happy with the sound. Then after a few months of regular use [6 hours a week], the 'line out' stopped working. Shortly after the effect loop stopped working. [no send, no return, and the footswitch quit lighting up as well] Then as i was playing today, one of my gain channels went out [the light on the footswitch stopped working too].

So right now i have a clean channel, and a distorted channel. I am mostly upset about the effect loop not working. i took the whole thing apart and cleaned it up, rotated the tubes, and tightened all the connections with no positive result. the fuse is good, and all the tubes light up. I noticed that the gain and effect loop channels went out slowly; they would flash on and off intermittently before they stopped working entirely. i got a copy of the schematic, and it makes little sense to me. I also took apart the footswitch, and everything was hooked up correctly.

Does anyone have any ideas about what went wrong? and/or why? And how it could be fixed? Or has anyone had similar problems?

Thanks!

[J M Fahey]

I guess you mainly have what we sometimes call "mechanical" problems, rather than strictly "electronic" ones, meaning that the suspects are connectors, switches, plugs, cracked solder and tracks, etc.
You'll need to "trace" an audio signal along it's path, from input jack to speaker, to find where it gets lost.
Please post the schematic so we all talk about the same.
Good luck.
PS: you will be working into an area powered with DEADLY hundreds of volts, no kidding, so maybe you will need to get the help of a seasoned Technician for the "hands on" part of the job, even if he is not specifically into the Guitar Amps area.

[drummer31337]

Thanks!

B52 AT100.pdf

it seems to me that there is a communication problem between the footswitch and the amp.

[J M Fahey]

Hi drummer.
I'm very sorry for writing this, can't find the best words to say it, but I'll try:
This amp is *very* complex (don't want to use the word "nightmare" but ... ), they put *everything* into it, the signal path zigzags in all directions, with a zillion options at your disposal (good), using a zillion relays and opto switches (bad for servicing), the switching logic is digital, etc.
Problem is, I don't know how to "remote control" you to check everything.
If it only were, say, "line out does not work", that's reasonable: the main suspect is JK3; VR15 is the third one, because I suspect these components are mounted on a small PCB joined to the main one through a small PC type connector. As you see, on such a simple section we already have 3 things to check ... scale that to the rest of the Amp.
As for the effects loop, main suspects are the jacks, but then you should have to check the relays ... but you say that the Leds do not work or do so intermittently, which makes me suspect the ICs driving them, or cracked tracks/solder or failing panel switches which are in parallel with Fsw jacks or the jacks themselves or the Fsw or the cable that joins it to the amp ............
As you see, possibilities grow exponentially with circuit complexity , so i feel it's above my head being able to give you useful insights.
If someone else wants to give it a try ...
Sorry and good luck.

[Enzo]

If the footswitch won't communicate with the amp, resolder all the pins on its chassis jack.

[Mark Black]

And if re-soldering the footswitch jacks doesn't help I would be suspicious of the CD4069 Hex Inverter chip used for the switching control functions. It's a CMOS chip (sensitive to static electricity), and in this design some of the inputs are ported directly to the "outside world". Also check to see if the +12VDC supply is present & proper at pin-14 of that chip.

In fact now that I look at it a weak or high-ripple +12VDC supply could account for pretty much all your symptoms...