Gibson Atlas Medalist Conumdrum

[benjamo]

Just got a Gibson Atlas Medalist in unworking condition: 4 out of 5 tubes missing, shredded power cord, fuse holder broken. Looked fairly clean under the chassis, so fixed the cord and fuse holder, replaced the missing tubes and tried it. Result was a god-awful, extremely loud hum through the speaker, even on standby, that increased with turning to "on". Figured it had to be the filter caps, so replaced those, and it worked, i.e., it played as it should, but for less than a minute. Then the hum returned, not quite as loud as before, and also did not increase when going from standby to on as it had before.

I'm not sure what the next step should be to diagnosing the problem. Any help gratefully accepted.

[The Dude]

Double check that you got the filter caps installed with correct polarity.

[Enzo]

Same as for any amp: isolate the problem.

I would check my power supplies, you may have replaced caps, but there is a whole circuit. Does the B+ voltage look remotely close to the specified levels and free of ripple? Is the bias supply clean and free of ripple?

Pull the phase inverter, V3. Does the noise stop? If not, the problem is in the power stage. if it stops, then the problem is earlier.

Note that the B+ feeds the power tube plates even in standby. The standby switch only removes B+ from V1,2,3.

Do you know for sure the tubes you installed are good?


Edit: Dude makes a good point, especially the bias filter caps, remember those are a negative supply so the positive end is grounded.

[benjamo]

Thanks for the tips - I'll give 'em a go.

[MattyX1969]

benjamo, I realize this thread is almost 2 years old, but I am seeking some info on this amp from someone who owns one. Did you get yours up and running?
Thanks,
Matt

[benjamo]

Hi Matt:
Yes, I got it working, but needed the intervention of a tech, who discovered that more caps in the power chain were bad. Replaced those, and it worked fine.