Bandmaster AB763, please help this newbie diagnose, i have all voltage measurments...

[Jimmybjj]

this is a continuation from this thread

http://music-electronics-forum.com/t18302/

Problem: low B+ voltage 370v, slight hum.

I am new to trying to solve my own amp problems and could use any advise and help you guys would be willing to give. As suggested by "Enzo" i also measured ac along the B+ line and got 88vAC. The only new parts I know of on the amp are a oversized "allen" output transformer, and the filter caps (2x 100uf 350v, 3x 22uf 500v)


Thanks for any help, Jim

[Jimmybjj]

trying to attach a image that's easier to read

[Bruce / Mission Amps]

trying to attach a image that's easier to read

Pull the power tubes out and try that voltage test again.
Although not the end all of trouble shooting,
from those lowish B+ VDCs, especially at the power tube node, I'd suspect one of these four things first:

a bad pair of power tubes or grossly under biased power tubes
a bad bias supply filter cap, resulting in not enough negative bias voltage
a bad main high voltage filter cap or setup
An open diode in the full wave rectifier, resulting in it operating like a half wave... which will still give you lots of B+ but without being able to sustain the load.

[Enzo]

We need two threads for the same problem in the same amp?

SInce you measure 88VAC on your B+, instead of a volt or two, I'd have to say your low B+ reading is due to a lack of filtration. Your filter caps need replacement.

[Jimmybjj]

We need two threads for the same problem in the same amp?

SInce you measure 88VAC on your B+, instead of a volt or two, I'd have to say your low B+ reading is due to a lack of filtration. Your filter caps need replacement.

I apologize, i just wanted to start a new thread when i had all of the voltages ready for anyone that was willing to take a look.

You were correct it was the filter caps, as Bruce discovered for my in the previous thread. I replaced the two larger caps and everything is almost in exact spec as the schematic.

I want to take the time for everyone who responded to either thread on this amp. I was a new guy with no posts and all of you guys took the time to try and help me. I can tell this is a great community.