Output relay on Ampeg B4

[caseyjones]

I was getting no output on my ampeg b4, so I searched and found that the A channel output had a relay in circuit with it, and it was welded closed. The B channel relay is fine, but there is no output from either side. Is this a thermal relay? I noticed the fan near the heat sink had some trouble starting on its own. I had to give it a spin to start it. could that be a cause? Any ideas of what could cause the relay to fail, or why both channels are dead?

[Jerrold Tiers]

Most likely there was a failure in part of the power supply, which sent the A channel to a high DC voltage. Also could be a regular output failure in the A channel, which normally causes DC output.

The high voltage (about 80V) DC will arc when the relay tries to open the DC that is going to the speakers, and the arc can weld the relay. it normally welds to the OPEN position, which is ground, though. That usually protects the speakers even if the relay welds.

In either case, the B channel isn't working because most likely the A channel failure is afffecting it also, and the relay won't close due to the DC condition.

Alternately, the relay power supply, which also powers parts of the preamp and amp, is bad and no relay is closing, and no signal is getting through.

[caseyjones]

Do you think if its a power supply issue, that its because of the transformer, or a chip or something else like that. Where can I test that? I dont have any schematics for this one.