Epi EA-300RVT

[amplover]

I have a Epi EA-300RVT on the bench and the OT,s get hot fast with a bad hum. Bias reading on pins 5 are -13 but should be -43. These pins are running to a PP driver transformer TF 1001D. The 250 caps are good as well as the 1.5K resisters and also grounds. If the transformer is bad, where can I find one?

Robin.

[Enzo]

Transformer? Your bias isn't even a third what it should be, forget the transformer. Let's get the 43 volts back.

The bias supply feeds the center tap of the phase splitter transformer, and flows through it to the grids. I bet the -53v at the first filter in the bias supply is also low. Low bias and a lot of hum? I bet if you flip your meter to AC volts and measure the bias at the grids you'll find maybe 25VAC there. All of that is ripple.

I'd bet my lunch money the filter caps in the bias supply are dried out. Those are C31 and C32, 10uf elytics. Don't know the voltage but I would use 100v parts if it were up to me.

In any case, the bias supply is simple - one diode, and a couple caps and a couple resistors. STart there.

[amplover]

Thanks Enzo. I should have looked a little closer. They are 70 volt 10uf and the D3 is a DI-405 200piv , 1 AD. I,ll change them out first thing. Thanks again Enzo.

[Enzo]

Well don't just change them out because I said so, use my clue as a place to look. FIND OUT if the bias supply is almost all ripple. I am betting on it, but it is always fun to get one past me.

Crummy caps could do it, or a shorted diode could leave you with bad bias and it would damage the caps all at the same time.