UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Another DC coupled amp (Ampeg bass combo)

ITAM805 · 17th Jan 2018, 6:40 pm

Cripes, what a mess this thing is, it's been got at

I fitted the new zeners and resistors and powered it through the lamp limiter - same fault? So I decided on a radical approach, I removed the 4 Mosfets, powered it up but the lamp still glowed and the power rails were still all over the place?

So I took the limiter out of circuit and just used the variac on it's own and wound it up slowly. I reached about 200V when magic smoke rose from the newly fitted R74 (0.33R) the source resistor for Q2 - even though the Mosfet was removed??

So I traced some wire links from R74 across the board to jumper plug J29 - some wires were cut from the plug had been hard wired to the board, but whatever clot did it wired it to a pin that goes straight to ground!

17th Jan 2018, 6:40 pm	#20
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Another DC coupled amp Cripes, what a mess this thing is, it's been got at I fitted the new zeners and resistors and powered it through the lamp limiter - same fault? So I decided on a radical approach, I removed the 4 Mosfets, powered it up but the lamp still glowed and the power rails were still all over the place? So I took the limiter out of circuit and just used the variac on it's own and wound it up slowly. I reached about 200V when magic smoke rose from the newly fitted R74 (0.33R) the source resistor for Q2 - even though the Mosfet was removed?? So I traced some wire links from R74 across the board to jumper plug J29 - some wires were cut from the plug had been hard wired to the board, but whatever clot did it wired it to a pin that goes straight to ground!