Sony XM-2165GTX

[Dr Zeus]

Got this amp on the bench which fails to power up properly. With the two rectifiers removed, the amp powers up well and has ~67vAC going to the rectifiers. With the rectifiers installed, this amp drags my PS down to ~9vDC. Presumably the output section has something wrong, but nothing obvious measures incorrectly on the DVM. This amp's PCB has surface mount components on the back side, and onlyhas traces on that side as well.

[nigelwright7557]

Have you checked the power supply capacitors ?

Check the supply with capacitors connected but not the amplifier if possible.

[Perry Babin]

With the power transistors clamped to the heatsink (clamped to the small heatsinks is good enough) and a current limiter inline with the B+ line, measure the DC voltage across each of the source resistors. They are 3-legged resistors. The center leg is common for the two resistors in the ceramic package.

Do any read more than 0.000v DC?

[Dr Zeus]

Im back at taking a look at this amp after moving to a new house

One channel's source resistors are measuring 0.090~0.120vDC. The other channel is measuring 0.010~0.012vDC. The amp has RV103 and RV203 pots which I turned fully counter clockwise and that made the voltage across the ALL emitter resistors about ~0.070vDC.

[Dr Zeus]

I think I just realized that the RV pots may be backwards! I turned them full clockwise and things are seeming to be slightly better.

[Perry Babin]

In virtually all amps, rotating the pot to one end some CCW, some CW) makes the bias current 0ma (0.000v across the emitter resistors). In a few amps, the bias pots are reversed from one channel to the other.

[Dr Zeus]

I've since fixed this amp. Im fairly certain after talking with the owner that he had tampered with the bias pots while testing the amp on his own bench - Lucky he didnt blow it up. For me, I probably would have done the same but it was my power supplys which saved this amp from over-draw a death.

Setting the bias pots correctly allowed this amp to play just fine.