Yamaha HTR-5630RDS

[Khron]

Ok, so, i got this 5.1 AV-receiver off ebay, as faulty.

Micom seems to be dead - despite it being powered fine, power button does nothing. No worries, i can still use the amps themselves, "hard-wired"

Here comes the nasty (and/or confusing) part: upon plugging the receiver in (after jumpering over the main trafo turn-on relay), the power rails all come up fine, just as they should (roughly +/-41v), but on the signal inputs, the feedback inputs AND on the amp outputs, i get a slowly rising (about 1v per second) negative DC voltage Ends up around -30v on all five channels.

Nothing gets unusually hot; the main heatsink starts warming up after a couple minutes, but not out-of-the-ordinary (the amps have 10/40/80mA idle current min/typ/max).

The amps themselves are two STK402-120 (front+rear left / front+rear right) and an STK404-120 (center).

The service manual can be found HERE (just ~100KB over the forum limit, unfortunately ); amp schematics are on page 59.

I've removed the five muting transistors, and the output relays, and replaced them with wire jumpers.

I'm stumped...

[Khron]

Re: Yamaha HTR-5630RDS



Mystery solved

The "signal ground" (amp input ground, feedback ground, muting ground etc) was AC-coupled to the "power ground" (output stage, main filter caps etc) through a 2R2 resistor and a 10nF cap.

Right next to those, there are two wire jumpers, TP101 and 102, parallel to each other, and with a hole in the PCB right underneith them. TP001 = signal ground side, TP002 = power ground side. With my multimeter probe on an output pad, i stuck the tip of a pair of tweezers in between the two jumpers (shorting them together) and ta-daaa! The +17v DC i had at the outputs (this time) instantly dropped to about -30mv

So... yeah...