Problem with Pioneer QX4000 - have some info and circuit diagram!

[ChopperCharles]

So, I have two QX4000 receivers. I bought one on eBay so i'd have a working unit to compare my dad's busted one against. (He passed and it has sentimental value). Dad's doesn't play at all, nothing from speakers.

WEELLLL.... the amplifier heat sink never gets hot in the bad unit. I compared voltage at all the terminals. There are two terminals that read 97 volts or so on the good amp. On the bad amp, one reads approx 100 volts, the other reads 32.

I traced the wiring back to something called the "muting unit". The circuit is simple, but i'm not electronic at all (software dev), so I don't know what it does or why it's called muting unit or pretty much anything.

What I do know, is it has a 100V input, which comes directly from the power supply (well, it goes through a fuse first), a 9v input from the power supply (also fused), a ground, and what's supposed to be a 100V output (all AC). I pulled the circuit diagram otu of the service manual and added these to it.



Soooo.... can you tell me what has gone bad to cause this to output such a low voltage? Bad cap? Bad transistor? Also... what exactly does this thing do? Why is it called a muting unit?? it has nothing whatsoever to do with the mute switch on the front panel.

That output connects to directly to the main amplifier boards, and I'm thinking that's why I'm not getting any sound at all. I've got low voltages on other boards - namely the quad decoder board, but they're the same magnitude lower, (Supposed to be 85 volts, is 20 volts) and I'm thinking they may all feed from the output of this muting board. My guess is fix this, fix the whole thing.

Thanks!

Charles.

[ChopperCharles]

Well, the 100V AC input is actually 50V DC. The output is less than 2V DC. Jumpered pin one and two together, and the receiver makes beautiful, beautiful sound. So.... anybody know a replacement for a 2sc1318 transistor?

Thanks.

Charles.