Advent 300: Good news, the power supply works. Bad news? 22v at output!

[jberger]

I have an Advent 300 receiver (a great little box especially as a tuner preamp). A few weeks ago I turned it on and the turn on thump was louder than usual and then it went silent.

Got the cover off and one of the internal fuses was blown (F401, on the -22v side), so I replaced that with the correct value, and checked the power supply voltage at the fuses, and it was +-22 v.

However, I found the same 22v at the speaker outputs!

No obviously burned components, and the fuse hasn't blown again.

Oddly enough, the tuner section still lights the stereo indicator; however, the LED-1 and LED-2 tuning indicators no longer light.

So, where to start? I have a transistor tester, meter, and scope, and the service manual with schematics.

 

[northpaw]

I'm still what I would call an "advanced amateur" with amp circuits, but with that caveat, the most likely cause of rail voltage at speaker outputs is a shorted output transistor.

Was there -22V on both channels? If there was, it would be a little odd as transistors on both channels would have shorted in the same event.

Anyway, first check Q316 (Left) and/or Q317 (Right) for shorts, as their Collector-Emitter pins bridge between the -22V rail and the speaker outputs with only a 0.15Ω resistor in between, so if they are shorted, they would pass the full -22V to the speaker outputs (assuming no speakers connected, so no current flow). Even if they are OK, you should check the other transistors upstream from them in the amp section, up to and including the differential pairs (Q300-302 and Q301-Q303) for damage.

 

[Ds2000]

Passing rail voltage to the speakers. Sounds like blown output transistors. Check entire circuit.

 

[jberger]

Thanks. I'm also following the thread at https://audiokarma.org/forums/index.php?threads/advent-300-no-worky.785414/ , as it has very similar symptoms, and some great explanations. Q402 appears to be toasted, and I don't have the 15v (don't remember if it was + or - right now).

 

[chazix]

Loss of the -15V supply will cause DC offset in both power amp outputs.