Help me diagnose a faulty amplifier.

[Maker]

C705 is fine, Q704's collector is at +15V but shorting it to the emitter doesn't do anything.

Looking at the diagram, the emitter appears to be connected to ground, but it's sitting at +22V from ground, unless I'm following it wrong? Shorting the collector to ground gets the brief illumination which fades out. The base is at 27V, looks like it's connected to the AC output of the transformer so that's probably about right.

 

[Seadog]

It is connected to ground. Check your measurement point. Is it 22V ac or dc?

The base can't be at 22V unless the transistor is blown or, once again, you're not metering across true supply ground (-ve)

 

[DSR]

Questions :-

How many LED's are we talking about ? I can see 5 on the circuit diagram but they appear to be wired as three in series and two in series so effectively two LED's. ?

Also how do they go out - do they go suddenly off or do they fade over a short time to off.

Do they all go off at the same time ?

 

[Maker]

22v measured with a DC meter, didn't think to check on AC and I don't have a scope to check the waveform.

I've desoldered the transistor, those measurements were just on the terminals where it goes. Should I put it back in and measure again?

There's a ribbon cable connecting the function board (Front panel) to the amp board, that's got GND, +15 and -15 labelled on it, I'm measuring off that ground which is also connected to the case, heatsinks, etc.

All the LEDs go off at the same time, and they come on full brightness and then fade out. I assume that's a capacitor draining.

 

[Seadog]

I've desoldered the transistor, those measurements were just on the terminals where it goes. Should I put it back in and measure again?

OK, that's what I'd expect to see. Refit and check again. I can't see that the -15V rail should appear on the board since it's for the power amp. There are only two capacitors, that I can see, that would influence this part of the circuit; C909 and C705.

What happens if you turn the source select switch to the "OFF" position?

 

[Maker]

I've put it back together now, I'll open it up again tomorrow.

The input board plugs into the function board, I assume -15V is only there because the phono preamp on the input board needs it.

 

[DSR]

Double check your transistor pinouts on the board and find a reliable 0V rail. Shorting the collector and emitter of Q704 should turn the LED's on permanently as @Seadog said.

You should have (measured WRT to reliable 0V rail) 0V on the emitter,

approx 0.7V on the base and 0-2V at the collector with the LED's ON.

You should see well under 0.7V at the base and something approaching 15V at the collector with the LED's OFF.

With the LED's fading slowly, it does look like a capacitor is either charging or discharging somewhere. It could be in the wider protection circuit or in the power supply.

IC801 should always give 15V out - check if it's getting warm - the 78 series have thermal and load limiting features. If it's getting hot it could be shutting down and turning the LED's off (and whatever part of the circuit that is causing excessive current). Also check the input and output voltages to it. There are series resistors and either of these could have gone high so allowing enough current to charge a capacitor but not enough current to power everything else.

D

 

[Seadog]

The schematic suggests 0.1V at the collector and 0.8V at the base for the on condition.