Valvestate 8280 shorting problems, need troubleshooting help

[rf7]

This amp had -43Vdc on the gate inputs to the Mosfet's (and on the channel output) on one channel. I pulled the Mosfet's (had same results) and then discovered that TR3's emitter leg was broken. I replaced TR3 and now I have +43V sitting on the gate inputs. I replaced TR4, but that did not help. All of the remaining transistors measure OK in circuit. If I jumper the base and emitter of TR3, then I get the -43 back on the gates again. It really seems like TR4 is not working, but it passes the diode test, and even has about .5V across it's base to emitter junction.

1. Does the TR3 and 4 circuit need the Mosfet's in place to bias properly?

2. I've re-flowed all the solder joints, and beeped out the pcb connections to TR3 & 4. Any ideas/things to check?

3. Also, the differential pair emitter voltage is .9V, it should be around .55V. Is this caused by my 43 volt problem?

4. Should I pull TR3 & 4, and then try to get TR1, 2 and 5 to be biased correctly? Or is that going too far?

Schematic

[Enzo]

Oh poop. Took me a minute, your voltages all seemed backwards. The lower channel is drawn upside down, of all things.

Just because a xstr tests good doesn't mean it is working. The engine in my car is good, but it doesn't work if I pull the fuel line off.

1. Well, this is a solid state amp, and that diffy pair relies on feedback from the output bus. Without th MOSFETs, what holds the output bus steady? SOlid state amps are heavily fed back and without that, they become unstable - there is no correction it can make. So I can see it slamming from one rail to the other at the slightest provocation. Like an op amp without a feedback resistor.

2. who knows, we'll see what we come up with.

3. This is the least of your worries.

4. No. See #1. The amp is a system, not a collection if individual parts.

Verify the MOSFETs are OK. Make sure their ballast resistors are not open. Make sure the three resistors serving TR3/4 are intact.

R16 and D1/2 bias TR4, are they OK? You mention a .5v B-E on TR4. Is that with your tester, or is that the voltage ther in circuit operating? COmpare to the voltage across those diodes in the other channel. I assume the other channel works?

You know in a trapeze act, there is always that other guy who just hangs ther swinging, his job is to catch people and throw them back, that's all he does, while the rest of the trapezers do stuff. Well, TR4 is like that guy. It sits there holding down the bottom of that driver circuit. TR3 controls the signal for the outputs.

AND

TR3/4 are like a rubber band stretched out in front of you. Sitting there stretched, the center between then centers at more or less zeroDC. If one of those xstrs open up or turns off, it is like letting go one end of the rubber band - the whole rubber band snaps over to the other hand. One of those xstrs opens or shuts off, and the voltage between them swings to the other rail.

Maske sure R2/3/4 are OK, be aware they are all in parallel.

I wish we were on the top channel, it is drawn normally, and I can intuit the circuit. The upside down lower one, I have to stop and think what I am looking at every time.

[rf7]

Enzo, thanks for the response. I'll check it out on Monday.

You're right, looking at the top half of the circuit does make more sense. I hadn't noticed until you brought it up. Duh!

[rf7]

Finally got back into this thing. I checked everything out, and put the MOSFET's back in, and , lo and behold, it's working. So, the broken emitter lead on TR3 was my problem. It threw all of the voltages off, and made me think that the output transistors were shorted. Now I know better. Once again, thanks Enzo!

Roy