Agreed. Check my earlier posts, that common zener reference is a good explanation, and there are indications it was messed with.
I pulled the outputs, and none are blown. FWIW this is a later iteration of the driver board where the emitter fuses were removed and replaced with a jumper to the board. There are now 5A slow-blow fuses for the B+ and B- on the power supply board.
I replaced the zener at CR805 a few days ago and it still tests ok (not shorted or open) in circuit.
R841 (390 ohm) measures 381 ohms in circuit.
I pulled Q805, Q806, Q807, and Q808, and all are ok.
I replaced the zener at CR805 a few days ago and it still tests ok (not shorted or open) in circuit.
R841 (390 ohm) measures 381 ohms in circuit.
I pulled Q805, Q806, Q807, and Q808, and all are ok.
You do not need to pull all these parts for debug. Let’s get a full set of node voltages in the circuit, preferably an annotated schematic. Pulling all this stuff for checkout just opens you up for introducing more problems. Most of us can quickly calculate what the voltages should be accurately enough to identify where the problem(s) are and what to do about it.
Relax. The transistors I checked are all socketed. I don't desolder parts unnecessarily for testing.
And yes, I'll get the voltages. Thanks.
And yes, I'll get the voltages. Thanks.
If you are not sure about what I mean by circuit nodes, let’s just start by measuring all the transistor terminals of all the transistors in one of the channels while in circuit.
Circling back to bring this to a conclusion.
The reason for the the rail voltage at the bases of Q805, Q806, Q807, and Q808 was shorted Q803 and Q804. I replaced both and also reinstalled the original stabistors (CR803 and CR804) after testing with a 9V battery and 1k resistor and finding that they were fine (as JM Fahey predicted). The receiver now biases up fine.
The reason for the the rail voltage at the bases of Q805, Q806, Q807, and Q808 was shorted Q803 and Q804. I replaced both and also reinstalled the original stabistors (CR803 and CR804) after testing with a 9V battery and 1k resistor and finding that they were fine (as JM Fahey predicted). The receiver now biases up fine.
Check make sure CR809 isnt open. It’s supposed to limit the current through Q803 to around 50 mA under fault conditions (by limiting the drive from the diff pair to 2Vbe). Either it didn’t limit, or 50 mA is too much for Q803. Could be either, but hard to believe 2 watts for short periods of time would have hurt it.
Then Q803 and/or 804 could get shorted again if you short out the speaker wires, depending on what you ended up using in there. What did you “replace” them with? Any modern VAS transistor I would end up using would take 50 mA at 40 volts without breaking a sweat. Original types obviously couldn’t.
Why replace in quotes? I went with Toshiba TTA004's.
Anyway, I'm calling this done. Thanks for the assist.
Anyway, I'm calling this done. Thanks for the assist.
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Because you didn’t use original types (unobtainium). The Toshiba TTA types will be fine, I’ve run them and similar types that high quiescent (with a little heat sink). I bought a bunch of the Sanyo/ON types (C3902 and PNP) for these purposes before they went away. Those modern 160 volt/ 1 amp TO-126’s are all pretty similar.
Glad to see you resolved this. I myself wonder why those failed? Abuse via slipped probe, design at the power limit of the transistor, ?????