D
Dave
- Jan 1, 1970
- 0
Mark D. Zacharias said:Dave wrote:
The bias diode is essentially two diodes in series. (Sometimes three,
actually). In this case two, looks like. A diode check function of a DMM
would be OK for a static check, and if it became necessary you could use
the DMM to help match a replacement pair of diodes to replace it with.
However, using regular diodes might not work as well in terms of correctly
responding to heat changes.
The proof is in the operation, and in this case that means the actual
voltage drop across the device in-circuit, while the amp is running. Bias
diodes opening up is uncommon but it does happen, and when it does,
catastrophe is the result.
In your case the resistors may have burned because of a momentary short at
the speaker wires, etc although this would ordinarily have destroyed the
output transistors as well.
If the bias diode is two diodes in series, why does the parts list call it a
varistor? It's a funky-looking thing that looks like one of those
connectors you put at the end of a wire to connect it to a screw. Don't
think I'd likely find another one. It's physically screwed to the heatsink
the outputs use.
Has technology evolved so that a momentary crossing of speaker wires could
not destroy expensive output transistors? You see all sorts amps with
"protection" circuits in them nowadays and lots of amp module IC's which
boast both thermal and overcurrent resettable protection...
I measured D405 vs D406 in circuit with no power applied and the value,
although useless quantitatively, was the same for both channels. I'll check
it in operation when I get all the resistors replaced (and Q417).
Bonus for me: the one transistor used in that amp which I've got in stock
is 2SC945, which matches Q417 the bias transistor. I like BD Enterprises
which you directed me to, they seem to stock all sorts of transistors nobody
else does, or their search facility immediately suggests an alternative if
they don't have the original.
Dave