On 19 Jul 2006 12:36:33 -0700, "
put finger to keyboard and composed:

I have an old Traynor 25B that has performed perfectly for many years
(around 20 I'd say) until now. When I switch it on, there's this loud
humming reminiscent of a mains hum, only much much louder. Playing with
the dials does absolute nothing, and feeding it a signal mixes it in
with the hum. After some troubleshooting I think I have narrowed the
fault down to the power amplifier circuitry. The preamp and power
supply is not to blame, I've tested them both. There are three diodes
that are destroyed as soon as I switch on the amp, and I can't figure
out what's causing them to blow (there are two 1.6A fuses in parallel
right after the transformer, but they don't blow).
As far as I know, the only way the diodes could be destroyed in this
application is through exceeding their maximum forward current rating
(200mA). Maximum reverse voltage is 100V, and I'm only getting 50 from
the power supply.

Here's an excerpt of the schematic:
http://img258.imageshack.us/img258/4232/ampkh8.png

The three diodes in question are the three 1N4148 placed in series in
the middle of the image.

I would be very grateful for any help you could give me. My next course
of action would be hauling the beast to a repair shop, money I'm hoping
I could save by repairing it myself.

Transistor MPSA55 is operating as a 6mA current source. That's the
maximum current that the diode string should ever see. Other than the
MPSA55, I would suspect a problem with either of the TIPs, in which
case I'd change them both, as well as the MPSA18, MPS8599, and MPSA05.
I'd also check both 0R51 5W resistors.

- Franc Zabkar
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