Hi

On 19 Jul 2006 12:36:33 -0700, "
wrote:

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).

Often the way with fuses around solid-state power amps - the
semiconductors tend to protect the fuses, rather than then the other
way round....

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.

So one end or the other of this diode string has ended up 'tied' to
one of the power rails ? - possibly by the demise of one of the
associated transistors...
You might be able to find which one has blown by checking continuity
from the power rails to the collectors of the transistors - with the
power off...


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.

Another (probably cheaper) way to get the thing up & running might be
to simply replace the power stages with (for instance) an IC power
module - probably under £15 - and you already have the +/- power
rails...

Bearing in mind that any decent tech is going to want £25-£50 just to
take the back off the thing - then the 'drop-in' replacement looks
more attractive....

Regards
Adrian