On May 16, 1:09 pm, Richard Rasker wrote:
Op Wed, 09 May 2007 22:20:35 -0700, schreef stratus46:

snip

Hm, as far as I can see, an MJ15003 might be a reasonable
replacement.

It wasn't a great transistor but was about the best there was in
the
early '70s. Are you sure the emitter resistors are 3.9 and not
0.39 ohm?

I know, I thought it was very high too. And perhaps the silver ring
has
picked up a bit of a brown tarnish over many years of use - I'll
check it
out as soon as I get round to working on this beastie again.

so could you run one of the remaining devices into a curve tracer
to get
an idea of the parameters?

The remaining transistors seem pretty ordinary, and don't appear to
be
precisely matched - or if they ever were, this is no longer the
case.
In particular, Vbe varies from 0.7 to 0.85 volts between separate
devices,
when fed with a constant current (10mA). Perhaps this is a clue as
to why
just one transistor failed.

I used to run 30KHz at about 1 watt to set the bias set for
crossover
distortion. Very obvious on a scope. Those were the days.....

Yup, that's a nice way of getting it right - although I also keep
an eye
on the absolute current. And when repairing blown end stages, the
first
tests are usually performed with a 75-150W light bulb in series
with the
mains supply. If anything causes a sudden current increase in the
end
stage, the lamp will often prevent real trouble.

Anyway, thanks for your information, and I'll let you know how
things
work out.

Richard Rasker

When I repaired consumer audio back in the early ' 70s, a real quick
check was to turn the amp on without the power transistors and run it
with just the drivers. It wouldn't deliver any power but it would make
the power supply rails so you could see it the remaining system worked
before you went to the trouble and expense of installing the new
output devices. Then run it at low power at 30 KHz and finish it up.

I agree on the MJ15003 but I suspect those are much tougher
transistors than what failed. I would bet you could do just fine with
half as many 15003s as the originals and still have more output
current capability as the original setup. It would however alter any
overload detection circuitry. Heck, load 'em all up.

Definitely interested in the rest of your story.

GG