"Ralph Mowery" wrote in message
.net...
In article ,
says...

I have a Heathkit IP-27 bench supply that I got at the dump. Its
problem is that the current limiter doesn't work due to a bad
transistor. Unfortunately, the transistor is a germanium one and
replacements are $30!

I found a forum post where the guy had substituted a silicon transistor
(TIP32C) and he said a germanium diode in the base circuit should also
be replaced with a silicon one. My question is: why does the diode need
to be replaced?

Here is the relevant part of the circuit diagram (transistor Q1 & diode
D7):
http://imgur.com/a/43wmd

Thanks,
Bob

Without looking, I would say the diode is a form of temperature
compensation and the material of the 2 devices should be the same.
That would be true if they are mounted on the same heat sink.

It looks like a very simplistic version of the current limiting circuit. The
diode Vf is pretty much a reference voltage generator that the sense
resistor volt drop is compared to - the fact that the diode also provides
dome degree of temp-comp, is probably incidental.