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R!

wrote in
oups.com:

I have an old 8 gernamium transistor, protable AM radio that is noisy
on weak stations when cold. It works reasonably well when set it in
the sunshine and warms up. The problem seems to be the RF section
since the noise goes away when the volume is turned down. I'm
suspecting the germanium transistors may be the problem and wondering
which one might be replaced with a silicon variety to cure the
temperature problems?

I'm not sure what all 8 transistors do. Two are in the output stage,
and another is used as a audio driver that drives the audio input
transformer.There are four RF coils, the usual oscillator (red) and
mixer (yellow) and white (1st IF) and (black (second IF). But that
only requires 6 transistors, and there are eight total. The detector
is a diode, so they didn't use a transistor for that. I haven't
figured out what the other 2 transistors do.

I'm thinking of replacing the oscillator transistor with a high gain
silicon variety to try and eliminate the temperature problems?

Any other ideas?

-Bill

I was just thinking are you shure it is the oscillator ?

Tune near the bottom of the band take another radio and try to tune to
the oscillator probably 455Kcs above tuned frequency, listen for a dead
spot with a little background noise, that changes when the dial is
rocked slightly on the defective radio. Do this while the radio works
then chill it and see if you can still tune the oscillator or not,
don't
change the dial on the now cold radio.

I had a problem similar to this and it was a bad IF transformer
worked fair when warm got worse as temperature droped and guit
completely at aproximately 25 deg. F.

If it worked with gernamium transistors at one time when the defect is
repaired it should function without major modifications, which would be
necessary with the change to silicon transistors.

Hope you understand what I am saying.

R!