Ancient_Hacker wrote:
wrote:
I have an old 8 gernamium transistor, protable AM radio that is noisy
on weak stations when cold.
By "noisy" do you mean there is lots of background hiss, or do you mean
the sound is rough and distorted?
Yes, lots of background hiss and roar at low temperature. At 80 degrees
F, it's not too bad, but at 55 degrees it whistles and squeals and the
weak stations are lost.
It works reasonably well when set it in the
sunshine and warms up.
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?
No. The radio was designed to work with germanium transistors. Every
stage you change over to silicon will need to be rebiased, retuned, and
reneutralized. Not an easy job.
I'm not sure the design is the best. They used a couple resistors to
set the bias on the output stage, and when the battery voltage falls a
volt or so, the output has some crossover distortion. I fixed that
problem with a couple diodes in place of a resistor so the bias current
stays above zero as the battery voltage falls. I can now drop the
supply voltage a couple volts with no crossover distortion.
I'd start with replacing the electrolytics. Quite likely they're at
about 20% of their original selves. Weak electrolytics can make the
radio motorboat, or sound tinny and noisy.
Yes, that could be a problem, but the AGC seems to work well, so that
capacitor must be ok.
-Bill