klem kedidelhopper wrote:

On May 21, 10:05 am, "N_Cook" wrote:
klem kedidelhopper wrote in message

...



I have a Grundig "Transistor 305" portable radio. It uses germanium
transistors. I've had this thing for over forty years and although I
don't use it often it has been a nice radio. In 1971 I replaced the
transistor in the FM detector circuit and I have had no problems with
it until just recently. Last month I spent three days in the hospital
and I brought the Grundig with me. I noticed that after it was on for
about a half hour it would start to cut out. I turned it off and then
later turned it on again, and after a period of time it happened
again. After several similar episodes I stopped using it. I wondered
about some type of interference from the hospital but that was wishful
thinking. In any case being in no position to test it I decided to
just put it away. Yesterday I put it on the bench and took a look at
it. Sure enough after a brief time it cut out again. I injected a
signal, (my finger on a small screwdriver) onto the wiper of the
volume control and ascertained that there is nothing wrong up to the
speaker. So now I need to get into the IF's and detector. My problem
is my old signal generators are not working and all I can come up with
for 10.7MHZ. is a CW signal out of my MFJ249 SWR meter. The MFJ puts
out a clean signal however it is CW, and it's amplitude is not
adjustable. I don't think that an unmodulated signal would be a
problem in this application but I don't know what the amplitude of the
MFJ is and I would hate to blow up an otherwise good IF amplifier with
an excessive signal. I''m admittedly a bit rusty having not trouble
shot an IF amplifier/detector circuit in many years so does anyone
have a feel for how I might signal trace this radio with what I have
on hand? Thanks very much for any advice. Lenny

Any 4 pin Ge transistors in there? if there is then the usual simple fudge
is worth a try

Well the AM is not working either when this set fails. And since the
set uses the same transistors but different coils for each stage that
doesn't really help me to track this down. I could look at the CW
signal from the 249 with my 50MHZ. scope or my spectrum analyzer but I
still don't know what a safe signal amplitude would be. Does anyone
know what the proper typical amplitude of an IF signal would be so
that I would be able to simulate this and inject this signal into the
IF amplifiers and detector stage? BTW except for the outputs, they're
all 4 pin GE transistors. What do you mean by the "simple fudge"?
Thanks, Lenny


Check the collector voltages on the IF and mixer stage. Use at least
a 1 meg resistor between the probe and the transistor to keep from
disturbing the circuit. I saw a lot of this in old germanium transistor
car radios. Noise induced into the circuit would momentarily restore
the transistor enough to work for a while, but of there was zero
collector voltage, one of the IF transformers was open. If it was high,
the transistor was bad. I troubleshot those radios and repaired most of
them in under five minutes with that simple method.


--
It's easy to think outside the box, when you have a cutting torch.