On Fri, 20 May 2011 15:21:01 -0700, klem kedidelhopper wrote:
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
Start with the first RF amp. It's probably a superhet so there will be a
455 osc and mixer. After that a couple rf amps and the the detector.
Google a superhetrodyne receiver. This one uses ICs but the basics are the
same
http://www.seekic.com/uploadfile/ic-...7173226815.gif
Should give you some insight as to where the signal is being lost in the
path.
--
Live Fast Die Young, Leave A Pretty Corpse