On May 22, 12:38*am, "Michael A. Terrell"
wrote:
klem kedidelhopper wrote:
On May 21, 8:58 pm, "Michael A. Terrell"
wrote:
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. 
The IF's are AF124,and 126. And to answer a previous question I did
use cold spray on everything but the detector, which is inside a can.
* *I may still have a couple new ECG160 in stock. *I'll try to get into
the old shop and check, if I can climb through some of the junk that's
piled up to be hauled off.
* *http://www.nteinc.com/specs/100to199/pdf/nte160.pdfis the datasheet
for the NTE version.
* *I only used freeze mist as a last resort. *On early transistor
equipment it seemed to cause more failures. *Thermal expansion & cooling
inside the transistor case caused metal fatigue.
It didn't seem to have any effect. I plan to open the can and hit the
detector too.
* *AM or FM? *A bad diode shouldn't affect both bands.
The "whiskering" as well as the method to remove it are
both kind of interesting, sort of like what happens to nicads it
seems. Well the transistors seem to have lasted forty some odd years
so I can't complain much. The NTE 160 is a germanium and it seems to
replace all of these however it does not have a fourth can lead.
* *A short between the shel & the collector was a common failure.
I don't know if that could make much of a difference. To check one of
these for whiskering, would you simply look for leakage between any
junction and the can? I never thought of this.
* *Using an analog meter would caue open juctions to start working
again, for a short time. *All this did was make you waste time, since
they invariably quit again. *You probably have a crack weld inside the
can, or a fractured bonding wire. *That is why I monitored the collector
voltages, with added isolation and used a VTVM, and later, a FETVM.
On another note, considering the bias issues, I really would not want
to use a silicon transistor if I don't have to.
* *It's not that difficult to change. *If it uses the typical two
resisotor bias, you need to raise the E_B voltage to get .6 volts
instead of .1 volts. *Since the silicon parts have less leakage
currents, raise the value of the bottom resistor in the string. *Tack a
trimpot or use a resistors sub box to find the new value, and solder it
in.
--
It's easy to think outside the box, when you have a cutting torch.
Good idea about the VTVM. I don't get to use my RCA Senior Voltohmyst
much anymore. I bought it as a kit from the RCA distributor at the
time, Bruno NY in Manhattan and built it when I was 17. It still works
great. I use an Energiser for the ohmmeter but I really do worry about
the battery leaking. I've been meaning to replace the battery with a
little isolated DC supply for a long time but just have never gotten
to it. Lenny