Old Gernanium Transistor Repair

[[email protected]]

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

 

[Homer J Simpson]

I have an old 8 germanium transistor, portable 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?

It could be a lot of things. You need to find a way to test which.

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.

Usually two for output, two for audio amps = 4.

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.

Actually 7. Some radios had non working transistors added to bring the count
up.

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?

You might need to alter the bias on each transistor you replace.

 

[Jim Land]

[email protected] wrote in @h54g2000cwb.googlegroups.com:

I have an old 8 gernamium transistor, protable AM radio that is noisy
on weak stations when cold....

Try over on alt.antiques.radio+phono... they'll enjoy hearing about your
germanium transistors.

 

[Michael Black]

[email protected] wrote in @h54g2000cwb.googlegroups.com:


Try over on alt.antiques.radio+phono... they'll enjoy hearing about your
germanium transistors.

I don't know. There has been debate in the past over there about what's
relevant to the newsgroup, and some have felt transistors aren't old
enough.

Michael

 

[Dave Plowman (News)]

Usually two for output, two for audio amps = 4.

Perfectly easy to manage with three - one driver only.

 

[Michael A. Terrell]

I don't know. There has been debate in the past over there about what's
relevant to the newsgroup, and some have felt transistors aren't old
enough.

Michael

Some of them think that Atwater Kent radios are too new for that
group. Ignore them, there are a number of transistor radio collectors
in that group. I have most of the "Sams Transistor Radio Manuals" in my
collection: http://home.earthlink.net/~mike.terrell/HWSTm.html The Sams
index is available online.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Ancient_Hacker]

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?

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'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.

 

[J. Todd]

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

Since warmer=drier, I might look for paper tubular
caps. When moist, they leak badly.

 

[Homer J Simpson]

Perfectly easy to manage with three - one driver only.

But in those days more transistors = more sales. I used to fix such things.

 

[[email protected]]

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

Replacing parts at random on electronics makes not one bit of sense.
There are hundreds of components and hundreds of connections, any one
of which may be to do with it. Trying to repair a radio by replacing
geraniums with silicon makes even less sense. Germanium trs are still
sold new as well as used.

If you dont know what youre doing, leave it alone.


NT

 

[Michael A. Terrell]

Since warmer=drier, I might look for paper tubular
caps. When moist, they leak badly.

Paper capacitors? Transistor radios use electrolytic and mylar film
instead of paper, but the electrolytics were poor quality, and they
don't age very well. The capacitors in tuned circuits are either mica,
ceramic or polystyrene.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[[email protected]]

"J. Todd" wrote:

Paper capacitors? Transistor radios use electrolytic and mylar film
instead of paper, but the electrolytics were poor quality, and they
don't age very well. The capacitors in tuned circuits are either mica,
ceramic or polystyrene.

Lytic caps are easily tested by piggybacking. Just clip a similar value
cap on, no need for any soldering or cutting. But really its farily
pointless unless you do some fault finding first.


NT

 

[Ray]

Get a can Of "Cold Spray" when it is working good, gently spray each
transistor to find which one is causing the trouble. Then replace it with a
new germanium transistor.

 

[Michael A. Terrell]

Lytic caps are easily tested by piggybacking. Just clip a similar value
cap on, no need for any soldering or cutting. But really its farily
pointless unless you do some fault finding first.

NT

Unless they are leaky, then "Piggybacking" is a waste of time. I
have an ESR meter, and a digital capacitor meter. On the other hand, I
would probably just replace all the old electrolytics first, then look
for other problems. It usually only takes a couple dollars worth of new
caps, and they should be replaced if you plan to actually use the radio.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[quietguy]

How many gangs on the tuning cap? Many of the 8 transistor radios had an
RF stage - if there are 3 gangs on the tuning cap that is a giveaway

David

 

[[email protected]]

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.

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

 

[Ken Weitzel]

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.


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.


Yes, that could be a problem, but the AGC seems to work well, so that
capacitor must be ok.

Hi Bill...

I haven't read the whole thread, so if I'm repeating anyone
else please accept my apologies.

Why not consider operating it in a warmish room, and spraying
local components one at a time (with a few minutes break in between)
with "freeze in a can" (whatever they call it in your part of the world)
until you find the component that causes it to act up.

And just for the heck of it, my bet's on a resistor.

Take care.

Ken

 

[[email protected]]

Replacing parts at random on electronics makes not one bit of sense.
There are hundreds of components and hundreds of connections, any one
of which may be to do with it.

There are about about 50 components. This is little portable radio
about the size of 2 packs of cigarettes (5.25 by 3 by 1.25). I like it
because it's easy to carry around and sounds better than the smaller
versions.

Trying to repair a radio by replacing geraniums with silicon makes even less sense.
Germanium trs are still sold new as well as used.

If you dont know what youre doing, leave it alone.

Well, if I leave it alone, I can only use it on hot days. I'm trying to
figure out a way I can use it on hot and COLD days?

Maybe you have a suggestion to overcome the temperature problems?

-Bill

 

[[email protected]]

How many gangs on the tuning cap? Many of the 8 transistor radios had an
RF stage - if there are 3 gangs on the tuning cap that is a giveaway

David

It's just a little pocket size radio (5 X 3 X 1.5) with 2 sections for
the tuning cap.It has the usual four RF coils and input and output
audio transformers. But that only requires 6 transistors, and there are
8 used. I suppose I can trace out the connections to try and figure out
what the extra 2 transistors do.

 

[Homer J Simpson]

Well, if I leave it alone, I can only use it on hot days. I'm trying to
figure out a way I can use it on hot and COLD days?

Maybe you have a suggestion to overcome the temperature problems?

The real way to fix it is with a signal tracer and signal generator. Isolate
to a section and fault find there.