Probably simple AM radio repair
 

Hi,

I acquired a Tandy Chronomatic 246 clock radio that is in great condition.
The only problem I have is AM reception below 900 kHz is almost non-
existant. I can turn up the volume to max and touch the internal antenna
to hear stations at low volume. The FM band works great.

I did replace a leaking 4.7u electrolytic cap that was nearest to the
adjustable yellow choke and tuning cap, but it made no difference. I also
adjusted the chokes a little to see if I could get better reception near
900 kHz, but no luck.

I'm suspecting there might be a broken wire in the internal AM antenna?
The wires in position on the antenna a

Red-Black-Blue White

If I touch the white wire, much better reception. But it is soldered to
the PCB securly.

Is there a way to test the antenna, like ohm it out? Any other ideas?

Thanks,

Dave

"Dave" bravely wrote to "All" (22 Jun 05 05:18:25)
--- on the heady topic of "Probably simple AM radio repair"

Da Subject: Probably simple AM radio repair
Da Xref: aeinews sci.electronics.repair:51188

Da Hi,

Da I acquired a Tandy Chronomatic 246 clock radio that is in great
Da condition. The only problem I have is AM reception below 900 kHz is
Da almost non- existant. I can turn up the volume to max and touch the
Da internal antenna to hear stations at low volume. The FM band works
Da great.
Da I did replace a leaking 4.7u electrolytic cap that was nearest to the
Da adjustable yellow choke and tuning cap, but it made no difference. I
Da also adjusted the chokes a little to see if I could get better
Da reception near 900 kHz, but no luck.

Da I'm suspecting there might be a broken wire in the internal AM
Da antenna? The wires in position on the antenna a

Da Red-Black-Blue White

Da If I touch the white wire, much better reception. But it is soldered
Da to the PCB securly.

Da Is there a way to test the antenna, like ohm it out? Any other ideas?

Da Thanks,


Dave,

The loopstick antenna can often be optimized by moving the pickup coil
along the length of the ferrite rod. However, I don't think this is
the real problem. There might be a break in the tuning capacitor or
its trimmer. Perhaps the rf xformer/mixer cans has an open coil.
Perhaps the local oscillator/mixer (transistor, IC) is stalling below
900KHz. This might be the supply bypassing electro for that circuit
being open, try tag soldering a good one across it. It should be
obvious where the problem lies by signal tracing with a scope or
demodulator probe. Was this an existing problem or a sudden event?

A*s*i*m*o*v

.... That was a fascinating period of time for electronics

"Dave" wrote in message
...

I'm suspecting there might be a broken wire in the internal AM antenna?

Unlikely. Probably someone unskilled "tuned it up". You need a signal
generator and basic skills.

N

NSM wrote:
"Dave" wrote in message
...


I'm suspecting there might be a broken wire in the internal AM antenna?


Unlikely. Probably someone unskilled "tuned it up". You need a signal
generator and basic skills.

N

Part of routine maintenance. Tighten all the screws :)

(OP - this an old joke; don't even think about it!)

Ken

"Ken Weitzel" wrote in message
news:ONhue.1778478$6l.340393@pd7tw2no...

Part of routine maintenance. Tighten all the screws :)

I saw a few tube radios brought in for repair after that was done. Then
there was the guy who disassembled his record changer and brought it all in
to me in a bag for me to figure out.

N

Don't forget that electrolyte is very corrosive, and has a tendency to creep
where you don't want it to - it loves things like RF cans . . .

If the decoupling cap that you mentioned really leaked, and it was near an
RF can, suspect that it's gotten inside, and eaten the windings .

Geoff


"NSM" wrote in message
news:gZiue.79315$9A2.64480@edtnps89...

"Ken Weitzel" wrote in message
news:ONhue.1778478$6l.340393@pd7tw2no...

Part of routine maintenance. Tighten all the screws :)

I saw a few tube radios brought in for repair after that was done. Then
there was the guy who disassembled his record changer and brought it all
in
to me in a bag for me to figure out.

N

"Asimov" wrote in
:

"Dave" bravely wrote to "All" (22 Jun 05 05:18:25)
--- on the heady topic of "Probably simple AM radio repair"
Dave,

The loopstick antenna can often be optimized by moving the pickup coil
along the length of the ferrite rod. However, I don't think this is
the real problem. There might be a break in the tuning capacitor or
its trimmer. Perhaps the rf xformer/mixer cans has an open coil.
Perhaps the local oscillator/mixer (transistor, IC) is stalling below
900KHz. This might be the supply bypassing electro for that circuit
being open, try tag soldering a good one across it. It should be
obvious where the problem lies by signal tracing with a scope or
demodulator probe. Was this an existing problem or a sudden event?

Thanks for the advice everyone! I should mention after tweaking the chokes
around the variable tuning cap, reception is acceptable to 750 kHz, but
there are a few stations below that I'd like to listen to. I bought the
radio with its current problem at a garage sale, it doesn't look like it's
ever been opened. I'll inspect the variable cap and chokes.

I do have a 5MHz function generator and a decent scope, is there some
advice on the net for testing receivers by injecting signals? I am mainly
a digital troubleshooting type and don't really know that much about tank
circuits and such.

"Dave" wrote in message
...

Thanks for the advice everyone! I should mention after tweaking the
chokes
around the variable tuning cap, reception is acceptable to 750 kHz, but
there are a few stations below that I'd like to listen to. I bought the
radio with its current problem at a garage sale, it doesn't look like
it's
ever been opened. I'll inspect the variable cap and chokes.

I do have a 5MHz function generator and a decent scope, is there some
advice on the net for testing receivers by injecting signals? I am mainly
a digital troubleshooting type and don't really know that much about tank
circuits and such.

Peak the IF at 455 KHz.

Adjust the coils to peak at 600 KHz, and the trimmers to peak at 1500 KHZ.
Repeat as required.

Not a big deal really. BTW, if the ferrite rod is cracked it won't work
well.

N

Dave:

Where are you located?

There are many antique/tube type clubs out there that could probably
figure out what's wrong in a few minutes. Those of us brought up on
tube type am-fm radios can do what's needed in our sleep, but writing
out the process would take ten times that long, even for just an am
radio.

Regards,

H. R.(Bob) Hofmann

Dave wrote:
Hi,

I acquired a Tandy Chronomatic 246 clock radio that is in great condition.
The only problem I have is AM reception below 900 kHz is almost non-
existant. I can turn up the volume to max and touch the internal antenna
to hear stations at low volume. The FM band works great.

I did replace a leaking 4.7u electrolytic cap that was nearest to the
adjustable yellow choke and tuning cap, but it made no difference. I also
adjusted the chokes a little to see if I could get better reception near
900 kHz, but no luck.

I'm suspecting there might be a broken wire in the internal AM antenna?
The wires in position on the antenna a

Red-Black-Blue White

If I touch the white wire, much better reception. But it is soldered to
the PCB securly.

Is there a way to test the antenna, like ohm it out? Any other ideas?

Thanks,

Dave