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 Post subject: RCA 68R4 FM Band?
PostPosted: Dec Mon 04, 2017 5:27 pm 
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Joined: Jan Mon 11, 2016 1:19 pm
Posts: 278
Location: Spring Lake Mi
RCA 68R4...replace all cap's. I do not have an FM signal generator, the one I have goes to 30 mh. The problem is that FM is way off, 107 FM comes in at 96 on the scale. No stations come in after 96 on the scale, but I do get the stations below 107 FM, below 96 on the scale. Tried adjusting osc not enough to go that far on the scale.
Am is very good and accurate on the scale..........
Hmmmmmmmmmmm

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Jim KA8ZXJ
Spring Lake MI 49456


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 Post subject: Re: RCA 68R4 FM Band?
PostPosted: Dec Mon 04, 2017 5:44 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: San Jose, CA USA
The first thing I would check is FM oscillator trimmer cap C33. If this is a rotating-style ceramic trimmer, corrosion of the silver film on top tends to put these out of commision. Try replacing with another trimmer, or simply trim with a few fixed caps if you have a good selection of small value micas on hand.

This may not be the problem, but it is what I most often see when this type of symptom comes up.

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Tom K6VL


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 Post subject: Re: RCA 68R4 FM Band?
PostPosted: Dec Mon 04, 2017 9:27 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 304
+1, my 68-R3 had the same problem, it was that trimmer cap.


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 Post subject: Re: RCA 68R4 FM Band?
PostPosted: Dec Tue 05, 2017 1:16 pm 
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Joined: Jan Mon 11, 2016 1:19 pm
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Location: Spring Lake Mi
Thanks for the comments I shall check this trimmer out.

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Jim KA8ZXJ
Spring Lake MI 49456


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 Post subject: Re: RCA 68R4 FM Band?
PostPosted: Dec Wed 06, 2017 4:49 pm 
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Joined: Jan Mon 11, 2016 1:19 pm
Posts: 278
Location: Spring Lake Mi
Tom Albrecht wrote:
The first thing I would check is FM oscillator trimmer cap C33. If this is a rotating-style ceramic trimmer, corrosion of the silver film on top tends to put these out of commision. Try replacing with another trimmer, or simply trim with a few fixed caps if you have a good selection of small value micas on hand.

This may not be the problem, but it is what I most often see when this type of symptom comes up.


Tom, thanks for your advise, replaced the C33 trimmer and put in a 10pf cap it works just fine, also giving me a headache was that the dipole antenna in the cabinet has issues. I have the radio connected to a FM antenna in the attic, radio works fine the radio antenna is cutting in and out going to check into that.

Thank you

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Jim KA8ZXJ
Spring Lake MI 49456


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 Post subject: Re: RCA 68R4 FM Band?
PostPosted: Dec Wed 06, 2017 5:47 pm 
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Location: USA
BTW, like some others, my signal generator doesn't directly go into FM band. But I use the harmonics where weaker signals can be heard on multiples of the tuned frequency (e.g. 30mhz should have a harmonic at 90mhz). You may want to check if your signal generator does the same.


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 Post subject: Re: RCA 68R4 FM Band?
PostPosted: Dec Wed 06, 2017 6:17 pm 
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Location: San Jose, CA USA
Is the built-in antenna made of copper wire (like a twin lead folded dipole), or does it use aluminum foil? If it uses aluminum foil, the problem is that the connection point between the wire lead and the aluminum foil goes bad due to oxidation of the aluminum. You may be able to fix it by driving a new sheet metal screw through the foil into the wood, and connecting the wire there, although this new connection may also go bad over time. Sometimes best to replace the foil antenna with a wire antenna. Be creative.

These comments assume, of course, that the antenna is the problem. If the real problem is that the radio does poorly unless signals are really strong, that's a different story. Do you think it's the antenna, or is the problem that the radio has trouble with weaker signals?

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Tom K6VL


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 Post subject: Re: RCA 68R4 FM Band?
PostPosted: Dec Wed 06, 2017 9:43 pm 
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Joined: Jan Mon 11, 2016 1:19 pm
Posts: 278
Location: Spring Lake Mi
Tom Albrecht wrote:
Is the built-in antenna made of copper wire (like a twin lead folded dipole), or does it use aluminum foil? If it uses aluminum foil, the problem is that the connection point between the wire lead and the aluminum foil goes bad due to oxidation of the aluminum. You may be able to fix it by driving a new sheet metal screw through the foil into the wood, and connecting the wire there, although this new connection may also go bad over time. Sometimes best to replace the foil antenna with a wire antenna. Be creative.

These comments assume, of course, that the antenna is the problem. If the real problem is that the radio does poorly unless signals are really strong, that's a different story. Do you think it's the antenna, or is the problem that the radio has trouble with weaker signals?

Tom, good questions I have not pursued it yet but what was happening (twin lead TV type dipole) I would get station's all across the FM band, when listening to 107.4 or any station on that end of the dial, reception would quit but I could get the lower stations below 96. That was connected to the antenna in the radio via PG clips could have been just a bad connection, it did happen several times until I connected it to the antenna in my attic, no problem there. I am working on the cabinet, needs some restoring. I don't think there is a serious problem with the antenna. Sure am happy to get this one going. I have been working two radios and got both of them restored this week.
Thanks again.

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Jim KA8ZXJ
Spring Lake MI 49456


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 Post subject: Re: RCA 68R4 FM Band?
PostPosted: Dec Wed 06, 2017 9:44 pm 
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Joined: Jan Mon 11, 2016 1:19 pm
Posts: 278
Location: Spring Lake Mi
AJJ wrote:
BTW, like some others, my signal generator doesn't directly go into FM band. But I use the harmonics where weaker signals can be heard on multiples of the tuned frequency (e.g. 30mhz should have a harmonic at 90mhz). You may want to check if your signal generator does the same.

Great idea I'll will look into that, it does go to 30mhz.
Thanks for the tip.

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Jim KA8ZXJ
Spring Lake MI 49456


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 Post subject: Re: RCA 68R4 FM Band?
PostPosted: Dec Sun 10, 2017 10:14 pm 
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Joined: Jan Mon 11, 2016 1:19 pm
Posts: 278
Location: Spring Lake Mi
Got the radio completed, the internal FM antenna works fine, and refinished the cabinet looks great. On to the next radio a Motorola 65T21..........................

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Jim KA8ZXJ
Spring Lake MI 49456


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