RCA 68R3 FM not working

[Jon F]

Hi, I have an RCA 68R3 that I recapped. All of the tubes are also tested good. The AM and Phono-in jack both work really well. However the FM band is silent, just a very faint static that does not change with tuning.

What could be the issue with this radio?

Thanks!

Link for reference, same model as mine:

http://www.radioatticarchives.com/radio.htm?radio=2677

[DavGoodlin]

The schematic is on Nostalgiaair and shows two 6BE6 tubes.
One is for FM, the other for AM. Try swapping these because one may be failing to ocsillate at FM frequencies.
The FM 6BE6 measures 235 volts on the plate, pin 5. You may need one of the Pomona adapters to measure voltage on this tube since it may be hard to reach in the tuner can.

[jr_tech]

I would first clean and de-ox the pins of the FM local oscillator tube (and socket) very well.

It is also quite possible for the FM local oscillator tube to test good (especially if you are using a simple emission checker), and still not oscillate at the high frequencies necessary to tune the FM band... I would try a substitute.

jr

[Tubejunke]

What year was the radio made? Mid 40s radios often had a partial FM band that went only from about 21 to 99 MHz (the #s on my Philco dial). Of course you should still be able to bring something in in most places.

[bob91343]

I concur that the likely problem is that the local oscillator isn't running. Sometimes it's due to the tuning capacitor sliding contacts being corroded, and sometimes it's due to the B+ voltage being too low. The comments on tubes and sockets are pertinent.

[stromberg67]

Originally Posted by Tubejunke

What year was the radio made? Mid 40s radios often had a partial FM band that went only from about 21 to 99 MHz (the #s on my Philco dial). Of course you should still be able to bring something in in most places.

Pre-war FM sets left off the first digit of the frequency on the dial scale. They tuned 42.1 through 49.9 mHz. Some earlier models, but not many, tuned a band of lower frequencies before the band allocation was changed.

[Jon F]

Thanks for the help guys, I was happy to find both 6BE6's did not test good, I replaced them along with a couple other tubes on the set, however the FM did not come back. There is no static nothing, just like a dead band.

[Jon F]

Originally Posted by DavGoodlin

The schematic is on Nostalgiaair and shows two 6BE6 tubes.
One is for FM, the other for AM. Try swapping these because one may be failing to ocsillate at FM frequencies.
The FM 6BE6 measures 235 volts on the plate, pin 5. You may need one of the Pomona adapters to measure voltage on this tube since it may be hard to reach in the tuner can.

I am a little confused about this measurement. The paperwork shows the 6BE6 for the FM on the front of the chassis and the AM on the rear of the chassis. However, I get about 295VDC on pin 5 for both of these tubes, but only on the front(FM?) tube when I have the switch to AM and on the rear (AM?) tube when I have the switch to FM.

Can someone confirm which tube is which?

Thanks!

[Reece]

With the radio playing in AM, pull first one 6BE6 and then the other: the one which makes AM stop is the AM oscillator. Also: carefully clean the band selector switch and turn back and forth a dozen times.

[jr_tech]

I'm betting the rear 6BE6 is FM... that "can" that it is setting on with the antenna terminals sure looks like an "FM module"

jr

[Reece]

Me too! The top of chassis drawing in the schematic is confusing as it does not tell you which side is the front; there are no shafts sticking out or speaker shown to positively identify it. However, if you have the actual radio in front of you that's not a problem.