RCA 8x-71

[lowell]

Hi I am trying to get FM on an RCA 8x-7. I have re-tubed and re-capped it and the AM is working. When I attempt to tune the FM it will NOT tune... and then when I switch back to AM the AM no longer works because I've attempted to tune the FM. I posted all this on Antique Radio forum but amp no longer getting help there. All voltages and grounds on the schematic seem good. I figured as a last ditch effort I'd post here one more time before telling the owner of the radio that it is forever an AM-only radio. I'm wondering if there is something simple I'm missing. IE I do not have an external antenna hooked up. I figure since AM is working then everything is good except for the FM tuner or receiver...

here's schem: (pg.5-6)
http://www.nostalgiaair.org/PagesByM...1/M0015251.pdf

[Enzo]

You are trying to pick up FM without an antenna?

COnnect three feet of wire to the antenna terminal and try again.

I don;t know why the AM would care at all what you did on the FM band, other than that the tuning won;t be at the same point on the dial.

[pontiacpete]

Lowel

Try this forum:

http://groups.yahoo.com/group/funwithtubes/

there are lots of experienced radio enthusiasts that are very helpful.

pete

[Robert M. Martinelli]

You are trying to pick up FM without an antenna?

COnnect three feet of wire to the antenna terminal and try again.

I don;t know why the AM would care at all what you did on the FM band, other than that the tuning won;t be at the same point on the dial.

Also, check solder joints, clean/retension tube sockets and clean rotating switches, that radio's very old, FM signals are very weak so they're prone to all kind of losses along the "road".

I took a look to the schematic, and there are a lot of test points/voltages/frequencies to be checked, though my first guess would be the I.F. ( Intermediate Frequency ) OSC/Mixer section. If you have a frequency meter or better still a scope, check for the 10.7 MHz FM IF frequency ( AM uses a 455 Khz IF ).

HTH

Best regards

Bob

[Steve Conner]

Have you tried replacing V1, the 19J6? Maybe it just barely oscillates on AM, and not at all on FM.

Apart from that, the most likely cause is a poor switch contact or a leaky capacitor, or some mechanical thing like a seized tuning capacitor.

I recently fixed up an old Grundig 3088 radio. The tuning capacitor was jammed solid and needed soaking with WD-40, then it needed about half a dozen capacitors replaced, plus a few resistors that had been burnt by excessive current draw through the shorted capacitors.

[lowell]

Also, check solder joints, clean/retension tube sockets and clean rotating switches, that radio's very old, FM signals are very weak so they're prone to all kind of losses along the "road".

I took a look to the schematic, and there are a lot of test points/voltages/frequencies to be checked, though my first guess would be the I.F. ( Intermediate Frequency ) OSC/Mixer section. If you have a frequency meter or better still a scope, check for the 10.7 MHz FM IF frequency ( AM uses a 455 Khz IF ).

HTH

Best regards

Bob

thanks Bob,
Could you please explain how to do this? Do I connect the scope ground to the chassis and the other lead to...where? Does the osc/mixer section naturally oscillate at 10.7mhz? Does it have to be tuned to that frequency or should it always be oscillating at that frequency regardless?

Steve I think I may carry on w/ replacing all the components regardless. I also may try a new 19J6. What exactly is that tube doing if you don't mind?

[Enzo]

Also, spray cleaner in the bearings of the tuning cap. If a section there loses contact, the circuits won;t function.

[Robert M. Martinelli]

I just took another look at the schematic diagram, and there's also a cathode current table, so you can also measure if all the tubes are working properly; if some cathode current is less than expected check for off-spec resistors, as per Steve's advice, if a resistor drifts on the high side or goes open the current through it will be either reduced or zeroed.

The schematic also tells you each tube's purpose, and it also includes the testing/alignment procedures, plus some advice on lead dressing for some critical components.

Anyway, before messing with those procedures, I would make myself sure the first tubes are working ( again, check/clean/re-tension the sockets, as the receiver's very old ), all the switches are cleaned and fully operational, there are no leaky/defective caps around, and the parts you replaced, if "critical" ones ( see schematic ) have the "right" lead dressing.

Hope this helps

Best regards

Bob

[lowell]

hey guys what is a diode probe? The alignment procedure calls for one to be in series w/ the o-scope when aligning.

[Enzo]

Call it an RF Probe. Or even a detector probe. Google RF probe and see if anything clicks.

At least I'd assume that is what they mean. Basically a diode in series rectifies the AC waveform - it "detects" it.

[Steve Conner]

Yes, it's basically a way of measuring the strength of RF signals that are too high frequency for the service guy's ancient tube scope. Such as the local oscillator when it's in FM mode, I suppose.

These days you can easily make a diode probe and use it with a DMM. The only possible gotcha is that the RF voltage inside a tube radio might be too much for a semiconductor diode. The LO can put out some pretty hefty voltage on the lower bands. The original diode probes used a diode tube like the 6AL5.

[Enzo]

Steve, you never owned an old VTVM, did you? All those Eico and Heathkit VTVMs had RF probes. They didn;t come with them, they were an add on option kit. The probes were these fat things about the size of an EL84 on the end of your probe cable. Had a plain old 1N34 diode inside with a cap and resistor.

Yeah, I imagine that 10MHz signal was tough to catch, even if your scope had whole ONE MEGACYCLE bandwidth.

Google RF Probe for a whole bunch of them, mostly the same

Here is a collection of signal tracers, many with RF probes. SOme home made RF prones even.

http://oak.cats.ohiou.edu/~postr/bapix/SigTrac2.htm

[Enzo]

Check out this tracer with a tube out in the probe!

http://oak.cats.ohiou.edu/~postr/bapix/SigTrac1.htm