Having trouble with a Westinghouse radio
 

I'm working on a Westinghouse AM/FM radio from 1950. Model number is H-334T7U. I have a copy of Riders but the schematic lacks most voltages at tube pins. The AM works fine but the FM is being a PITA! You can hear a station but cannot tune anything else. Sometimes you have to touch he trimmer cap to start it working but only 1 station(sort of). All the tubes are good, most replaced. All new caps, electrolytics, paper and 2 domino caps. The voltage checks in riders are right on but there are only a few listed. I tried to align it with a signal generator. All the IF cans seem fine. I'm at a loss and about to pull my hair out. Any suggestions?

First measure all the plate and screen voltages. My guess is that the local oscillator isn't oscillating. I had a Harman Kardon like that, which had separate power supplies for AM and FM; the FM was running low and the local oscillator would quit near the top of the range.

The circuit of your unit is pretty standard. Check the local oscillator if you can (it's running at 100 MHz and up so ordinary meters are no good for that). If it's old, the tuning capacitor may have corrosion at its contacts. Spray the spring fingers on the rotor of the FM oscillator section. See that adequate power is reaching the tube. I don't have the diagram in front of me, but the oscillator often is a separate tube, like half of a 12AT7. Sometimes it's a pentode.

It's a 12AT7. The problem is, I do not know what the voltages should be. The one they show is right on but I don't know the rest. I need a better schematic. Cant check the oscillator as of now. I will check the tuner though. It's pretty dirty.

Per the schematic on NostalgiaAir, the FM oscillator is the 12BE6. Sometimes they don't want to oscillate on FM frequencies even if they check good on a tube tester. Try subbing some other known good ones. Also, the voltages are shown on this schematic so you could check. Also, clean the bandswitch and all tube pins/sockets.

The FM oscillator is a 12AT7. Check the tube and socket pins for tension and grip. Clean the tube pins with a light sanding.

The schematic for H-334T7U clearly has the 12BE6 marked as "AM Conv./FM Osc." The 12AT7 is marked "FM RF - Mixer." The circuits bear this out.

http://www.nostalgiaair.org/PagesByM...5/M0024205.pdf

You're right. I saw the markings, I just didn't believe it until I looked at the circuit. I wouldn't count on a 12BE6 for reliable operation at 100 MHz. It is connected as a triode in the FM mode.

I've suspected the 12BE6 even though they test OK. My tester has 3 tests for that tube and it only passes 1. Can someone verify this for me?

Sure sounds like the local oscillator is not. Your Westinghouse uses the other half of the 12AT7 as the RF amp (instead of an oscillator) while using the 12BE6 as the oscillator for both AM and FM. Could this have been done to avoid a patent infringement on the "Armstrong system" ? Maybe some kind of upgrade is in order to get the 12BE6 to run at an abnormally high frequency. I would try swapping in as many different 12BE6's as possible.

My Motorola 79XM1 uses a 12BA7 (AM-FM osc-conv) with a 12BA6 RF amp, but barely functions on FM. I am not sure if this was the "Armstrong system" either. The tube characteristics show a 12BE6 and 12BA7 have an awful lot less Gm (gain) than 12AT7 as well.

My 1950 Zenith H724Z (a very common set with good reason) uses a 12AT7 as the FM AND AM oscillator with 6BJ6 RF amp. It has a gold sticker on the back proclaiming "the armstrong system". The selectivity is great for such an early FM.

BTW I will NOT own an RCA with FM solely because of Sarnoff's mistreatment of Maj. Armstrong.

The Pilot FM tuner T601 uses a 6BE6 as a full pentagrid oscillator and converter. So it can be done. Also, be sure to check for dirty band selector switches.