[Ricky L. Adkins]

Hi all, it's been a while since I have been able to work on any radios, but now I have this Philco model 52-944 that is acting strange on the FM band.  I have replaced most all wax paper caps and the radio works fine on AM, but could not get anything on FM band.  I checked the band switch and all is good there.  I get nothing on FM band unless I touch the coil (L2) with the blade of a screwdriver...then FM jumps to life and plays until I remove the screwdriver.  I am not grounding it with the screwdriver, just simply touching the L2 coil.  I know there is probably a good reason why this is working like this, but I'm not sure where to start.  Any ideas out there as to what this needs?  I do not have the old Line cord FM aerial that attaches to the power cord...I guess the previous owner got rid of this.  But I do have an FM antenna attached to the proper terminals (1 & 2).  Thanks in advance for any ideas.

Rick

[RodB]

I would start with measuring pin 5 of the 12AU6. It should be around 100 volts. FM front end circuits of this era are very sensitive to plate voltage problems. Since the power supply is driven by a selenium rectifier there may be a problem with it. I always replace a selenium rectifier with a modern silicon diode (1N4007). L2 is in the FM oscillator so you can check all solder connections associated with those parts. I assume you have the antenna connected to pins 2 and 3 of the antenna terminal block.

[Ricky L. Adkins]

Thanks for the info. I have checked the voltages on the 12AU6 and they look good. I may have made a silly mistake...I think I have the FM antenna attached to pins 1 & 2. I will make this change and also check all of the solder connections as you pointed out. I'll let you know what I find. Thanks so much for your tips,

Rick

[Ricky L. Adkins]

On the Philco 52-944, The only way FM will receive is if I attach a 16" piece of wire from point 4 on L2 to point 3 on L2. I'm stumped. I have solder the points just to make sure they were connected good, but that did not help. Antenna is connected properly. I have not tried replacing the small "resistor-like" capacitors in this part of the circuit, so maybe they are bad. The ones I'm talking about are in the 100 and 220 picofarad ones.

[RodB]

Have you measured the continuity of L2, without the jumper, between pin 4 and ground? The caps are ceramic and very rarely are bad. When they are bad it's usually one of the leads causing problems.

[Ricky L. Adkins]

The continuity checks good to ground from pin 4 of L2.  All tube voltages look good as well.  I have not tried adjusting any of the trimmer capacitors, since that can really put things out of alignment real quick.  I did find that one of the ceramic capacitors is reading 53 picofarads, when it should be 3.3 picofarads.  It goes from terminal 2 (FM antenna) to the trimmer capacitor on L1.  I currently do not have any replacements that size, but will eventually replace it.  I have also noticed that if the jumper from pin 4 to pin 2 on L2 is just a short wire, FM still doesn't receive.  The length of wire seems to be acting like an antenna, which makes me think the problem is between L2 and the antenna terminals.  I may have to start removing one end of resistors and capacitors in this area and verifying their value against the schematic.  I'll get back with more info once I do additional troubleshooting.  Thanks or your help,
Rick

[RodB]

BTW Rick, I looked at the schematic a little closer last night and L2 is a rf transformer, not the oscillator coil like I previously said. You're on the right track. The 16" antenna is bypassing the rf amplifier and feeding the signal to the mixer/oscillator. Have you tried a different 12AU6? Check out C6 at 100pf. The signal is not getting from the rf amplifier to the oscillator. Yes, don't tweak any FM trimmers, you could loose your signal altogether.

[Ricky L. Adkins]

Thanks RodB, I think I have another 12AU6. If that doesn't help, I will replace C6 capacitor. I see what you are saying. I will let you know how it goes tomorrow. It never fails to amaze me that there is so much expertise here at this forum. I'm a chemist by trade, retired now, and I just picked up tube radio as a hobby about 5 years ago...now my collection is just about crowding me out of my garage, Lol. Hopefully I will be able to help someone else the way you all have helped me. I have found several good books on radio theory...so I'm trying to learn a little more as I go. Thanks for your help,

Rick

[Ricky L. Adkins]

Thanks again for pointing me in the right direction.  The radio is working great now.  Capacitor C6 was bad.

[RodB]

Score one for the home team!

[Ricky L. Adkins]

Yeah!