Hello All,
I have replaced all paper and electrolytic capacitors in this radio. I also replaced some burned resistors (68 ohms and 10 ohms). The AM broadcast band works fine but all I can hear is the amplifier when the band switch is in the FM position. No FM hiss, no static, no nothing. The FM band is quite dead. What should I look at? I have checked every resistor and every capacitor and cannot find anything wrong.
Please help if you can.

Thanks,

Dave

I don't see this schematic on here. However, first you should check tubes, especially if there are tubes used only for FM. If the set was used on AM a lot, the FM tubes could have become poisoned.

Second, if any of the FM IF transformers have permanent silver mica capacitors, you need to check these. Often the FM IFs use just the interwinding capacitance, but occasionally they will have external capacitances, such as in the detector IF.

T.

Schematic is here.

http://techpreservation.dyndns.org/schematics/Philco.htm

That 14X7 is the detector tube. Could be the diode section is bad. Tack in a signal diode on the bottom of the socket with tube still in place. Test the FM now.

I do not currently have a djvu program installed on my computer, so I cannot view the file.

It is unlikely that the detector tube is at fault, as these handle incredibly small amounts of current, and have to be practically dead to not work. However, if no other fault can be found, it would be good to try a substitute tube.

It is better to check for an oscillator, RF, or IF tube that is only used for FM. Especially critical is the oscillator.

T.

Thomas.
This radio uses exactly the same detector tube for both AM and FM. There are only two diodes in it. The AM broadcast band works fine. This also uses a converter tube instead of two separate tubes for oscillator and mixer. Maybe it no longer works at the higher frequencies. 14F8 is the converter tube.

Thanks,

Dave
:I do not currently have a djvu program installed on my computer, so I cannot view the file.
:
:It is unlikely that the detector tube is at fault, as these handle incredibly small amounts of current, and have to be practically dead to not work. However, if no other fault can be found, it would be good to try a substitute tube.
:
:It is better to check for an oscillator, RF, or IF tube that is only used for FM. Especially critical is the oscillator.
:
:T.
:

Dave. That detector tube is a duel diode. One for Am the other FM. It's just an outside chance the FM side is near dead.

Warren,
I believe that the diodes are switched from the AM (detector-AVC) to FM (discriminator). I had asked this question before and I remember Edd explaining how this works in this radio.
Just a second, I'll find it. Here is the thread:
http://www.nostalgiaair.org/forums/Messages/180/M0085180.htm

Yes, it really is a ratio detector. I think that when someone "worked" on this radio before they got the connections wrong on the FM RF amplifier tube. So I'm going to be exploring that possibility today. When I was restoring the chassis, I noticed that what was in front of me didn't quite match what was on the schematic. The 47 ohm resistor connected pin 7 to pin 2. But on the schematic pin 7 and pin 2 are connected to each other, and just one end of the 47 ohm resistor connects to the jumper. If that's connected incorrectly I don't think there's any signal to listen to. That area of the chassis is extremely difficult to get to, to work on it. My "micro" diagonal cutters wont fit in there.

Thanks,

Dave

:Dave. That detector tube is a duel diode. One for Am the other FM. It's just an outside chance the FM side is near dead.
:

Thomas,
I mis-spoke. The converter tube is two triodes in the same envelope, and not a pentagrid converter. So I take back what I said about it.

Dave
:Thomas.
: This radio uses exactly the same detector tube for both AM and FM. There are only two diodes in it. The AM broadcast band works fine. This also uses a converter tube instead of two separate tubes for oscillator and mixer. Maybe it no longer works at the higher frequencies. 14F8 is the converter tube.
:
:Thanks,
:
:Dave
::I do not currently have a djvu program installed on my computer, so I cannot view the file.
::
::It is unlikely that the detector tube is at fault, as these handle incredibly small amounts of current, and have to be practically dead to not work. However, if no other fault can be found, it would be good to try a substitute tube.
::
::It is better to check for an oscillator, RF, or IF tube that is only used for FM. Especially critical is the oscillator.
::
::T.
::
:

Check that tube. It needs good emission in order to work at those high frequencies.

T.

Hello Thomas.

Lizardtech.com has a DjVu solo 3.1 reader. Works fine.

The radio in question has the loctal tubes. Probably he does not have any to sub in. This was just a try and see.