Been working on a Philco 46-480 radio chassis that has the modern FM band on it. I got the AM and SW working but nothing on the FM. I've been checking what components, and circuits I can access (the big 4 wafer band switch is hard to trace and see components on and physically blocks access to many parts of the FM circuit. There are no voltages on the schematic for the FM circuit when in use.
I could use some ideas on how to see if the FM oscillator is working (I don't have scope) or any similar tips to get to the bottom of this problem. I tried a different mixer/osc tube already and have verified as many of the component values that I can reach......

Forgot...Here's the link to the schematic....
http://www.nostalgiaair.org/Resources/602/M0013602.htm

Assuming that you have an RF signal generator and a high-impedance DC voltmeter, you could try performing the first two steps of the FM alignment procedure and see if the set responds. That takes the mixer/osc tube operation issue entirely out of the equation. If the set responds to the procedure, you have a problem in the mixer/osc stage; if not, the trouble is in the IF or discriminator transformers.

Before I did the above, I think that I would first check the FM portion of the IF cans for continuity and also the discriminator portion of the discriminator/detector transformer. Since this set uses the same tubes for FM as for AM, the problem has to be in a discrete component or transformer that is switched in for FM operation but switched out for AM. If you do a visual comparison on the "AM mode" v.s. "FM mode" Rider drawings you can quickly put together a hit list for specific parts to examine. If you can exonerate those, then you can move into looking for more esoteric causes such as the legendary "tube that will oscillate in the BC range but not in the FM range".

Actually, the statement that "the set uses the same tubes in AM mode as in FM mode" is not 100% true: I believe that in FM mode both diode elements of the 6H6 tube are used, whereas in AM mode only one of the two is used. So, if you had a 6H6 tube that was dead on just one side it would prevent FM from being demodulated but would demodulate AM OK. It seems like a stretch to think that this could actually happen, though: not only would half the tube have to fail, it would have to be the half of the tube that isn't used to demodulate AM.

I've checked the continuity of the FM coils as best I can...The way I understand this circuit---That since the AM/SW signal (when choosing AM/SW on the band selector) just passes thru the FM coils and moves on to the AM coil portion of the transformer cans..Wouldn't it be fair to assume that if the AM/SW works, the FM coils are OK AFA continuity, since the signal has to go thru them first? Just sayin'...
I don't have replacement tubes on hand to try 7H7's and 6H6 replacements.
I've went thru the circuits pretty well and it may come down to the tubes...I don't have a signal generator so I'm pretty limited on ways to check the osc.

The bandswitch shorts out the AM section of the IF transformer stack when in FM mode, and vice-versa. So one section in the stack could be open and it wouldn't affect the operation of the other- the set would just be dead in one mode. Take a look at the sheets that show the FM mode connectivity v.s AM.

I measured the voltage on Pins #1 & 8 (grids) of the 7F8 mixer/osc tube. On the AM and SW positions, even with no station, I get a neg voltage of at least -.750vdc, and up to -2.0vdc with a station. On the FM position (of course no signal right now just amp hiss) I don't get any voltage on either grid. Does this give any clue? I'm not very strong in all radio theory, and have been able to fix 95% of the radios that come in the shop, but this one is a bugger.

No voltage at the oscillator grid in FM mode probably just means that the reactance of your voltmeter probe has killed the oscillator. At the high frequency that FM requires the circuit is much more sensitive to being detuned than it is at the AM frequency.

No DC voltage at the mixer grid means that you have no signal making it to the discriminator/demodulator so that a negative AVC voltage can't be developed which would drive the grid negative- no clue there.

Thanks for your help so far CV...I just noticed that both the fixed vanes and the rotating FM tuner vanes are at ground (they are not rubbing)....On the AM which works, the rotating vanes are at ground, and the stationary vanes are infinite resistance. Is having both FM tuner vanes at ground normal?...Or, what am I seeing here?

The AM tuning cap sections are coupled to their coils via fixed caps in series- so ohming across the vanes will give infinite DC resistance. The FM tuning caps are tied directly to their (very short) coils- so as far as DC resistance is concerned they appear to have a dead short from stator to rotor vanes.

OK CV..That explains it very well....Thanks!!! Now, back to the drawing board...

:OK CV..That explains it very well....Thanks!!! Now, back to the drawing board...
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:Does this look familiar. I started a post last month that just kind of died with the change of the month. I had a B+ problem that I traced back to the RF section of my 46-480. Been working great since I built it in 2009 but recently went DOA.

This is the 2nd time I've had this RF section out now. At this point I have the bandswitch completely disassembled and im cleaning it and replacing everything on it. I have two new Loctal sockets on order because these are shot.

I can only say this about Philcos design of this section of the 46-480... Poor guy on the assembly line!

Johnny

Yep Johnny, that's the offending switch in your pic...The thing that radio repair nightmares are made of. The guy I'm doing the repair for may just want the FM section hardwired in, bypassing the band switch, so only FM can be used...I may be able to pull that off, but either way I have to get the FM band working. I'm a little bit leery taking that switch out, since it isn't my radio, and there may a chance I won't get anything working...But it would clear the way to look at the components on the switch and get at T2 and the mixer/osc tube socket...Let me know how you do with your non-working FM...I hate those loctal sockets..Sometimes the tube pulls the center 'hole' of the socket up with the tube, and, you feel like you're going to break the tube trying to wiggle it out..Keep in touch..I'll do the same...

Brian I can tell you that back in '08 or '09 when I got this radio I never did power it up. I knew I was restoring it. When I was looking that bandswitch over I knew I didn't want to pull it out but there was at least one resistor and one capacitor under it that I could barely see and could of easily overlooked them. They were both fried. This section is so crammed together it's one big short waiting to happen. Good luck!

Johnny

:Yep Johnny, that's the offending switch in your pic...The thing that radio repair nightmares are made of. The guy I'm doing the repair for may just want the FM section hardwired in, bypassing the band switch, so only FM can be used...I may be able to pull that off, but either way I have to get the FM band working. I'm a little bit leery taking that switch out, since it isn't my radio, and there may a chance I won't get anything working...But it would clear the way to look at the components on the switch and get at T2 and the mixer/osc tube socket...Let me know how you do with your non-working FM...I hate those loctal sockets..Sometimes the tube pulls the center 'hole' of the socket up with the tube, and, you feel like you're going to break the tube trying to wiggle it out..Keep in touch..I'll do the same...
:

Johnny--I can see a couple of the components you mentioned that are tucked 'under' the switch...I've been able to check 95% of the other parts on the switch by 'creative' DVM probe extensions. I have a feeling it's something in the I/F cans, but it could get sticky if I mess one up...The customer says he is OK with only AM..but the repair guy in me can't resist to keep going after it. Never got a peep out of the FM yet...I don't have a scope, and would love to know if the oscillator working.

..I don't have a scope, and would love to know if the oscillator working.
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If you have a comm receiver with BFO that tunes 9.1 MHz you should be able to use it to "sniff" the mixer product in FM mode. This would tell you if the oscillator is working. You would need to make a crude RF probe to connect the comm set's antenna input with a short metal wand that you could hold in the vicinity of the UUT oscillator. Even a lowly S-38 would work.

Thanks for the tip CV...I used to occasionally have a receiver around with BFO, but none 'in stock' right now.
I'm going to try to start tearing into the I/F cans I can access, maybe there is a mica cap inside one that has gone south.

:Been working on a Philco 46-480 radio chassis that has the modern FM band on it. I got the AM and SW working but nothing on the FM. I've been checking what components, and circuits I can access (the big 4 wafer band switch is hard to trace and see components on and physically blocks access to many parts of the FM circuit. There are no voltages on the schematic for the FM circuit when in use.
:I could use some ideas on how to see if the FM oscillator is working (I don't have scope) or any similar tips to get to the bottom of this problem. I tried a different mixer/osc tube already and have verified as many of the component values that I can reach......
:

::Been working on a Philco 46-480 radio chassis that has the modern FM band on it. I got the AM and SW working but nothing on the FM. I've been checking what components, and circuits I can access (the big 4 wafer band switch is hard to trace and see components on and physically blocks access to many parts of the FM circuit. There are no voltages on the schematic for the FM circuit when in use.
::I could use some ideas on how to see if the FM oscillator is working (I don't have scope) or any similar tips to get to the bottom of this problem. I tried a different mixer/osc tube already and have verified as many of the component values that I can reach......
::
:
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:::Been working on a Philco 46-480 radio chassis that has the modern FM band on it. I got the AM and SW working but nothing on the FM. I've been checking what components, and circuits I can access (the big 4 wafer band switch is hard to trace and see components on and physically blocks access to many parts of the FM circuit. There are no voltages on the schematic for the FM circuit when in use.
:::I could use some ideas on how to see if the FM oscillator is working (I don't have scope) or any similar tips to get to the bottom of this problem. I tried a different mixer/osc tube already and have verified as many of the component values that I can reach......
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