The PHILCO Phorum
48-1264 No FM - Printable Version

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48-1264 No FM - RWood - 05-22-2016

Hello,

New member here with a console that I've recapped and have the AM and Phono inputs working well, but have nothing at all on the FM. I've read other threads on this model and the FM problem and have tried the things I've read about, but with no luck. I do not have a working signal generator and have not yet learned to use one (I do have a Hickok 288AX that needs to be recapped).  Anyway, here's what I've done so far:

New power supply caps
Changed out wax and paper capacitors
New tubes
Replaced C412 (100pF) and C413 (220pF) as suggested
Checked pin voltages - all look good and are within 20% of the target voltages
Cleaned all bandswitch contacts (emery cloth and deoxit)
Cleaned socket and pins of the 7F8
Reflowed solder on most socket pins and terminal strips
Checked all coils for restance per the Philco service manual - no shorts as far as I can see

I could use some guidance about what to try next. Thanks!

Richard

RE: 48-1264 No FM - klondike98 - 05-22-2016

Welcome to the Phorum!
Icon_wave

Some of the folks here are pretty good troubleshooters so hopefully you'll be able to get it going. Here's a link to the Service Bulletin for that model so people can help you out more easily.

RE: 48-1264 No FM - Ron Ramirez - 05-22-2016

I would check C310 first (in the schematic Bob linked above). This is a special 3-legged capacitor which, if not replaced properly, will kill your FM reception.

It may be replaced with a conventional .01 uF capacitor, but you have to be careful that you connect it properly. You may have to connect a jumper wire to take care of the now-missing third leg, as discussed in the following link:

http://philcoradio.com/tech/ftcap.htm

Hopefully this will be helpful.

RE: 48-1264 No FM - Ron Ramirez - 05-22-2016

See the drawing below:

[Image: http://www.philcoradio.com/images/phorum/481264.jpg]

C310 should be connected as shown above in blue.

A jumper wire should be added as shown above in red.

RE: 48-1264 No FM - RWood - 05-22-2016

Thank you both! This place is great.

I did have C310 connected in the wrong place but fixing it did not bring back the FM. I've got the jumper in there, too.
I'll link to a photo below of the current state of the chassis......let me know if I should take any close-ups.

Ideas for next steps? Anyone have voltages for 7F8 when FM is selected?

Richard

[Image: http://i32.photobucket.com/albums/d26/RWoodVa/IMG_0792_zpsmcktzxri.jpg]

RE: 48-1264 No FM - klondike98 - 05-22-2016

Unless someone else has a different thought, I'd start working my way through the Trouble Shooting steps in the Service bulletin.

RE: 48-1264 No FM - KCMike - 05-22-2016

Just a quick question, Did you have the FM antenna connected? I see it is built in to the cabinet.

RE: 48-1264 No FM - RWood - 05-22-2016

Yes, antenna is connected. It's a 2-wire connected to the two wider pins of the 4-pin socket. I did try the cabinet antenna, too, just to make sure. Still nothing.

I would do the troubleshooting steps but I don't have a working signal generator. I was hoping for a few "try this" ideas like in Ron's post. If there are any more ideas please let me know. If a signal generator is the way to go, I will shelf this chassis and get to work on the Hickok.

RE: 48-1264 No FM - KCMike - 05-22-2016

You will probably have to get a signal generator or fix the one you have since I see you have did the first trouble shooting test of the voltages. Maybe someone here with more knowledge than me can give you an idea what to try next without sig. generator.
Only other thing I can add, you say you put in new tubes. Do you have tube tester are you sure they're good? Mainly the ones in the FM section. Good luck.

RE: 48-1264 No FM - klondike98 - 05-22-2016

ah yes..I forgot you said earlier that the signal generator was not working. I guess I'd go to the FM circuits trouble shooting section on p. 6 and take a look at the caps and resistors that are listed in the "possible cause of abnormal indication". See if any resistors are open and caps connected to correct spots.

RE: 48-1264 No FM - RWood - 05-24-2016

Thanks, that helps. I could, for starters, check each component that is listed on the Possible Cause side.
I frankly did not realize that zooming that part of the service guide would give such sharp detail of the layout. I would have used that during the recap!

Another thought, since this is not being done for alignment purposes, could I use the cheap sine-wave/square wave generator from my old CEI trainer/breadboard? (Anyone remember Cleveland Institute of Electronics?) It looks like I need a 9.1mc modulated output. Could I try this just to see if/where signal is passed?

RE: 48-1264 No FM - Radioroslyn - 05-24-2016

Well I think if you just want to just poke around a bit you could use an AM signal signal generator and a tracer hooked to the plate pin of the sec IF amp tube 7W7. Should work fine with an AM signal till you get to the FM det. So either at the FM bc frequency or at the IF.

Terry

RE: 48-1264 No FM - RWood - 05-26-2016

Thanks, Terry.

Well, I used my crude CIE sine wave generator which did give me audible signals when probing the grids of the power tubes, the phase splitter and the audio amp triode.

The radio failed the very first test for the FM circuit, though: no signal when probing the grid of the 7R7 tube (pin 6).
I retested the tube, which is strong on the pentode and good but not particularly strong on the two diodes. It was a new-in-box National Union.

The other possible causes revealed nothing, although my meter won't test the 100pF cap (C323) and I am out of that value so I'll need to order some and replace it. The C325 is new and is oriented correctly. Z302 does ohm out correctly per the SM with no opens. C324 is a new .01uF (not a .008uF) and is connected right. C205 is another 100pF cap on the volume control and I could not test that. R318 has drifted a little high to 118K Ohms but is not open. R319, the 6.8M, I could not test that high.

The last possible cause: 'Z302 misaligned' has me a little nervous and I don't want to make things worse, so I will leave that alone for now, get a couple 100pFs in there, put fresh 100K and 6.8M resistors, and also test the 7X7 which last night I didn'y remember to test. It was a new-in-box Tung Sol.

Does my reasoning sound good so far or is there anything else that comes to mind?

RE: 48-1264 No FM - Radioroslyn - 05-26-2016

As I posted before you will need a RF signal generator capable of emitting an amplitude modulated signal at the FM bc band and at the receivers FM IF frequency.

In order to trace this generated signal you will needs a signal tracer with audio and rf inputs as your Fm detector in your receiver can't demodulate AM. Use the tracer's probe in the RF or Diode setting.

You can start at the Fm ant post at 88mc and trace it though to the plate of the mixer tube. After that you will need to inject a signal at the receiver's IF frequency and trace it though the IF stages.

The caps  you mention aren't very critical they look to be a filter to filter rf out the audio before it going to the amp. Short of being shorted I don't think their your issue. The FM IF transformers can't be open because they are in series with the AM IF and if either was open the set would not work on AM or FM.


Terry

RE: 48-1264 No FM - RWood - 05-26-2016

Okay, since it wasn't a modulated signal, it wasn't picked up by the 7R7. I need to get that Hickok going. Thx