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Help on 48-482 FM - Printable Version

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Help on 48-482 FM - jerryhawthorne - 11-12-2008

I'm working on the above and have finally got the FM working well after alignment of the IF. I went to adjust the FM detector xformer and in the first step, I got no response from adjusting the trimmer. I used my scope/counter and looked at the detector oscillator and it was running at 10.4 MC and the trimmer did nothing to change the frequency. I measured the resistance between the yellow and white wire coming out of the oscillator section of the xformer and got about 6 ohms. I am not the brightest bulb when it comes to electronics but it appears from the schematic that should read infinite due to caps in the can. Am I wrong? Tuning in an FM station and turning either the trimmer or adjusting the coil has no affect on the signal reception. I don't see how it can work with the IF frequency running 9.1 MC and the detector oscillator running 10.4 MC. Can anyone out there shed a little light on this?
Jerry

Re: Help on 48-482 FM - codefox1 - 11-14-2008

Have you cleaned those adjustable caps? There might be enough crud to short them out, or the mica may be split or "diseased" I usually open 'em up all the way, and spray de-oxit, wait a few hours until the solvent dries and then retune. You might have to take the can off the transformer to do a thorough cleaning job.

Re: Help on 48-482 FM - jerryhawthorne - 11-14-2008

This particular can has a single trimmer and an adjustable coil. The trimmer is not the normal type I am use to seeing. It is not the compression type with a mica insulator. It appears to me to be more like a wiper, perhaps a single turn? I have had the can off. I wish someone could take a look at the schematic and let me know if I am wrong in that the two wires coming out of the can from the oscillator side should read very low or infinite in circuit. It appears to me they should read infinite not just a few ohms as they do. Sorry, I don't know how to post the schematic from N/A on the forum.
Thanks for any help!
Jerry

Re: Help on 48-482 FM - jerryhawthorne - 11-15-2008

Well, kind of hoping someone out there knows something about the FM1000 circuitry. Am I correct in my assumption that the trimmer on the detector can should adjust the oscillator frequency of the can to the same frequency as the IF (9.1MC) and the FM1000 combines the two and selects the null heterodyne to remove the IF carrier leaving the FM "signal"? I am not too bright at this but trying to understand. As indicated above, adjusting per the proceedure does not provide a zero beat. Adjusting does not change the detector oscillator frequency which remains at 10.4 MC. Need some major education here!
Jerry

Re: Help on 48-482 FM - Ron Ramirez - 11-16-2008

Some info on the FM1000 circuitry:

http://www.philcorepairbench.com/tube/fm1000.htm

http://www.antiqueradios.com/forums/viewtopic.php?t=34479&sid=782135d163f07e69503a2bf1f9274ac9

http://www.antiqueradios.com/forums/viewtopic.php?t=95266&start=0&postdays=0&postorder=asc&highlight=&sid=9b4d323b57779af4fac27df4907cc80d

http://www.audiophool.cjb.net/Misc/philco_48-482.djvu (requires a DJVU reader)

http://www.antiqueradios.com/forums/viewtopic.php?t=67688&sid=e742f330ac3c106c2529b58a749472fb

Terry Judkins, where are you??? Your expertise is needed here...

Re: Help on 48-482 FM - jerryhawthorne - 11-16-2008

Thanks Ron, the links were quite helpful. I did learn a lot but a lot of it was over my head. Particularly with respect to the oscillator in the FM detector can. Should this run at 9.1 Mhz? Should it be adjustable via the trimmer in the circuit? Again, mine is running at 10.4 and the trimmer does nothing. I do receive FM stations but the sound quality could be better. Is it safe to say the FM 1000 tube is OK (I have no tester) if I am receiving FM? I do have excessive hum on the FM band only. Everything has been recaped.
Jerry

Re: Help on 48-482 FM - Terry Judkins - 11-16-2008

Going from memory but here goes.

The IF frequency is 9.1 mHz and the FM1000 detector oscillator nominal frequency is the same. I believe that the procedure has you align the IF using a signal generator set at 9.1 mHz then align the detector. If your radio is receiving stations then your IF must be set high at 10.4 mHz. The detector will demodulate the signal however distorted when maladjusted. The cap should change the frequency of the oscillator. It is very 'touchy' though. When adjusted properly the sound is comparable to any ratio detector or discriminator. Try getting the IF aligned to the correct frequency then the detector may work as designed. The cap may be at the limit of adjustment.

My first 48-482 ( I have restored three.) had a broken core in the FM1000 transformer. I removed the transformer and replaced the core with one from a TV IF. I had no trouble aligning the detector after reassembling the transformer.

All paper caps in the set should be replaced. In all three radios there were many leaky caps. Care should be taken to place the new caps and leads as near as possible to the original locations. Stray coupling can cause many and sundry problems that are very difficult to find.

I assume that you have read my posts on 'the other forum' re other pitfalls so will not repeat here.

Re: Help on 48-482 FM - jerryhawthorne - 11-17-2008

Thanks so much for your response Terry. Today I went through the IF alignment by the book, using the load each step of the way. It was peaking prior to alignment at around 9.2. It is now peaking at 9.1. The local oscillator is running at about 9.15. I get stations somewhat distorted. I hook my counter/scope to the detector oscillator and without a signal in, it is steady and clean at 10.4. With a strong signal, I can see some "modulation" in the detector oscillator signal. The adjustable detector coupling coil seems happiest all the way out (perhaps because of the high frequency), but doesn't peak. My questions are:
1. With no signal in (off station) shouldn't the detector oscillator run at 9.1 and be adjustable in frequency by the trimmer cap? My cap has no change on the frequency of the oscillator.
2. The cap does not appear to be a standard multi-turn compression type of trimmer. Mine seems to turn hard but doesn't have stops. Is this normal?
I'm beginning to believe that the oscillator is my problem and perhaps the trimmer is at fault. Should I be looking for another detector transformer or trimmer? Even if the slug in the coupling coil is broken or the coil is misaligned I would think I should still be able to adjust the oscillator section, am I wrong?
Yes, all caps have been replaced, talk about a bear. I was also careful about lead dress and location.
Your help is highly appreciated, I am new at this with no formal education in electronics.
Jerry

Re: Help on 48-482 FM - Terry Judkins - 11-17-2008

Yes, as I recall part of the adjustment procedure is to zero beat the detector oscillator with the 9.1 mHz If signal. Apparently your cap is kaput. I really do not remember what the cap looks like and don't relish taking one of my working sets transformer out to see. Icon_smile Maybe you can find a suitable replacement. I have some small 20-120 pf caps about the size of a pencil eraser that I got from Ebay. i suspect though that the suspect cap is more like 5-30 or so. You might find one at Mouser. As proof that the cap is bad you could parallel a small value silver mica cap or even a 'gimmik' (two insulated wires twisted together).

Re: Help on 48-482 FM - jerryhawthorne - 11-18-2008

Thanks again Terry. Today I took the cover off the FM detector can and fired it up. With the cover off it did change the frequency slightly. I put a 20pf cap across the trimmer and voila, shifted the frequency of the oscillator from 10.6 to 8.5 pf. Looks like I need to post to see if I can find a replacement used xformer (philco pn 32-4004) or maybe a defective one with a good trimmer. Unfortunately the schematic does not give a value for trimmer. At least I feel I am making progress. Still amazes me that the set receives FM fairly well. Thanks for your help.
Jerry