FM on SW

[Gregb]

Looking for suggestions, I am working on a no-name 50's vintage 6 tube radio record player. I have not been able to track down a schematic and it has been recapped sometime in the past. I have recapped it and replaced a couple resistors. The AM works great with plenty of sensitivity and it sounds good. The shortwave works good as well but I am getting lots of FM interference, some stations are coming through almost good enough to listen to. Seems to be the same four or five stations fairly close together and tuned at four or five spots across the dial. I have never seen this before in 30 years of working on these old radios.
Anyone have any ideas?
Thanks

Gregb

[Electronic M]

How close are you to the towers? Within a few miles there may be strong enough carrier harmonic radiation to receive/interfere well.

[Gregb]

I would say I am within 5 miles of a number of stations but this is the only radio I have ever had this happen to and I have seen a lot of radios.

Gregb

[jr_tech]

At what frequencies on your dial do you pick up the FM stations?

jr

[DavGoodlin]

The SW bands seemed to be available on many "home entertainment" sets made up North instead of FM which had limited range. Most of my SW sets only tune as high as 18-22 MC. I cant imagine an image frequency in that range being any sufficient amplitude.

Maybe there is an issue with the detector stage causing it to turn phase modulation into audio

[jr_tech]

When I was a kid, I regularly used a crude 6 tube Hallicrafters S-41G that belonged to my Father. (yes, I still have it, of course). This set has only a 2 gang tuning capacitor, hence had *no* tuned RF stage. Selectivity was poor, but sensitivity was ok, when using a long wire (approx 100 ft) outdoor antenna. It was great fun for a geeky kid!
Video "buzz" and garbled audio from a nearby ch 6, as well as garbled audio from several FM stations could easily be heard when tuning around above 20 mHz or so. I was told that this was caused by "local oscillator harmonics" generated by the radio.
The math seemed to confirm this theory... When tuned to, say 21 mHz, the local oscillator of a set with a 455 kHz IF amplifier would be tuned to 21.455 mHz. The fourth harmonic of 21.455 is 85.82 mHz which is right in the range needed to convert a segment of ch 6 (82-88mHz) to 455 kHz, which could then be amplified by the IF amplifier and heard. By tuning around a bit above this, several FM stations (FM band is 88 to 108mHz) could be heard.

jr

[Electronic M]

Also the IF has a limited bandwidth. If you tune an FM signal such that it varies in frequency along the slope of the IF you will actually convert the signal in the IF system from FM to AM which can then be detected by a standard AM detector stage.

They used to use that trick to make 441line AM sound pre-war TV sets work after 525lline FM sound became the standard in mid 1941.

[Gregb]

I will try tonight to see exactly where on the dial I am tuning them in, I have to reinstall it in the cabinet. The strange thing is the tuning is very sharp but if I am careful I can tune an FM station so good it is not to bad to listen to, a little distortion but not bad.

Gregb

[jr_tech]

Originally Posted by Electronic M

Also the IF has a limited bandwidth. If you tune an FM signal such that it varies in frequency along the slope of the IF you will actually convert the signal in the IF system from FM to AM which can then be detected by a standard AM detector stage.

Yes ! I should have mentioned "slope detection" which worked fairly well on the old S-41G (sorta mushy, but understandable).
Just tried it on a more modern receiver (Icom 8500) and it did not work so well. I found an FM station that did not have IBOC, RBDS, or SCS carriers hanging off the sides of the main FM carrier, switched to "AM mode" and tuned around, using several different bandwidths. Sounded terrible... Possibly the steepness of the "skirts" of the IF response of the 8500 are too steep to allow "slope detection" to work very well.
Another factor may be that modern FM transmitters are quite clean, with very little "AMing" of the FM carrier present.

jr