Interesting FM Problem

Needs clarification. "The volume knob doesn't affect it". No, it shouldn't, the balance knob should. "Changing the input selector doesn't help". Is it the preset channel/station? If it really is the input selector, the FM/AM/PHONO/TAPE/CD/AUX, then it clearly is a fault after this selector.

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beslutning at undlade det.

I presume this is a Marantz 2270. That receiver has diodes in the power amp which possibly, through slope detection, could amplify FM radio stations. I used to own an Accuphase integrated amp. where a FM station a mile away used to come through the speakers regardless of input. Chuck

Slope detection counts on an actual slope of a tuned circuit. As the FM signal moves back and forth over that tuned circuit, the slope of the tuned circuit means the signal varies in strength according to the slope of the tuned circuit, which then gives an amplitude signal that can be demodulated.

I'm not sure how that occurs though overload.

I certainly have seen speakers modulated by interference, a friend was running his ssb transmitter next to the stereo, and the speaker wires were acting as antenna, the output transistors were rectifying the RF, and it was all strong enough to drive the speakers directly. Awful sounding, but it happened. But that was a a signal that varied with amplitude, not frequency.

Michael

Even broadcast-FM signals often do contain a significant amount of amplitude variation, by the time they arrive at a receiving antenna. This can occur as a result of multipath: delayed reflections of the signal from trees and buildings and mountains. The primary (direct) and secondary (reflected) signals join at the antenna, reinforcing or cancelling in strength depending on the effective path lengths (e.g. measured in wavelengths).

There's enough wavelength variation within the deviation range of an FM broadcast signal (e.g. +/- 75 kHz) that the reinforcement and cancellation shows a significant amount of frequency-specific change. As a result, the amplitude of the "FM" signal varies somewhat. This amplitude variation can create audible distortion... FM detectors and stereo-multiplex decoders don't have perfect AM rejection.

Some higher-end FM tuners have "multipath" meters or oscilloscope outputs (electrical or visible) which let you see this, and orient your antenna for the least amount of multipath on any given channel.

I suppose it's possible that rectification of a strong FM signal, in an amplifier's output stage, could be detecting enough of this multipath-created amplitude variation to be audible.

The "fix" I'd try would be the same one I'd use for any stereo which was picking up inappropriate radio transmissions through the speaker wiring... I'd try ferrite common-mode and differential-mode chokes on the speaker wires (first) and other power and signal cables (later).

Far as I figure, for it to be multiple FM stations there has to be a source of 200 kHz around somewhere.

I suspect the original story is garbled, someone mis-explaining the conditions. We see that here every so often.

That said, if the synthesizer was badly designed, or out of proper operation, it could put out sidebands every 200KHz (or whatever the steps are), which would put multiple channels into the IF, so long as the front end selectivity was wide enough to pass them.

That used to be a real issue. People would want synthesizers to go with existing transmitters that would multiply up a 6 or 8MHz crystal to

146MHz. So the reference frequency was really very low, and that was hard to filter out of the signal out of the phase detector. So there could be such spurs.

If the steps are 200KHz, that would be harder to be sloppy with, but if a cpacitor or resistor went bad, the synthesizer might not be filtering out that reference frequency, and the sidebands would be there on the local oscillator.

Michael

Your theories sound very plausible. The audio level was so low that, even though at the time I was an audio technician, I never tried various filtering techniques. This station was probably the worst technically operated FM station in the Twin Cities. It was run by a religious group and I presumed that it was low power so I was surprised by the interference. We did have audio bleed from FM stations into mixers that we sold to businesses in downtown Minneapolis. There was a club within a block of the IDS tower, a building with many FM stations' antennae on top, where a contractor installed a mixer that was bought from my company. The FM interference was extremely noticeable so I was brought in to try to solve the problem. I don't remember exactly what steps I took to resolve the issue though it was very time consuming .

but I noticed today that it seems like all the radio stations at once are coming from the left channel. The volume knob doesn't affect it, as it is t he same level no matter where you turn it to. Changing the input selector d oesn't help, and neither does moving the tuner. What is going on? "

** The fact it is only one channel and no controls have any effect indicate s RF signals are entering one of the power amplifies via the speaker lead

- probably travelling up the feedback loop into the input differential pair and being rectified / demodulated

Simple test, does it still happen with no speaker leads connected to the am p while listening via headphones ?

The earth on each speaker output terminal should have a 100nF ( film or cer amic ) cap to the nearest chassis point plus the output zobel may be faulty .

The same channel may also be suffering from parasitic oscillations at a few MHz - which only a scope would reveal.

... Phil

stand if it is AM, but FM ? He says it is about ten sttions all at once, th ough that is probably subjective, it could be "only" five.

in around 100 mHz that have a measely 75 kHz deviation ?

?

AHA. I shall ask.

snipped-for-privacy@gmail.com skrev den 20-11-2014:

Oh, I misunderstood. I read it as "today, whatever radio station I'm listening to, it only comes from the left channel".

Sorry :-)

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beslutning at undlade det.