Interesting FM Problem

[jurb...@gmail.com]

On a forum, I read this :

"I am the proud new owner of a 2270. It seems to be in pretty good shape, 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 the same level no matter where you turn it to. Changing the input selector doesn't help, and neither does moving the tuner. What is going on? "

The thread contiues with some details, these are FM stations. I can understand if it is AM, but FM ? He says it is about ten sttions all at once, though that is probably subjective, it could be "only" five.

What, in a regular audio reciever could demodulate multiple FM staions up in around 100 mHz that have a measely 75 kHz deviation ?

Could this have something to do with this digital stuff in the signal now ?

[Leif Neland]

Følgende er skrevet af jurb...@gmail.com:

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.

--
Husk kørelys bagpå, hvis din bilfabrikant har taget den idiotiske
beslutning at undlade det.

[chuck]

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

[Michael Black]

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

[David Platt]

In article <alpine.LNX.2.02.1...@darkstar.example.org>,

Michael Black <et...@ncf.ca> wrote:

>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.

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).

[jurb...@gmail.com]

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

[Michael Black]

On Thu, 20 Nov 2014, jurb...@gmail.com wrote:

> 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

[chuck]

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 .

[Phil Allison]

jurb...@gmail.com wrote:

> On a forum, I read this :
>
> "I am the proud new owner of a 2270. It seems to be in pretty good shape, 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 the same level no matter where you turn it to. Changing the input selector doesn'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 indicates 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 amp while listening via headphones ?

The earth on each speaker output terminal should have a 100nF ( film or ceramic ) 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

[jurb...@gmail.com]

>"Simple test, does it still happen with no speaker leads connected to the amp >while listening via headphones ?"

AHA. I shall ask.

[Leif Neland]

jurb...@gmail.com skrev den 20-11-2014:
> On a forum, I read this :
>
> "I am the proud new owner of a 2270. It seems to be in pretty good shape, but
> I noticed today that it seems like all the radio stations at once are coming
> from the left channel.

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

Sorry :-)