How to eliminate FM RF coming thru turntable?

A corrosion-formed diode can indeed rectify an AM station's signal so as to produce recognizable audio. In my experience what you usually get is a mixture of several stations, some louder than others.

However, it takes a lot more than a diode to pick up recognizable audio from a FM signal.

Hmm, yes, you have a good point there.

But still, if we understand the facts correctly (recognizable audio from fm, being introduced through the phono input, and present (I assume) when the fm tuner is turned off), SOMETHING in the phono path is somehow demodulating the signal. Not sure what that could be.

Regards,
-- Al

Thanks for the suggestions -- will give them a try. Just completed a year long renovation. The equipment was originally in the center of the house and rarely if ever had an RF problem. Now, equipment is housed in a wood cabinet against an exterior wall. A new FM antenna is about 10' from the turntable on the eaves. Disconnected the cable to the antenna and still have RF signal. The same station comes in when the night signal changes. It makes no difference whether the tuner is on or off with the RF. Again, during the day, phono is dead-quiet.

This is just a wild guess, but since it's only happening at night, could it somehow be related to the fact that lights are on (vs. during daytime)? I'm no electrician, but it seems that the difference between night and day might be the clue here. I wonder if the house wiring is somehow acting as an antenna/repeater (from the renovation you mention) is causing the issue, but only when the circuit is closed such as when lights are on.

Again, a wild-assed guess at best but logic dictates ruling it out as a cause.

Are you using a tube or solid state phono stage?

I had similar problem and after trying many great tube phono stages, eliminated the trouble only by switching to ss phono stage. Cheers,
Spencer

Are you really sure that it is an FM station you are hearing? There are two things that seem to point to AM -- the fact that it is being demodulated into understandable audio, as discussed above; and the fact that the problem occurs only at night.

My suspicion would be that the day/night difference would be caused by differing signal strengths before/after sunset. As you probably realize, AM stations typically propagate much further at night than during the day. With FM, I believe that would only be true to a slight extent if at all.

If the other suggestions don't help, you might try this: There are kits that are available (I used to have one) that provide small value capacitors on rca plugs, for the purpose of optimizing the capacitive loading that is applied to the cartridge. Or you can solder up something like that yourself. If you were to add say 50 or 100pf of capacitance (connected at the preamp input with a y-connector), without going outside the range of total capacitance that the cartridge is specified for, the extra capacitance might load down the rf pickup sufficiently to kill it.

Also, you could try to find the kind of ferrite-based rfi filter that has a donut-hole in the center, through which the phono cable would be routed. That might do the trick as well. I think some of them are specifically marketed for audio applications.

Regards,
-- Al