UK, vintage fm receiver talk sport bleed over

[dr pepper]

I have an old tube fm receiver I'm doing some work on.
I get talk sport alll over the dial, and it gets louder where local stations are.
At first I thought the IF strip was goosed, but looking up frequencies for talk sport they only transmit on AM.
It picks up FM stations and they are just audible in the background, but talksport is loud.
I could imagine a harmonic getting through the front end, but the discriminator wouldnt detect it.
Shorting the Am aerial and oscillator coil doesnt change anything, so its not the Am side bleeding over.
N.B. the transmitter is 40 miles away, however its a powerful one at 500kw.

 

[BradtheRad]

This phenomenon is more common than we'd expect. I've seen a couple pieces of audio equipment which were not radios yet played a strong AM station.

It's easy for a diode (somewhere in the circuitry) to act as an AM detector. Then an amplifier downstream makes the broadcast audible.

 

[dr pepper]

Yes with an audio amp thats can be expected, but on a Fm receiver connecting a piece of wire to the Fm antenna socket?

More investigations, I made a loop of copper 100mm dia and soldered a 5-50pf cap across the ends, connected this to the rx to see what happens, it works, kinda Fm stations tune in sort of where I'd expect but they sound badly over modulated and they appear twice on the dial.
Sounds like enough gain, but somethings up with the front end and the If system.

 

[betwixt]

Or, being AM it might simply be finding its way to a non-linearity in the audio stages. Try a small (~100pF) capacitor across the volume control and see if it makes any difference. If it does, experiment with different positions for the capacitor around the input of the audio amp stage.

Brian.

 

[dr pepper]

100pf across the volume pot doesnt change anything.
However I found something, theres a cap/coil mounted to a plate, this looks like a fm preselector for 88 - 108mhz.
There was no continuity to ground on the plate, rusted screws.
Fixing that removed loud am stations but the distorted over modulated sound persists.
I had this before it was grid emission, but not this time new tubes dont change it, nevertheless this is probably something I'll be able to find.

 

[dr pepper]

The distortion problem was one side of the winding on the ratio detector, it had a short ringing one side got 4 rings and the bad side 0 rings.
Works well now except the oscillator wanders all over, but thats how these sets are for the first hour or so, no afc.