Cossor 524 rustling noise on FM

[PaulR]

Only on FM and only when a signal is applied. It varies with signal strength

[PJL]

I meant the 12V DC on the pot as this is bad.

[ms660]

Looking at the schematic there should be a decent DC voltage across the pot track when tuned to an FM signal and less so when the receiver is switched to AM and tuned to a signal. Is C48 (Trader sheet ref.) ok ? As that would form a DC path from the pot slider to ground via R27 if it was leaking and could in theory result in a noisy pot depending on how leaky it was.

Lawrence.

[PYE 405]

With the "Radio" link in place, there will be a DC voltage at the hot end as it is directly coupled to the FM/AM audio output stages via the selector switch. Removing the link for "Gram", then there should be no DC voltage.
If the link is in place for "Radio" when applying an AF input to the pot, the DC may well be removed if the output impedance of the AF source is low. In other words, it will short it out. Thus, it will stop the rustling too if it is appearing before this connection.

Crossed posts with Lawrence above, but agree with the noisy pot possibility. Certainly as the DC increased with signal strength, it will become more pronounced if the pot was faulty.

[PJL]

On AM it should be slightly -ve.

[frsimen]

Could this be leakage from the HT on the valve holder? Such leakage will cause the loudest noises with the volume control near the mid setting. At higher and lower settings, the impedance seen by AC component of the noise at its lowest, so the noise will be reduced.

Try cleaning the EABC80's valve holder with a solvent to remove anything which may be causing leakage. If that doesn't help, try a different EABC80.

Paula

[PaulR]

Originally Posted by PJL

On AM it should be slightly -ve.

Yes it is.

I have just tacked in a 0.1uf cap between the feed and the hot end of the volume pot and the rustling has gone. I had already replaced C48.

I will check the suggestions tomorrow regarding the EABC 80 and its holder, but if all else fails I can always leave the capacitor in place. It seems that the pot may be dirty, not enough to affect the normal operation but sufficient to react to the voltage across it. If so I will leave well alone and make the capacitor permanent.

Many thanks to all for the suggestions and discussion. It is so helpful for me with limited experience to be able to talk the problems through.

Paul

[PYE 405]

If you decide to leave the capacitor in place, you might need to substitute the DC resistive path presented by the volume pot. In other words, fit a 470k resistor on the DC side of the cap to ground.
This will maintain the correct DC loading for the FM/AM audio output stages.

[PaulR]

Ok, thank you for that

[Silicon]

The attenuation of the rustling noise at the volume control may be due to the shunting effect of the low output impedance of the signal generator.

[Station X]

One post moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=198683