Quad FM4

[Mark Packer]

Hi all,

I hope you can help. I have a Grey Series RCA Quad FM4. It powers up and tunes in but there is a constant hum through the signal cables.

I'm pretty sure I replaced the snubber cap in the PSU some years ago.

What might be causing the hum? Is it an easy fix?

All suggestions gratefull received...

 

[Zombie]

Ground loop or defective elco?

 

[Barrymagrec]

There are several variants of the FM4 but given the age of them now PSU electrolytics would be the first port of call. I think early versions had quite small bridge rectifiers - if one leg of the bridge had gone down that would produce high levels of 50Hz hum.

 

[chartz]

Also look for bad, cracked ground solder joints on the audio side, especially the socket one. Use a magnifying glass.

 

[Mark Packer]

Thanks guys, I’ll check it out for the solder joints on the sockets. I’m pretty sure it’s not a ground loop unless something’s defective. I tried it powered via my 34 and the result was the same, with hum...

 

[chartz]

I had three FM4s with their original PSU caps (1982 to 1989) and they didn't hum before I finally replaced them last year just for peace of mind.
But they can go bad indeed, although 78XX regulators will take a lot.

 

[337alant]

Always worth changing the Rifa X2 cap and battery whilst you are in there
Alan