Fisher 400 hum when listening to FM

My 400 has recently developed a hum when I'm listening to FM. It's not present when I have it set to AUX. The receiver has been partially restored with the Metalbone kit. I tried swapping out the 6HA5 tube, but the hum's still there. I would appreciate any thoughts on the matter as I am not technically inclined.

Hum level change with volume changes?
Happen when tuned in on stations?
Any changes to the tuning tube?

The hum is present when tuned into a station. It gets louder as volume increases, gets louder when I turn on the bass boost. I haven't noticed anything about the tuning tube. I'll check it out next time I listen to the radio. Thanks for replying.

The tuning tube, I'm guessing, is right behind the faceplate? I see nothing odd happening there, no changes.

Schuh -- One critical clue is if the hum is there when the selector switch is set to mono FM. In that mode, the multiplex circuits (that decode the stereo information) are completely removed from the audio path. If there is no hum in that mode, then service to the multiplex section is indicated. If the hum is still there, then it is originating in the FM tuner circuits. If that is the case, have the tubes carefully tested -- particularly for any heater/cathode shorts. Tubes in the tuner section with this condition would still allow the FM section to "work", but can induce an audible hum in the process.

When you test the tubes, note the socket they came from so that you can replace them in exactly the same socket they came from. This will help preserve the alignment of the tuner section.

There are other conditions that can cause FM tuner hum, but tubes with heater cathode shorts is one of the most common.

Good luck with it!

Dave

Also if the tubes have shields make sure they are on tight. If they are missing replace them.

Thanks for all the help. It does hum when in mono. I'll look into getting those tubes checked out.

You can mark the tube with it's position using a sharpie to no ill effect.

audiodon said:

You can mark the tube with it's position using a sharpie to no ill effect.

Click to expand...

I use a Wite-out pen. This is a ball point pen version of the old Wite-out liquid, available at any office supply. The white ink won't burn or rub off. Absolutely the best tube marker I have found.

I wonder if there isn't an H-K short in one of the tubes, probably in the detector area. Does the signal bypass the multiplex circuit entirely when in mono mode, or does it simply switch in some sort of filter that cuts the 19kc pilot off? If it doesn't bypass the multiplex circuit, then a failure in either of those tubes could also make it hum.

Which tube positions do mean, V1 and V2?

I have swapped out the 6au6 and 6ha5 tubes in the tuner section, and it still hums. I also hooked up a TT to the phono section, and I am getting the same hum there. There is, however, no hum when using the AUX input. Any suggestions?

It's possible that the phono is humming for a different reason...they may not be connected.

Do you have all of the metal lids in place (especially the bottom plate)? I've had amps that hum like crazy until all of the covers are on to shield components.

The fact that it doesn't hum with aux at least tells you that the stuff after the input selector switch is all ok.

All the shields are in place. I did notice that when I touched the 6ha5 tube a kind of hissy static occurred. This happened with both of the 6ha5 tubes that I used.