home stereo hum on FM only

[bxd20]

Hello.

Was just given an old all-in-one home stereo. Zenith Digilite, the one with knobs not sliders. It's one of those all in one units that had a turntable on top, 8 track player built in, AM/FM/FM stereo, AFC toggle. I am guessing its manufacture between 1970-1975, forgot to check date on it.

Here's the only problem. Only on FM, it hums (well, admittedly I didn't test AM thoroughly). It does not hum on tape mode. It has inputs on the back, I connected a modern MP3 player and it sounded great. I have tried connecting an external antenna to the FM ant screws and that helped pull in more stations but made no difference in the hum, so I don't think its antenna related. And it doesn't seem power supply related, if tape and input modes don't hum, correct?

Your help is really appreciated - for how old it is and how small the speakers are, this thing sounds great!

Brian

 

[Resqueline]

It could be just a dried out electrolytic capacitor, in the radio section power supply (power amp may be separate). Did you try to search for a diagram?

 

[bxd20]

No I haven't searched. Sorry for the newb question, but when you say search do you mean on these forums, or on the internet, or looking for schematics at my library??? What's the best way to find the diagram?

My areas have tended to be PCs, radio control cars, fixing vacuums and washing machines..... radios or TVs are a new world to me.

Thanks
Brian

 

[davenn]

No I haven't searched. Sorry for the newb question, but when you say search do you mean on these forums, or on the internet, or looking for schematics at my library??? What's the best way to find the diagram?

My areas have tended to be PCs, radio control cars, fixing vacuums and washing machines..... radios or TVs are a new world to me.

Thanks
Brian

hi Brian,
Resqueline meant search the net. You may be suprised what a google search comes up with
It may also be worthwhile the talk to some in the trade in your area, they may have old service manuals around their workshops.

but before that see if the hum is present on AM mode as well on the receiver. Then you can almost garantee that the prob is with some filter caps in the receiver section
They dry out with age and or heat and what was once , say, a 100uF cap may now only be a 5uF cap

cheers
Dave

 

[shrtrnd]

Resqueline and daveen have you on the right track.
One other thing to look for, is something physically across your FM antenna input
terminals. The hum you're hearing definately sounds like the AC hum from the input
power. But I had a guy once, that somehow ran his power cord over the wires for the
radio input, and the AC hum from the power cord, was induced into the signal.
Any power wires somehow make their way across your FM input antenna wires?

 

[bxd20]

I confirmed this weekend that AM does not hum, only FM Stereo. the FM setting doesn't work, at all.

The design of the antenna screws is such that the AC cord was apparently MEANT to server as a crude antenna, it has a little wing you screw onto antenna input.

But I did disconnect that, and connected my little fm antenna instead, and the hum hasn't changed.

The hum is directly proportional to the volume. Crank the volume and you crank the hum.

But not on AM or using inputs - just on FM stereo.

 

[Resqueline]

Well, then the FM part is bust..
In the absense of a diagram perhaps you could shoot a good picture of the radio part, if you're up to measuring & changing components that is?

 

[bxd20]

I plan on hunting for a diagram -- just extremely busy, will probably take a week to get to this.... will try a picture if I can't get a diagram.

Thanks for everyone's help!

 

[shrtrnd]

Like Resqueline so eloquently put it, the problem is in your FM circuit.
I'm betting on an old electrolytic cap that dried-up, and will need to be found & replaced.