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Heathkit FM-3 Tuner - Humming and High Voltages

I disconnected the mystery part. As you suspected, nothing happened. I reconnected it.

I disconnected the cap by 6C4 plate and the humming became louder. Again, the humming dwindles (but does't disappear) when I make contact with the potentiometer without a knob.

Somewhere along the line of tests...the radio has disappeared. It was loud and clear.

I swapped all the tubes. The hum persists. No radio...

I checked voltages. All seem good on 6C4. Except pin 6. It's only 0 with the unit turned off. Turned on, the voltages range sweeps with knob from 28v to -69.
 
Not the electrolytic cap, the cap between the output jack and the 6C4. It should shut it up, but if it doesn't, its probably some problem right at the jack itself. Open ground connection most likely but maybe just the cap leads laying right next to some AC wiring.

The electrolytic is power supply filtering at that stage. if its not filtering sufficiently, its going to hum quite a bit. Also will do this whether or not the 6C4 is present or not. Unhooking it will make it hum a lot louder. Maybe it needs a larger cap here, or the cap installed isn't much good. Value won't be terribly critical, 4.7 or 10uf should be just fine so long as its at the same or greater voltage as the original.

pin 6 is the grid, it should be sitting at zero volts. There is a DC blocking cap between the volume control and the grid, and a 1M to ground.
 
Dig22 said:
I checked voltages. All seem good on 6C4. Except pin 6. It's only 0 with the unit turned off. Turned on, the voltages range sweeps with knob from 28v to -69.
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Pin 6 is the grid, it should always be 0 VDC. Is the 1M resistor to ground good? Are the solder joints on the resistor good? Is the bottom of the level pot well-grounded? Are you sure this isn't a measurement error?
 
I unhooked the .05 capacitor from the top of the 47 to ground and the hum was drastically reduced, as was the output. Also, the radio signal used to be strong; now it's weak.
I'll try replacing the 6C4 socket this weekend. One tab is broken in half, another is sheared at base and soldered in.

I've repaired 3 Heath-kit tuners in the last year and they are fine; this FM3 is driving me batty.

I'll report back after socket replacement. Have a great holiday and thanks to all for their input.
 
While I'm replacing the socket, should I replace the .01 ceramic disc? I have .01 2kv on hand; is that a good replacement?
 
There's no 0.01uF at the 6C4 in the schematic. I see two .05uF caps at the output jacks.

Jack
 
TriodeLuvr said:
There's no 0.01uF at the 6C4 in the schematic. I see two .05uF caps at the output jacks.

Jack
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Hi Jack,
Thanks for responding.
From the manual: The .01 ceramic goes from pin 6 on 6C4 to center lug of potentiometer. see attached. Which would be right? The schematic or the manual? They don't always appear to always be in sync.
 

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Dig22 said:
Hi Jack,
Thanks for responding.
From the manual: The .01 ceramic goes from pin 6 on 6C4 to center lug of potentiometer. see attached. Which would be right? The schematic or the manual? They don't always appear to always be in sync.
Click to expand...
My mistake, they do agree. Sorry for the confusion, there's a .01u on the grid, don't know how I missed it. In answer to your question, this only needs a voltage rating of 50V at the most. You can use almost any type in that location.
 
if pulling that stops it, the hum is coming in from the pot. If the other jack is quiet, that means the hum is coming from the pot. Either its bad, or the wiring to it is routed wrong, but its coming from somewhere between the good output jack and the center terminal of the pot. If the pot is not grounded to the chassis it may cause this problem.
 
Hmmm,, this is a Heathkit and you have the manual,,, start from the beginning and trace thru testing as directed along the way,,, If it were mine, I'd start by gutting it and starting from scratch,,, You will get rid of the horrible soldering and learn to read a schem!!!! I gutted and rebuilt a pair of WA-P2s, and they came out fine...
Best of luck...
 
gadget73 said:
if pulling that stops it, the hum is coming in from the pot. If the other jack is quiet, that means the hum is coming from the pot. Either its bad, or the wiring to it is routed wrong, but its coming from somewhere between the good output jack and the center terminal of the pot. If the pot is not grounded to the chassis it may cause this problem.
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I replaced the pot with a new Alpha 1meg switch. The wiring seems to be correct. The 6C4 socket has broken tabs and one is soldered in. ould that be it? I'm replacing socket this week.
 

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TriodeLuvr said:
My mistake, they do agree. Sorry for the confusion, there's a .01u on the grid, don't know how I missed it. In answer to your question, this only needs a voltage rating of 50V at the most. You can use almost any type in that location.
Click to expand...
Hello Again,
I've changed the 6c4 tube socket and fixed a ground connection to the 2 @150V capacitor. It greatly reduced hum, but doesn't eliminate it. There's definitely an issue with pot and pot wiring. I'll replace all. Any recommendations for best potentiometer switch to use in these old Heathkits? I think I installed an audio taper and it seems to be much louder on one channel. Do they make a 1 mg linear taper switch pot? If so should I try it? Thanks again.
 
Dig22 said:
It greatly reduced hum, but doesn't eliminate it.
Click to expand...
Does the hum still decrease when you touch the chassis?
Dig22 said:
I installed an audio taper and it seems to be much louder on one channel.
Click to expand...
What does this mean? There's only one channel, correct?

About the control, a linear taper will probably work better as a level control. Don't know where to suggest, other than the usual sources like AES and eBay. I haven't searched for something like this in years.

Jack
 
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Its mono. If using both outputs, the one after the 6C4 will be louder than the other and the control will adjust only the 6C4 output.

linear v log won't make it louder or not, it just changes how the control responds to movement. All the way up is still all the way up.
 
gadget73 said:
Its mono. If using both outputs, the one after the 6C4 will be louder than the other and the control will adjust only the 6C4 output.

linear v log won't make it louder or not, it just changes how the control responds to movement. All the way up is still all the way up.
Click to expand...
Thanks!
 
TriodeLuvr said:
Does the hum still decrease when you touch the chassis?

What does this mean? There's only one channel, correct?

About the control, a linear taper will probably work better as a level control. Don't know where to suggest, other than the usual sources like AES and eBay. I haven't searched for something like this in years.

Jack
Click to expand...
The hum does not decrease when I touch it. It used to, before I switched out the socket, and fixed grounding on 2 160V capacitor.
 
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