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Heathkit FM-3 Tuner - Humming and High Voltages

Dig22

Dig22

Active Member
Hi,
I recapped Heathkit FM-3. Photos and schematic are below.

It sounds great, but there's a bad humming coming off the output next to the antenna terminals. The other output is clean. I haven't spotted any broken connections but most resistors were way over value so I swapped them. I haven't changed the tubes yet because the voltages are high and I don't want to damage any good tubes. I'm not sure whether or not that should be a real consideration.

On the Tube A, 6X4, Pin 1 was 255, 25v over the higher end of the range. Pin 5 was 238V, 28v over. Pin 6 was 255v, 25V over. Pin 7, 276, 26V over.

And on the 20uf 350V capacitors, one was spot on, one was 37V over.

So where else can I look? Is it a transformer issue? A grounding issue? A bad tube? Have I missed something really obvious? Lastly, the board looks filthy. I'll clean it when I've resolved the issues.

Thanks in advance.Slide1.jpegSlide2.jpegSlide3.jpegSlide4.jpegSlide5.jpegSlide6.jpegSlide7.jpegSlide8.jpeg
 
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Is the one that hums coming from the 6C4 or the top of the volume pot?

Can you measure the heater voltage? If its over 6.3 volts, thats probably why the rest of the voltages are high. If you have a variac, use that and dial it down until the heaters are at 6.3 vac and see if that brings the rest of it back where it belongs. Its often higher line voltage causing the issue. Handily on this you can cheat and just fit a couple ohm resistor between the power switch and the transformer. Or between that fuse holder that I know you're going to add and the power switch. Not sure how many ohms, maybe 10 at 10 watts? Gonna depend on how much needs to drop and what the current draw is.
 
gadget73 said:
Is the one that hums coming from the 6C4 or the top of the volume pot?

Can you measure the heater voltage? If its over 6.3 volts, thats probably why the rest of the voltages are high. If you have a variac, use that and dial it down until the heaters are at 6.3 vac and see if that brings the rest of it back where it belongs. It's often higher line voltage causing the issue. Handily on this you can cheat and just fit a couple ohm resistor between the power switch and the transformer. Or between that fuse holder that I know you're going to add and the power switch. Not sure how many ohms, maybe 10 at 10 watts? Gonna depend on how much needs to drop and what the current draw is.
Click to expand...
Yes. it's the output off the 6C4.

I have a Variac and with the tiniest drop, the heaters lowered to 6.3V.
The voltage on 20uf 350v cap lowered to 260V (about a 13V drop)

On the 6X4:
pin 1: 252v / pin 2: 2.5v / pin 3: -.3v / pin 4: 6.3v / pin 5: 225v / pin 6: 240v / pin 7: 260v

The tubes are in bad shape. Is it safe to try my newer tubes without damaging them? And you think the hum will disappear with voltage drop?
 
Dig22 said:
Hi,
I recapped Heathkit FM-3. Photos and schematic are below.

It sounds great, but there's a bad humming coming off the output next to the antenna terminals. The other output is clean. I haven't spotted any broken connections but most resistors were way over value so I swapped them. I haven't changed the tubes yet because the voltages are high and I don't want to damage any good tubes. I'm not sure whether or not that should be a real consideration.

On the Tube A, 6X4, Pin 1 was 255, 25v over the higher end of the range. Pin 5 was 238V, 28v over. Pin 6 was 255v, 25V over. Pin 7, 276, 26V over.

And on the 20uf 350V capacitors, one was spot on, one was 37V over.

So where else can I look? Is it a transformer issue? A grounding issue? A bad tube? Have I missed something really obvious? Lastly, the board looks filthy. I'll clean it when I've resolved the issues.

Thanks in advance.View attachment 2960158View attachment 2960159View attachment 2960160View attachment 2960161View attachment 2960162View attachment 2960164View attachment 2960165View attachment 2960166
Click to expand...
Was it humming before the capacitor changes ? And which capacitor have been replaced ?
 
Does the output of the 6C4 hum if the level pot is at minimum (wiper at ground)?

Jack
 
I did
peterh said:
Was it humming before the capacitor changes ? And which capacitor have been replaced ?
Click to expand...
I replaced the three electrolytic capacitors: 1) I replaced the cap can with 20uf (x 2 ) 350v with 2 separate 22 350's. 2) I replaced the 10 25V with a 10 50V. 3) I replaced the 2 160v with 250v (the closest thing I could find at local shop)
I didn't plug in before hand because I've had paper capacitors smoke and nearly catch fire.
 
Last edited:
It might be a grounding problem or a poor connection around the 6C4. Some of the solder joints look terrible.

Screen Shot 2023-08-16 at 4.47.01 PM.png

Jack
 
also make sure the body of the pot itself is well grounded to the chassis. wouldn't be a bad idea to ohm check the volume pot to see if its good too.

if its quiet before the 6C4 and hums after, its a pretty small area of components that must be causing it.
 
Hello Again.
My FM3 is still humming. I've swapped all the out of spec resistors. Changed suspect wiring that had chassis corrosion on casing. Swapped out the potentiometer. Changed the electrolytic caps. Tightened grounding screws. Changed the wire to potentiometer switch and AC power cord. Added a fuse. What else is there left to check? I must be missing something.

I've try new tubes as well. One of the 20 350V electrolytic caps is still 30 volts higher than spec.

Lastly, I've attached a photo of a part on the chassis connected to the 6Bq7. What is it? I don't see the part listed in manual. or in the layout drawing or schematic. It's connected to ground. There's continuity through both tabs.

Thanks again.
 

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  • FM3_IMG_0164.jpg
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looks like its on pin 5? Probably a choke of some sort, meant to keep RF noise out of the heater supply?

dunno, if its not humming before the 6C4 but hums after there just aren't many things. Does it stop if you ground pin 6 of the 6C4 ?
 
gadget73 said:
looks like its on pin 5? Probably a choke of some sort, meant to keep RF noise out of the heater supply?

dunno, if its not humming before the 6C4 but hums after there just aren't many things. Does it stop if you ground pin 6 of the 6C4 ?
Click to expand...
Yes, the mystery part is on pin 5.

It doesn't stop if I ground pin 6 on 6c4, though it dampens a bit if I touch the casing on the wire (something I don't normally do).

I don't know how to check if the humming is before the 6C4.
 
I'm pretty sure that's a capacitor, the type I remember seeing in military gear from WWII. Looks like someone was trying to bypass the filament line. It measures shorted due to the filaments and transformer winding to ground. Again, the solder on all those joints looks terrible. You need to clean this up before proceeding.
Dig22 said:
When I turn the knob all the way right, the hum almost disappears
Click to expand...
Have you replaced the 6C4?

Jack
 
TriodeLuvr said:
I'm pretty sure that's a capacitor, the type I remember seeing in military gear from WWII. Looks like someone was trying to bypass the filament line. It measures shorted due to the filaments and transformer winding to ground. Again, the solder on all those joints looks terrible. You need to clean this up before proceeding.

Have you replaced the 6C4?

Jack
Click to expand...
Hi Jack,
I've tried replacing tubes and it doesn't effect the hum.

I don't see the mystery part in Heath-kit manual layout or schematic (though I'm not great at reading schematics). Should I try removing the part? It's a ground to pin 5 on the 6BQ7, which grounds three separate lines to various places. The schematic and layout are posted earlier in this chain.

I'll trying cleaning up solder joints this evening.
 
Dig22 said:
It's a ground to pin 5 on the 6BQ7
Click to expand...
The schematic shows pin 4 is ground and pin 5 (where the capacitor is located) is filament AC. If the capacitor was actually shorted, the entire unit would be dead. I don't know why this part was added, but it's extremely unlikely to be contributing to the hum.

Does the hum disappear if you remove the 6C4? Also, maybe try adding a larger cap across the 2uF temporarily to see if that has any effect.
 
The other output is upstream of the 6c4. If no hum there, it's at the 6c4 stage


Leave the whatsit at the 6bq7
 
TriodeLuvr said:
The schematic shows pin 4 is ground and pin 5 (where the capacitor is located) is filament AC. If the capacitor was actually shorted, the entire unit would be dead. I don't know why this part was added, but it's extremely unlikely to be contributing to the hum.

Does the hum disappear if you remove the 6C4? Also, maybe try adding a larger cap across the 2uF temporarily to see if that has any effect.
Click to expand...
Still hums when 6C4 is removed. Pin 4 on 6BQ7 is grounded. I've retested the mystery part on 6BQ7. It connects at pin 5 which has continuity with chassis and pin 4. Any harm if I disconnect the mystery part from pin 5?
 
probably not, but I very strongly doubt it has anything to do with the hum.


so 6C4 out and it hums. Is the cap from the top of the 47k to ground good? Does unhooking the 6C4 end of the cap between the 6C4 plate and the output jack stop it?
 
gadget73 said:
probably not, but I very strongly doubt it has anything to do with the hum.


so 6C4 out and it hums. Is the cap from the top of the 47k to ground good? Does unhooking the 6C4 end of the cap between the 6C4 plate and the output jack stop it?
Click to expand...
 
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