Magnavox 142 chassis, smoking resistors, help!

[Sam Lysinger]

Given my fervor in discovering that 142 amps do indeed have treble when you replace bad HF filter capacitors, I got one unit running like a champ and the second is now making me fight to get it running correctly.

I have attached a picture of the two misbehaving resistors (511), they like to smoke after you get about 90v on the Variac. I have also attached the schematic. These resistors are found between the 6.3v filament winding legs, then connect to the cathode resistor and from there to ground. Before smoking, there is an audible 60hz hum from the speaker. A 60hz hum is audible with the Variac set at 60v (.1vac measured at the output transformer).

I am sure I have miswired something, I did replace one tube socket and initially miswired it, luckily I miswired a filament wire to the unused pin one. I have swapped the driver and power tube (and tested the existing set of tubes for shorts), no change in the hum or the smoke.

I have included pics of the chassis and the ground wiring due to the 60hz hum being audible

All advice and insights are greatly appreciated!

 

[gadget73]

Those shouldn't be carrying any current worth speaking of, just the 6.3 volts that divides across the two. If they are smoking, something is shorted to ground that ought not be. They do connect to the cathode of the output tubes to elevate the heater circuit by a dozen-ish volts DC for noise reduction. If the heater was grounded on one side, the cathode current would flow through those resistors to ground, and I'm betting thats why it smokes.

Chase out your heater circuit and see whats going on. Fair bet you'll find something is grounded to chassis. Check the connections out to the pilot lamp too, those are on the same winding.

 

[cademan]

Make sure the electrolytic can capacitor C1B (triangle noted 10uF) is not shorted.

 

[crispycircuit]

As you replace them make sure the polarity is correct

 

[gadget73]

a short in that cap won't cause any current to flow through those balance resistors. It does need to not be bad though.

 

[cademan]

If it were shorted, 1/2 of the 6.3 v would be shorted to ground. As mentioned earlier, also make sure neither leg is shorted to ground.

 

[gadget73]

The heater circuit has no ground connection anywhere else so there is no current flow. Could short that on purpose and it wouldn't matter. The output tubes would roast but the heaters would be perfectly content.

 

[cademan]

I was thinking through the chassis center tap but you're probably right so its most likely a short in both heater wires together somewhere.

 

[Sam Lysinger]

I finally got back to this disagreeable amplifier and have resolved the issue thanks to everyone's input! First step was to look at all the heater leads and they were fine, I could not measure less than 224 ohms to ground from any filament pin. I pulled all the power tubes and the driver tube and ran the Variac up to 90v and let it sit. No smoking filament lift from ground resistors, but still had the 6-hz hum in the speaker. I pulled the rectifier tubes and still had 60hz hum in the speaker, with no tubes! Weird... I desoldered all the leads in the tube socket I replaced. No change. I then was looking at the heater leads to the first power tube while things were on and shoved a chopstick to get the leads off the chassis, no change. I measured 135mv on the output windings of the audio transformer and 5vac between the plate leads of the output transformer. Weird... Then I was poking with another chopstick (great for finding cold solder connections) and moved one of the plate lead wires of the output transformer and the 60hz went away. Upon magnifying lens inspection, the lead was not re-attached right up to the insulation and the exposed part of the lead was close enough to a heater pin that moving the wire back brought the hum back. I applied some heat and move that wire up so that the insulation was right at the solder lug and have taped the lead down. We are back in the saddle! I suspect that when 300v b+ was to close to the heater pine, there was drama. Thanks again!

 

[cademan]

It really does have to touch. Being close won't cause smoke.

 

[gadget73]

yeah, that'll do it. Probably one strand of wire poking across.

 

[GordonW]

It really does have to touch. Being close won't cause smoke.

Dunno about that. I've had one overly-close pair of output tube terminals (plate and heater) occasionally arc- once it started, it would go on, until the amp was power cycled.

And it might or might not start again immediately, on re-start.

Bent them apart, cleaned off the soot from the arc- no more problems.

Regards,
Gordon.

 

[cademan]

I can see that on the high voltage side, but the low voltage?

 

[gadget73]

One of the HV leads was touching or nearly touching the 6v heater lead, so arcing is a possible.

 

[Sam Lysinger]

yeah, that'll do it. Probably one strand of wire poking across.

It was one strand when I used a magnifying lens to check it. When I cleaned it up, I missed one strand. Those two 100 ohm resistors are toast. They are out at the moment and the amp is running fine. I want to get a few hours of smoke free operation and then I will carefully put new 100ohm resistors in it.

 

[gadget73]

Value there isn't too critical if you don't happen to have any 100 on hand. Anything up to probably 1000 ohms will serve fine, just need 2 of them.

 

[Lavane]

Ya, I've used 220ohm, 390ohm, etc. I have three boxes of resistors. One box for ohm resistors, one for K ohm& M ohm, and another for high wattage resistors. All in small baggies. I'm cheap. I just reach in and grab a bag, if its under around 500 ohm, I call it good for for heaters.