TV-37 vertical hum?

[Tom Albrecht]

Originally Posted by miniman82

Thanks Tom, this is a series string set so technically AC is everywhere already? In any case this has vacuum rectifiers not diodes, so that’s not it. I thought the 6.3v transformer I put on the CRT filament was magnetically influencing the tube due to where I had it mounted, but the rolling remained after I moved it elsewhere.

It's not the AC voltage that causes the problem, it's the AC current flowing through the wire that generates a circulating magnetic field around the wire. Any of the wires in the series filament string (or the AC line wiring to the switch, etc.) that pass close to an unshielded part of the CRT can cause this kind of problem. Also any of the wires in the rectifier (or voltage doubler if it has one) up to the first filter capacitor can have high AC current on them and can do a similar thing.

[miniman82]

I really don’t think it’s magnetic directly to the CRT, there’s nothing near it currently and the wiring is being shielded by being under the chassis. This is AC getting into the deflection or video circuit that I can see with my scope moving slowly through the waveforms. If I look at the vertical oscillator waveform for example instead of being nice and flat I can see a half cycle of AC distorting it slowly moving from right to left. This corresponds with the waving seen in the picture tube, so I think the AC is getting in at a grid someplace and getting amplified because there’s no way such a small signal would have such a large effect on the geometry of the picture directly. You’re talking about maybe 2.5v of AC against the hundreds of deflection and ultor volts seen by the tube.

[Tom Albrecht]

I agree that if you see the disturbance on the vertical deflection waveform, then direct magnetic influence on the CRT is not the problem. In cases where I have seen magnetic coupling from wiring, there is a cutout in the chassis to accommodate part of the CRT bell which exposes it to the under chassis wiring. I believe the specific case where I saw that problem was a Sentinel 400TV.

Thinking about the chassis and CRT mounting in a TV-37, there should not be any wiring exposed to the CRT.

If you’ve checked the power supplies and found no ripple that should be causing a problem, let me bring up one other semi-obscure problem that can cause this: The TV-37 has an aluminum chassis, and over time the riveted ground connections develop some resistance. Sometimes you need to hardwire a soldered ground wire between most or all of the ground connections on the chassis to fix the problem. In particular make sure the filter cap grounds are hardwired to this new ground wiring. The problems you can get from these bad ground connections are a little hard to predict, but AC hum getting into things is certainly a possibility.

I know you’re quite experienced, so that’s why I’m bringing up these less obvious causes of this kind of problem.