TV-37 vertical hum?

[miniman82]

This is the second set I’ve had do this, last one was a VT-71 and I never figured out what caused it so I sold the set. Wanting to avoid Déjà vu here…

Has good sound and picture, but there is 60hz crawling vertically up the screen. This causes the image to be slowly distorted, like a barrel is being rolled behind the screen. I swapped all the tubes around from another set thinking a H-K short was to blame but no dice, I even powered the CRT filament from a DC bench supply, still does it. Positive and negative supplies have only .1v ripple, so I have a hard time blaming those. I need a hint, I’m losing it here lol.

[Tom Albrecht]

A couple of cases I've seen over the years:

1. If a wire with high current AC, such as a filament supply wire, runs close to the neck or funnel of the tube, this can happen. The magnetic field from the wire is enough to cause this kind of problem. So check wire routing of AC line and filament supply wires and keep them away from any unshielded part of the CRT.

2. If you replace a selenium rectifier with a diode, there can be crossover current spikes when the diode switches. These can get into various things and cause a small "tiddle" on the picture that travels slowly up the screen. From your description, I don't think this is it, since you describe yours more like a rolling barrel distortion. In any case, when you see this problem, adding snubber caps across the diodes often helps.

[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.

[Username1]

.

I agree with lead dress, and will add check outside foil on the paper caps you replaced.

.

[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.

[timmy]

Track back from there recheck everything maybe a defective cap
What about the mica dominoes

[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.

[miniman82]

There is ripple in both power supplies, it’s unavoidable with half wave rectification. Strange thing is it checks out exactly like another restoration I looked at so I didn’t think it was a problem, see the below link.

https://www.radiomuseum.org/forum/pi...l_details.html

But obviously if it’s being coupled into circuits and amplified it’s an issue, I’m just mystified how his works and mine has this problem with hum.

Timmy- it’s recapped.

[timmy]

Originally Posted by miniman82

There is ripple in both power supplies, it’s unavoidable with half wave rectification. Strange thing is it checks out exactly like another restoration I looked at so I didn’t think it was a problem, see the below link.

https://www.radiomuseum.org/forum/pi...l_details.html

But obviously if it’s being coupled into circuits and amplified it’s an issue, I’m just mystified how his works and mine has this problem with hum.

Timmy- it’s recapped.

I already figured it was recapped already but new caps can be bad as it has happened to me.

[Tom Albrecht]

These work very well and do not have problems with hum in the video. I’ve restored at least three of them over the years. Check your chassis grounds.

[miniman82]

Went through all ground points in DC and AC looking for drops, couldn’t find anything. Tried 3 times to attach a scope shot, but apparently the forum changed what it deemed acceptable and now I can’t figure it out.

[timmy]

Originally Posted by miniman82

Went through all ground points in DC and AC looking for drops, couldn’t find anything. Tried 3 times to attach a scope shot, but apparently the forum changed what it deemed acceptable and now I can’t figure it out.

What about a whisker heater cathode short in the crt. Put a diode across the 25z6

[miniman82]

Swapped CRT with a 3KP1, same result. I went as far as powering it on its own pure DC regulated supply, still did it. I feel like either the negative supply simply has too much ripple (unlikely given that another site showed exactly the same power supply conditions and his works fine), or that the vertical amplifier is being upset by the ripple when it shouldn’t be.

[timmy]

If you powered on dc no grid ac then it’s coming from an ac source ac oscillations in the vertical or someplace else