VT-71 'rolling band'

[miniman82]

So I decided to take a break from the CTC-4 to give the VT-71 more love. It's always had sync issues, and there could be a little more width. At first I thought there was something wrong with AGC, but it turns out to be doing what it's supposed to. Then as I was scoping waveforms, I noticed that when it would lose sync the waveform would grow slightly. Looking a little closer, there's a damn band rolling through the sync waveform and when it reaches the sync tops that's when the picture tumbles. It looks like a positive half sine wave to me, but a small one.

Power supply is the most likely culprit for a rolling band, but the power supply looks clean as a whistle to me. So is there anything else that can cause this issue in these chassis, like a signal beating or something? It only seems to be affecting the horizontal section, vert is solid.

[Tom Albrecht]

Have you replaced the selenium rectifiers with diodes? If so, that sometimes results in some 60 Hz (actually very high harmonics of 60 Hz) noise that is hard to get rid of.

In one set where I saw this exact problem, putting a 0.1 uF capacitor across each power supply diode did the trick. This functions as a snubber to reduce the rapid and harsh switching in a modern silicon diode compared to the original selenium rectifier.

You could also try bypassing each electrolytic in turn with a 0.1 uF cap to help quiet down 60 Hz spike noise.

[bandersen]

I recall in mine there were several 250, 500 and 1000 pF micamold caps between some of the tube filaments and ground. Those are really paper caps in disguise and I replaced them all with micas. Perhaps if those are bad they could be the culprit ?

[jr_tech]

Possibly Heater cathode leakage in one of the sync or sweep tubes that is adding a little 60 Hz signal to to the cathode circuit?
jr

[miniman82]

You might be on to something, Bob. I broke my own rule, because I left those filament caps in place! I figured they were in a low enough voltage circuit not to matter, but it makes sense to have leakage at that point. I'll try to find enough caps to replace them and see if that solves the problem. JR, I don't think I have a tube issue. I tested all of them when I first got this, and the only one that was bad was a 12SN7 in the vertical section which Bob was nice enough to give me a replacement for.

Tom: I have plenty of .1's, so I'll give that a try.

[Reece]

Also think about H-K leakage, might exist in a tube which tests OK and works OK otherwise.

[kx250rider]

Those are usually easy to find, and are almost always a leaky electrolytic. Or a leaky selenium could cause it, but more likely, a leaky selenium would have shorted out or passed enough AC to overheat the electrolytics and blow.

If it's causing the vertical to flip, I'd look at the main B+ supply, and see if it has AC riding on it. 99% sure you have a bad filter. OR, the set is getting a hot loop through the antenna line...

NOTE: Hot chassis; use an isolation transformer when poking for AC in the B+. You might read AC falsely if the scope or VTVM is plugged in opposite polarity to the TV, as well as posing the obvious risk of shock or damage.

Charles

[miniman82]

It's not the power supply, as far as I can tell it's perfectly clean as seen by the trace on the scope. Yes, I have an isolation xformer. No selenium, new caps.

I think the caps on the heater circuit will fix most of the banding, but what to do with the sync clipper being overdriven? I need to lose like 10 volts to the first grid, can I use a smaller C-44 to accomplish this?

[Username1]

I'll add my vote for H-K leakage too. I had one ona toshiba once caused over volt on tube heaters, then caused a white band to run through the screen almost like a ac hum rolls through the picture.