Portacolor got the wiggles

[AlanInSitges]

This is an HE chassis that's exhibiting behavior I have never seen before and I'm not even sure where to go about starting to look. Video

The lower-left corner of the screen is completely stable. As you move further to the right or up toward the top, it starts getting jittery, kinda, in that it's the height and width of the picture that vary, not the position. In the upper right-hand corner it's very pronounced.

So far I've determined that B+ voltages are correct and there's no variation of them. If it were only happening on either H or V I might at least start looking at voltages in those sections but since it's affecting both, and by the same amount, I think there is a risk of going down a rabbit hole and chasing the wrong things.

Another bit of information that's probably important: This is a US-market set, NTSC, designed for 120V 60Hz. I'm feeding it a standard NTSC-M signal from a Fluke PM5515 signal generator, and have tried with the output from a HLLY transmitter set for NTSC-M and being fed an NTSC signal from a DVD player, and the behavior is identical. So I don't think this is signal-related. The set is running on 120V, 50Hz AC. I've tried both a variac and a normal autoformer to step down my domestic 230V 650Hz current.

One thought I had was that there is some interaction between the 60Hz vertical scan frequency and the 50Hz power, but the jiggle is around 2-3Hz, so it wouldn't be an artefact of the two beating together or the difference, and it's affecting the horizontal too, so 🤷. I also haven't found any evidence of other 60Hz sets having problems like this on 50Hz current, looking around the various fora.

Can anyone shed some light on what's going on here?

[user181]

If your mains supply is 60 Hz, why are you trying to give the set 50 Hz? Is that all you have available in terms of your power conversion equipment?

You should use 120VAC, 60 Hz and see how the set behaves.

[AlanInSitges]

That was a typo, it's been corrected. I have 50Hz current, no option for anything else.

[old_tv_nut]

More clues please?

Does this set have a transformer? Is it sitting near a 50 Hz appliance? With a color picture, is the purity similarly disturbed? Does moving/turning the set make a difference?

[nasadowsk]

Check that you're not overloading the poor thing with signal (turn down the signal gen or the AGC on the set.)

Also, being a GE, the filter on the power supply is probably marginal...

[Electronic M]

50Hz linear power supplies need larger filter and doubler caps than 60Hz supplies do.

There are options for getting 60Hz in 50Hz countries...Perhaps the most straight forward is to buy a US market automotive power inverter (12VDC to 120V 60Hz) off the internet and locally buy a beefy 12V supply....Another option if you know people that work in industrial electronics is to get an old used VFD (variable frequency motor drive) and set its output to 60Hz.
Small gasoline generators and dynamotors are also options but probably harder to obtain.

[zeno]

GE sets of this era used to get bad doubler caps. It usually gets extreme
but yours may be just starting. The main filter cans were not a
problem on them ( 40 yrs ago !). Since 'lytic cans rarely short try
bridging apx 80 mfd across the doubler cap & see if it gets better.
Running at 50 cps in theory requires bigger caps but there is some
room there.

BTW bridging electrolytics is an old trouble shooting technique.
It can save you time & $$$. Also give you a look at the pix to see
if you want to go further with an old set.

BTW2 If you change a doubler cap be SURE you use one rated at
the same or higher temp.

73 Zeno
LFOD !

[AlanInSitges]

Thanks for the suggestions everyone, I dug back in today and here's what I've tried and the results.

Subbed filter and doubler capacitors: N.b., the B+ was fine before, in fact maybe 20V high. Swapping in the new filters and doubler made zero difference at all: B+ didn't vary more than a few mV.

old_tv_nut gave me an idea, I noticed the degauss thermistor had been replaced by some weird-looking thing I've never seen before. It measures the same cold as the original piece, and anyway I removed it from the circuit to be sure I'm not seeing magnetic effects from the degauss circuit. No difference at all.

I decided to tackle the vertical circuit next. Voltages on the tube are correct or a small amount high, output cathode is at 19.86, osc plate is at 210, output grid is at -18.2. None of the voltages vary even a tiny bit. The waveform on the grid of the VO looks exactly like the picture in the Sams, measures 211V P-P, and is completely stable, no change at all in amplitude or frequency. I tried to get a photo but at that slow sweep rate the camera wasn't cooperating.

Tell me if my thought process from here makes sense: it looks like the vertical circuit is operating as it should. What we're seeing on screen is pretty significant; if it's affecting the vertical it should be immediately evident in any of the waveforms or voltage measurements. Since this issue is affecting both the H and V equally, and in sync, and the B+ is both correct and stable, I'm thinking it could be a problem with high voltage. If the HV were fluctuating rapidly, it could cause exactly this symptom, no? The Boost voltage seems stable. The set was hissing a bit when I first brought it up but I blew out the HV cage and it stopped. I probed the HV and got 17kV. It didn't seem to be varying, but I don't know how accurate these meters are or how much variation would cause our symptom.

So that's where we are. This is my first color set. I'm suspicious of the HV but not sure where to go from here. Any other ideas?

[old_tv_nut]

The effect on both vertical and horizontal, plus it being stronger toward one corner, still indicates magnetic field interference to me. That's why I asked whether the set has a transformer or if there is other equipment near, or if moving and turning the set makes a difference. Is there a sound tone on your signal? If so, what happens if you use a signal with different audio?

[AlanInSitges]

No transformer, it's a portacolor series string set. There is a variac about a meter away, moving the set around doesn't seem to make a difference, also no effect on any other similar vintage sets on the same bench. As a test I plugged the set into an autoformer instead of the variac and got the same result.

There is a 1kHz audio signal, I have switched on and off the carrier but no change there either.

[old_tv_nut]

Are you sure removing the thermistor has completely disconnected the degaussing coil?

This is surely a puzzle!

[old_tv_nut]

Any progress on the wiggles?

[AlanInSitges]

Progress, no...but I have been working on it!

Thermistor completely removed, I also disconnected the degauss coil. Still doing it.

I did a test where I held up a piece of paper horizontally to the screen with a crosshatch pattern - vertically is really isn't moving hardly at all, it just looks like it is in the context of the whole picture. But with the paper there as a guide I can see it's pretty stable - it's maybe moving a little bit, but not nearly as much as horizontally. .

Horizontal, well, that's a different story. Voltages are weird on the oscillator, -4v on the grid vs -2, and plate is around 190v vs 165, though the waveforms look just like the pictures and frequencies are right. The curious thing is that if I put my scope probe on the grid of the H output, it immediately shuts down and the oscillator stops running. I'm not seeing any sign of the jitter in the waveform on the grid of the H osc, which is where my limited logic and experience tells me it would show itself. The frequency is stable and the waveform itself is completely solid.

I'm guessing (and it is a guess at this point) that the H osc shutting down when I probe it might be a clue.

ETA: High Voltage is correct and doesn't vary at all, from what I can see on the probe's tiny meter.

[old_tv_nut]

If your scope probe can shut down the oscillator, it might indicate that some coupling component connected to that point has gone open. Can you post a pic of the schematic in that area?

[AlanInSitges]

Here you go! https://i.imgur.com/nbdCpC6.png

I briefly considered C265 but given the -40V on the grid of the HO I dismissed it.