Magnavox CD4756 (T989 chassis) dark bands down left edge

[James_S]

A couple months ago I found a 1974 Magnavox 25" console with a T989 chassis in a thrift store for 10 bucks and took it home with me. Initially it had audio but no picture, I discovered the xray protection was triggering and tracked that down to a big 190M power resistor that is part of a voltage divider monitoring the EHT which had failed open then started arcing. I replaced that and that got the picture back. Then I found that after about an hour the picture would fade darker and darker, I found that the CRT heater voltage was slowly dropping as a large oil capacitor associated with a ferroresonant transformer was getting very hot, replacing that fixed that problem. Now there are a couple of significant problems remaining that are stumping me. The issues are these:

With the B+ set to the specified 120VDC the EHT is nearly 35kV, substantially higher than the 30kV max that is on the label inside the set. Also there are some darker bands at regular intervals down the left edge of the screen that appear to be some sort of ringing. If I dial down the B+ to about 112V the EHT is at a reasonable level but the horizontal width doesn't quite fill the screen.

If I adjust the horizontal centering the dark bands move with the picture. I tried tacking an extra 2.2nf capacitor across the 3 safety capacitors across the HOT and that brought the HV down and allowed the B+ to be increased so that the width filled the screen however the dark bands were still present. I tried replacing all three of the safety caps and that made no difference at all. I found multiple ground connections where the pins for the plug-in modules connect to the chassis that had cracked loose so I resoldered those and it made no difference at all. I found C36 in the horizontal drive section was bad, ESR over 25 ohms, so I replaced that and it made no difference. Finally I went through and replaced all of the electrolytic capacitors on the PCBs, something I typically avoid doing, that made no difference at all. I replaced a handful of resistors that measured well out of spec, no difference. I poked around with a scope and didn't find any smoking gun. I'm not sure if the dark bars are due to something in the video signal, or ringing in the yoke modulating the scan velocity but looking at the waveform across the HOT as well as at the CRT cathodes failed to find any obvious issue. Anyone ever see something like this before? I don't know what else to try.