What's going on with this power supply?

[AlanInSitges]

I have a Zenith 13X15 chassis, the little series-strung 12" portable that weighed as much as a Volkswagen. This is as usual for me a 220V CCIR model that was built in Spain.

After swapping the horiz osc tube it worked fine, aside from what I could swear was the typical horizontal twerking caused by a bad filter capacitor. Except this set is using 50Hz power and the signal source also has 50Hz vertical, so there should be no movement, just a stationary bend in the picture.

I finally replaced the filters in the power supply and it's still behaving the same way. I got frustrated tonight and taped a paper template on the screen in front of a crosshatch pattern so I could measure what I was seeing, set my DVM to measure voltage difference on the main B+, and waited.

Sure enough, after a couple of minutes it started, I dunno...wheezing? The picture would slowly increase in size around 1-1.5cm over the course of 20-30 seconds and then shrink back down. The B+ didn't vary much at all during this episode, maybe .2V, it was a few V high to begin with.

So I stuck a scope on the B+ line to see if there was anything interesting happening. The Sams claims there should be 1.3V ripple. Here's what I'm seeing: normal-looking ripple of about 5V and gradually morphs into double-humped ripple of almost 20V and back again every 20-30 seconds. Video: https://photos.app.goo.gl/hn7EiXpC9XML4FXMA



This is a very simple power supply circuit, and I'm baffled by what I'm seeing. Can anyone shed some insight? Thanks!