What's going on with this power supply? - Videokarma.org TV - Video

AlanInSitges · #1 07-13-2020, 05:59 PM

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!

maxhifi · #2 07-13-2020, 09:55 PM

I think it means something is drawing increased current periodically. Increased current would load the power supply more, producing ripple. If you remove the horizontal output tube plate cap (and suspend it so it doesn't short circuit to anything), I'd bet you a glass of beer the ripple goes away.

If this is the case, try changing the horizontal oscillator tube again and substituting a different horizontal output tube if you have one. Also make sure the components in the horizontal section are all good, capacitors and resistors.

old_coot88 · #3 07-14-2020, 12:21 AM

We had a set in the shop once with a similar "breathing" problem, but it was in the heater string (it was a series set). The heater string was cycling on-off, on-off on a regular basis. Finally traced it to the CRT heater that was 'blinkering' just like a turn signal blinker (but at a slower rate). Never run into that problem before or since.

JohnCT · #4 07-14-2020, 06:36 AM

Quote:

Originally Posted by old_coot88

We had a set in the shop once with a similar "breathing" problem, but it was in the heater string (it was a series set). The heater string was cycling on-off, on-off on a regular basis. Finally traced it to the CRT heater that was 'blinkering' just like a turn signal blinker (but at a slower rate). Never run into that problem before or since.

Wow. Weird.

Along the same lines, I'd try replacing that rectifier. Very few open and I've never seen one do what this TV is doing, but I'm old enough to know I'll never see every possibility.

John

old_coot88 · #5 07-14-2020, 01:11 PM

Is the vertical in the signal source specifically synced to the power line? Or is it independently generated and possibly running slightly off the power rate?

AlanInSitges · #6 07-15-2020, 05:25 AM

Quote:

Originally Posted by old_coot88

Is the vertical in the signal source specifically synced to the power line? Or is it independently generated and possibly running slightly off the power rate?

It's coming from a Philips PM5515-TX, which allows you to set any flavor of PAL/NTSC at various frequencies and frame rates, so I suspect that it's independent of the line frequency.

Do you think it could be a beat thing with the difference between the two? I wondered if it could have something to do with having the set plugged into a variac, plugged into an isolation transformer, but I removed those from the equation and same thing. N.b., the heaving is visible on regular broadcast TV as well, well regular meaning DVB tuner --> PAL modulator.

AlanInSitges · #7 07-15-2020, 05:27 AM

Quote:

Originally Posted by maxhifi

I think it means something is drawing increased current periodically. Increased current would load the power supply more, producing ripple. If you remove the horizontal output tube plate cap (and suspend it so it doesn't short circuit to anything), I'd bet you a glass of beer the ripple goes away.

If this is the case, try changing the horizontal oscillator tube again and substituting a different horizontal output tube if you have one. Also make sure the components in the horizontal section are all good, capacitors and resistors.

Thanks for this, I will order another osc tube and see what happens; I have another H output here but it's a 38HE7, so no plate cap. First step will be to swap it and see how it goes.

AlanInSitges · #8 09-13-2020, 12:22 PM

N.b., I replaced the rectifier and got the same result. Then I started going back over the work I had done changing the filter capacitors.

Everything was OK, but I realized that the little metal bracket that the filter cans had been mounted on (and that my terminal strip was now mounted to) wasn't very well screwed down to the chassis. There are three sheets of metal that come together and the bottom one wasn't getting caught by the screw. After messing around with it for a while I decided it must have been like that from the factory. So I ran a wire from the (-) of the main filter to a good chassis ground. The weird ripple went away and the heaving picture along with it.

Thanks to everyone for your input!