Hi - looking for a resolution to this problem of 4-5 white vertical bands on this Kriesler. Any suggestions? Picture in link below.

http://john116.mywire.org/images/Kriesler_white_stripes.jpg

You may also notice that the top of the raster is darker than the bottom. What does that tell you?

If you could check the chassis tag and tell me the "79-??" chassis number I should be able to give you a component reference for the bad part, or at least where to look.

For chassis series 79-4 and 79-5 it will be C4, 50μF electrolytic cap.
For 79-9 and 79-11 it is C3A. Same cap.

For later chassis this symptom should not be possible!

You'll need to replace it with a suitable can type electro, there is considerable ripple current stress on this cap. I have some suitable caps.

50hz ripple causes the raster dark/light problem?

The chassis is 79-10A

I have replaced all the electrolytic, but based on your comment above, maybe not the right type?


John

Ah, yes, the 79-10! A fully-featured "fringe area" design. Has a unique feature that rolls off the high frequency video if the signal is weak, to reduce the visibility of snow. Amongst other high-end features.

This feature caused some issues in deep fringe areas, where adding a masthead amplifier could actually make the picture appear worse!

C4 in this case is a 150μF can electro. What happens is if this has high ESR the horizontal AND vertical scan currents modulate the B+ which also supplies the video output stage.

What did you use to replace it?

An off the shelf electrolytic of similar and value and correct voltage rating, just a small axial lead component.

This should be OK in the short term. Is it possible you connected it incorrectly? Or did this fault appear after using the set for a while?

Did you also replace C1 and C2? What did you use? There is a very good reason the original caps are so physically large. Those 2 caps carry the entire B+ current for the TV.

This is why I normally try to re-form the original caps. I've found Ducon can electros usually reform well, UCC caps usually fail to do so.

The fault could be elsewhere but C4 is most likely.

Ian,

Replaced all electrolytics. Happened (as far as I know) from initial use after capacitor replacement, and is there from first turning the set on.

Assuming I've made no errors in capacitor replacement, would using low ESR types help? Or can types if I can find them? I left the original capacitors physically in place, but can't recall the brand of them at the moment.

John

C1 and C2 also replaced with off the shelf modern electrolytics.

If the old ones are still in place I’d try hooking back up the original cap C4 to see what it does

OK, here's what I'd do.

1. Re-connect original can electros, checking for any signs of electrolyte leakage around the insulators and seals. This will show up as a white powder. Any such cans will need to be replaced.
2. Remove yoke plug, 6CM5, 6AL3, CRT base, vertical and audio output valves.
3. Plug chassis in via a dim bulb of around 100 watt lamp.
4. Power up. Monitor the 230 volt rail - pin 5 of the yoke socket is convenient for this.
5. Lamp should light brightly, dim and 230 volt rail should settle at some value around 230 volts. If it's over that, good.

6. If the existing electros have been hibernating for many years it may take up to a day for the 230 volt rail to stabilise and the lamp to go dim. In any case, let it run for at least a few hours and check that none of the cans are warm. Warm cans will need replacing.

7. If you get this far, bypass the dim bulb and check the 230 volt rail again. It should be well over 230 volts, probably closer to 250 or more.
Let it run like this for an hour or so, checking cans for any sign of heat. If cans are cold, leave it run like this for a few more hours to ensure a good re-form.

If all is well, put everything back together and check the performance. I normally scope the B+ lines for ripple after re-forming old electros.

If you get any failures, I found some very good quality clamp-mount can electros from JJ in the Czech Republic. I've used them in TV restos for a few years now. They hold up well and are not expensive.

There are many people using modern electros in restorations that, although they meet the voltage requirements, are much too small to handle the ripple current demanded of valve TV power supplies.

Ian - thanks so much for that comprehensive reply. I shall try what you suggest and let you know how I go.

Here is a left field idea, if I wanted to do that deliberately, that is put vertical stripes on a screen, I would feed a modulate HF signal into the video amplifier at a harmonic of the line sweep speed.
There is about 4 stripes so times 4 x 16kHz?

Could be something in the video amp that is ringing like an undamped compensating choke or maybe cross couple from the flyback circuit into the video amp? That comes back to what Ian said, scope all the HT lines amazing what crap is there because the LF and HF bypassing is insufficient.
In the little TV sets I made, cross coupling through the HT rails caused all sorts of wierd effects untill I decoupled the sections.

The big shadow bar top to bottom definitely LF bypassing not good enough, exactely what Ian is saying, once more scope will show sawtooth ripple on HT line of many volts!

Just bubble thought thinking.

Fred.

Agree, Fred.

The only other path I can see is the blanking which is both H and V on that set. Or perhaps something around the brightness regulator V1b - a strange circuit that is! But I can't really see a failure mechanism that would cause that display other than via the +230v rail. Unless during the re-cap something was put in the wrong place. All bets are off in this case.

C42 and/or C48 AGC bypasses missing or wrong place is another possibility - I'd need a CRO to chase this further.... The clue here is, does the contrast control reduce or increase the shading as it's turned up?

I assume you are running a modulator with a flat field from a pattern generator into the set, John?

We have here another good reason for selective and incremental re-capping, i.e. changing a few parts at a time and re-checking the performance of the chassis. A fault caused by a part connected to the wrong place can be VERY hard to find.