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Short raster on Toshiba TV

My Toshiba 19" color TV set (Model CE910) has developed a problem where the raster is about an inch or so short at the bottom of the screen, leaving a black strip which cannot be adjusted out. This can be seen here:

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The problem occurs no matter what the video source and has actually gotten a little worse since the above picture was taken. It has been going on for quite a few years, there is a height control inside the set and every once in a while I'd just go inside and adjust it to get the full picture back. Now it's at the point where the height control is maxed out and the black stripe at the bottom is creeping up.

On a conventional TV I'd probably check the vertical output tube for starters, but this is a transistorized set and I'm not quite sure how to approach it. It actually has been quite reliable over the years despite the "Made in Japan" label -- aside from occasional tuner cleanings this is the first repair that has been needed. (Maybe electrolytic condensers are going bad? The set is about 28 years old.)

Any tips on a possible fix appreciated! (Quips about "buying a new set" can be sent to /dev/null...)

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  Roger Blake
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Reply to
Roger Blake
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It is probably is an electrolytic. From memory, the most common ones were c302, C307 and C310. If you have an ESR meter, check eletrolytics in the vertical area and in the metal case which contains the jungle chip. Chuck

Reply to
Chuck

Thanks, I'll take a look for those. Is there any place online that has a schematic and/or component layout diagram?

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  Roger Blake
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Roger Blake

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One possibility is the flyback tuning capacitor, in the vicinity of the H-sweep transistor and/or the E/W modulator diodes (one of those could be leaky).

The capacitor could be a resin dipped foil type - in which case it might be visibly bulged, sometimes its a resin dipped disc ceramic - they normall crack and show a small burn mark on the resin, but the symptoms are usually same as a SC sweep transistor.

If you have to delve deeper into fault finding, it looks as if the deficiency is on the LHS - the line pulse drives the beam from the middle to (your) RHS, at the end of that pulse is the flyback pulse which takes the beam rapidly to the LHS & generates the EHT - its the recovery of this energy stored in the resonant circuit that drives the beam from LHS back to where the next line pulse takes over

Reply to
Ian Field

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Look for a non-polar electrolytic around 10 uF or so, 50 to 100 volts. In the vertical drive to the yoke.

As old as that set is, suspect any of the electrolytic. Borrow an ESR meter from someone.

Reply to
tm

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The shadow on the LHS threw me.

IIRC NP electrolytic coupling caps in the vertical OP are comparatively rare except in portables - but if that model has one its a good place to start.

Another place worth looking is the CR timing network in the VTB charging circuit and/or the linearity network (sometimes includes a preset pot).

Reply to
Ian Field

There is something to be said for shotgunning the caps, especially the electrolytics.

Reply to
William Sommerwerck

I would suggest looking for a large lectrolytic, either polarized or non-polarized, in the connection from the verical output transistor to the deflection yoke. Odds are at least 10 - 1 that is the problem. And, please let us know what you do find, it adds to the total knowledge on this site.

Reply to
hrhofmann

I was thinking along those lines, particularly given the gradual nature of the problem (electrolytics slowly deteriorating).

You guys have definitely given me some ideas to go on, thanks! I'm going to open up the set and inventory the caps, particularly those in the vertical section if I can identify that area. The set has a single circuit board in the bottom where the majority of the components are located.

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  Roger Blake
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Reply to
Roger Blake

And unless you are skilled at carrying out such work, that 'something' is "don't" ...

Arfa

Reply to
Arfa Daily

I am admittedly more at home working with tube electronics, and mostly audio and radio for that matter, but am no stranger to replacing caps and other components on a printed circuit board. As long as I don't do anything stoopid to electrocute myself the worst that can happen to screw it up and having to replace the set...

Fortunately the vertical section is clearly marked on the main PC board. I've inventoried the caps in that area and will start there, though I was unable to find all the ones that Chuck mentioned ... The c302 is physically located in the adjacent horizontal section. I have not found c307 or c310, though the c3xx numbered caps do seem to be primarily located in the vertical area and those are the ones I'm targeting for replacement.

Also I need to retract my earlier "Made in Japan" quip - even though Toshiba is a Jap company, it turns out this set is actually made in the U.S. and contains a lot of US-made components, including the picture tube!

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  Roger Blake
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Reply to
Roger Blake

agreed.

I still fix the CRT based computer monitors I have. Any goofy geometry or strange scan lines are bad caps.

Reply to
Cydrome Leader

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One other weak point in the vertical deflection circuit is the solder joints. Resolder any connections on the vertical output driver and it's heat sink.

PlainBill

Reply to
PlainBill

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