TV repair question

I have a Sampo color TV from the early 80's which has what may be this problem from the sci.electronics.repair FAQ:

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"Vertical brightness or color bars These are typically more or less equally spaced possibly more evident at the left side of the screen. They result only in brightness or color variations, not deflection speed. Diagonal lines are straight and not squiggly. Note that the appearance of these bars differs from those caused by ringing in the deflection circuits where diagonal lines will show a squiggling stair-step appearance. The most likely cause is a dried up electrolytic capacitor in the scan derived power supply for the video or chroma circuits or video output. Check for this ripple with a scope or test/replace any suspect capacitors."

The bars I'm seeing are 1/2 to 1 inch wide, with 1/2 to 1 inch between them.

I have the schematic and can do my own repair work, but needed some help with the troubleshooting since I don't have a scope or signal generator. For a small fee, a local shop marked 10 caps on the schematic for me to replace. None solved the problem- in fact of the

10, 2 were not even on the board. On the second attempt, their tech claimed to have tracked a signal from the tuner to the final video driver and everything seemed fine. Their final answer was "the tube must be bad".

I'm not ready to give up yet. The colors and picture quality are fine, except for vertical stripes of increased brightness across the screen. If I can fix this set I'd really like to, as it is the last piece of "repairable" electronic gear I will probably ever own. Repair shops will not fix anything this old, for obvious reasons.

All advice appreciated.

Reply to
wn513
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You probably have more bad caps than what commonly goes bad. You will need some test gear and the knowledge to locate, and change them. If you invest in an ESR meter to test the caps directly, this would be a start.

When servicing these old sets, it is common to have to change many caps. Especially if the set was not used, many of the caps probably have to be changed.

If you need an extra TV set, you would be best off to buy a new low cost set. At least it should go trouble free for about 3 to 4 years, and it will have a minimum 1 year warranty with it.

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JANA
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Reply to
JANA

Thanks for your reply. This set has been used almost daily for 23 years. It's been in dorm rooms, living rooms, family rooms, bedrooms. No remote and a mechanical tuner. I have gotten my money out of it for sure. If I _can_ fix it cheaply I'd like to, simply "because I can". If the tube was obviously bad, it would be in the trash tomorrow. But I think the shop was just giving up on it, and the tube is probably fine. I can't blame them for not spending a lot of time on it, there is no repair money to be made.

Replacing every cap on the chassis was my fall back position. It probably wouldn't cost more than $10. I'll think about getting a meter too.

JANA wrote:

Reply to
wn513

If I'm understanding your description properly, the problem is called "jail bars." Is it like looking through a bunch of fence posts? If so, the problem is likely caused by one or more bad caps not doing their job properly, and it's causing some "ringing" where it's not supposed to.

It may not be an electrolytic that's causing the trouble, but more likely a ceramic or polystyrene low-value cap in the horizontal output transistor circuitry.

Reply to
Matt J. McCullar

From (fading) memory, this problem was usually a 'decoupling' cap around the collector of the line output transistor. It was a fairly common problem on this generation of TVs, and even earlier hybrid models

- where it was generally a decoupling cap off the screen grid of the PL 509.

Reply to
Farmer Giles

The schematic in the area of the horizontal drive and FBT looks sorta like this: (if you'll email me your address I'll send a picture)

Horiz. driver transistor, with 10pF across base-collector, emitter to ground, collector to one end of the primary of a transformer "T601 Horiz. Driver", and to a

680pF 500V cap in series with a 6.8K 1/2 W resistor connecting to the other end of the transistor primary.

T601 secondary side has one end grounded, the other end to the base of the Horiz. Output transistor, whose emitter is grounded, there's a 50 ohm resistor between base-emitter, a diode across emitter-collector, and the collector goes to the primary side of the FBT. Also across the collector to ground are a) 3 caps in series, value ".027(J), 630V". Not sure what "J" means... b) a 330pF 2KV cap in parallel.

Am I in the area of what you're calling the "line output transistor"? Does PL 509 refer to the flyback?

Reply to
Uncle Fester

Yes, the horizontal output and line output are the same thing - and a 'PL 509' was a line output (horizontal output) valve. I would replace the 3 caps in series - that's the area I think the fault will be (of course the FBT is also a possibility, but I think it more likely to be a capacitor around there).

Good luck.

Reply to
Farmer Giles

I'm having trouble finding these caps. My local parts house did not have them. Are these unusual values, and if so where might I look for them?

Reply to
Uncle Fester

J means 5% tolerance.

They are unusually high voltages, probably available from places that specialize in TV parts.

Reply to
mc

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