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Old 29th Jan 2007, 5:06 pm   #1
GMB
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Default TX100 chassis - pincushion distortion

Having established in another thread that my old Baird 059B3 uses the Thorn TX100 chassis and is reported as having the potential for interesting faults I thought I might mention a curious one that mine has....

A long starting fault is that it sometimes displays the picture with quite severe pincushion distortion with only verticals being distorted. The raster continues to fill the screen and it isn't exactly obvious what is going on. The text display is also affected but it just looks like the middle lines are short. I can't say if the top/bottom is being overscanned or the centre lines are highly non-linear (they repeat old programmes but not old test cards). I have seen this effect start suddenly and has sometimes seemed to vary with picture content but that may be an illusion. It stops doing it if the set is left unused for a long while.

It has been said that the LOPTs tended to go on this set - - why? Is my fault a sign that the LOPT-destroying problem is happening? I had been waiting for this problem to get more solid and perhaps for smoke to indicate the faulty part. When the set had been in use on a 12hr/day basis there had been signs of imminent breakdown of something but the death-blow never came.
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Old 29th Jan 2007, 5:24 pm   #2
Welsh Anorak
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Default Re: TX100 chassis - pincushion distortion

Hi
Don't worry - the LOPT's not to blame. It's most likely to be a dry joint on the E/W sub-panel by the LOPT, but it's worth going over the plug/socket connections there and the two E/W diodes. A few minutes with a magnifying glass and a soldering iron should do the job!
Glyn
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Old 12th Feb 2007, 9:57 pm   #3
Dr-Watts
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Default Re: TX100 chassis - pincushion distortion

There is a sub panel near the LOPT with a handful of Rs & Cs on it.
The tranny; can't remember what it is but it's a TO220 package, passes away.

TR4 goes leaky causing R19 to go O/C on the E/W correction board.
Also check TR8 (BC137) or R80 470 ohm changing value.

Let us know how you get on.

DW

PS: Beware the chopper transformer !
Especially if it's an early one.
Boy - can they cause some obscure and interesting faults.
The favourite is the chopper transistor blowing for no apparent reason.
I got this one on my TV Final exam...and after you've lived it you don't forget it.
It cropped up several times over the years !
Other than that, the good old TX100 is great
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Last edited by Mike Phelan; 1st Nov 2007 at 5:55 pm. Reason: S&P
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