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Old 15th Dec 2018, 12:46 am   #1
1100 man
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Default GEC 2000DST VDR questions

I finally got round to putting the GEC back together after rebuilding one of it's smoothing caps and varnishing the LOPT. It's been a 'round tuit' job for the last year....! The Thorn 1500 I've been using has developed some niggly faults and needs to be sorted out, hence the need for another TV to use!

Anyway, I got the GEC giving a great picture on both 405 & 625. I set the raster geometry up spot on. Next day, at switch on, the picture was much smaller both in width & height and the boost & EHT were much lower. The reduction in height is caused by the lower boost voltage although this is stabilised by a VDR.

After 15 mins or so, the picture, boost & EHT had increased back to the settings of the night before. My usual diagnostic tools in such cases would be a small hot air gun and a can of freezer. Unfortunately they are 150 miles away so I've got to make do.

Heating things up as gently as one can with a 2Kw hot air gun, suddenly the width & EHT increases to where it should be. I need to do much more thorough investigation over the weekend to find out what's going on.

There are, however three VDR's in the set, one of which stabilises the line timebase. When working correctly, the raster size does not change one iota from min to max brightness- I've never had a set that good!

I realise I don't know exactly how these VDR's work! Googling VDR's brings up modern components (varistors) that look like ceramic caps. They seem to be semiconductors and have zener like characteristics but in both directions.

These old ones are rods about 3/4" long with crimped on ends, one of which is green. They look like some form of carbon material. Looking at past threads on the forum brings up a 470V/ 10ma device with a green end.

Does anyone have more information about these things and exactly how they operate?

This chassis is the same as the Sobell 1000 and similar to the ST196.

Many thanks
Nick
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Old 15th Dec 2018, 5:07 am   #2
Focus Diode
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Default Re: GEC 2000DST VDR questions

Seems the component is acting up when cold, requiring the set to warm up fully before working as it should do.

These components, though generally reliable, do seem to be playing up as they get older.

I had an unusual intermittent fault on a Thorn 960 of creeping verticals at the top of the picture. I went as far as to replace the frame output transformer to no avail. The culprit turned out to be the VDR from the boost feed to the height control! The funny thing was the voltage was stable, hence why I didn't initially suspect it.

The original was two tone green. One of those all black types fitted in Thorn 14/1500 sets turned out to be suitable as a replacement.
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Old 29th Dec 2018, 11:57 am   #3
Vectamart
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Default Re: GEC 2000DST VDR questions

I have a few VDRs in stock. Do you know the type number? If you could confirm the circuit reference, I'll check if I have a service manual with the specifications of the VDR.
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Old 29th Dec 2018, 12:25 pm   #4
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Default Re: GEC 2000DST VDR questions

Many thanks for the offer. In fact, whilst heating things up, the width suddenly jumped back to the correct size and has been spot on ever since! I don't doubt the fault will only show itself again once the chassis is all back in the cabinet. So whether it was the VDR, I don't know.

All the best
Nick
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Old 30th Dec 2018, 11:56 am   #5
Focus Diode
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Default Re: GEC 2000DST VDR questions

Could be a dry joint, remembering the double sided PCB?
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Old 1st Jan 2019, 12:48 am   #6
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Default Re: GEC 2000DST VDR questions

Quote:
Originally Posted by Focus Diode View Post
Could be a dry joint, remembering the double sided PCB?
Actually, I've found the PCB's to be exceptionally good. I've replaced quite a few components and not had any trouble with print lifting. I scrutinised the boards when they were originally removed for cleaning & rebuilding and did not find any dry joints. I've been surprised how good these boards are!

I don't doubt the fault will show itself at some point but in the meantime, I would still like to know how these things work. I can't find any information on them.

Many thanks for the suggestion though.

All the best
Nick
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