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Old 26th Jul 2023, 9:26 pm   #1
Studio263
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Default Sony KV-1820UB / 2000UB help needed

I've been repairing a Sony KV-1820UB (like the KV-2000UB, just with a smaller tube and cabinet) and it was all going very well until one day the picture went green briefly and then disappeared altogether. Disconnecting the EHT transformer from the LOPT brought the rest of the set back to life; reconnecting it with the EHT unit disconnected from the winding showed that this was where the problem was. I have to say that I've never really looked at this part of one of these before, they have never given any trouble in the past. However, it turns out that the little red box is a can of worms!

For a start its a doubler, not a tripler (who knew?). The EHT is still 25kV though, not 19kV as many earlier Japanese sets used. The other potential fly in the ointment is that the H STAT control provides a variable supply that is referenced to the EHT output, not the chassis. To clarify, in a conventional set of this era the focus potential is up to around 4kV to 8kV in respect to the chassis but in the Trinitron the H STAT voltage is about 4kV to 8kV less than the EHT. For both these reasons a conventional tripler cannot be fitted.

Since a second hand replacement has proved to be unobtainable (who ever kept this bit when they scrapped the set when the tube went flat - they never used to go wrong...) I've been looking into making a new one but there are various problems with this. Firstly the unit is pretty small and its part of the structure of that part of the set. It also contains the H STAT control which would be difficult to replicate. To get around this I spent an (un)happy wet afternoon digging out the red resin sealant and the old components. One pair of nice wire cutters ruined, red bits everywhere but I got it done. The H STAT control is in a separate compartment and survived unscathed.

Because its a doubler there are only 4 diodes in there, luckily the Sony manual includes the circuit. This is the rub. At 25kV you need much bigger ones than the sort you use to make a tripler (Baird 700 etc). 15kV rating would seem to be a minimum (25kV in 2 stages). You can get them but they are huge! The situation with the capacitors is slightly better - because there are fewer stages they can be a lower value. 1nF is usual for triplers but measurements made before I broke it up suggest that the originals are about 330pF. Against this similar observations apply to the voltage - 15 or 20kV rating would seem sensible. These are again available but they are big.

The last problem is that because of the compactness of everything it will be necessary to pot it in resin before the power can be applied. If it doesn't work for some silly reason that's an expensive and time consuming mistake. Vacuum packed two-part potting resins are available though, so this may just be a case of careful / brave / lucky.

This all seems a lot of faff for what it usually a simple / trivial fault. Does anyone have any other ideas about how to get round it? Of course EHT units don't normally fail in Japanese sets, but this one has 'Sony - made in Great Britain' written on it. That'll be why then.
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Old 26th Jul 2023, 10:43 pm   #2
greg_simons
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Default Re: Sony KV-1820UB / 2000UB help needed

I can offer some scant help, there are plenty of 20kv piv diodes available from the usual sources, I've used a potting product called silcoset 101, not cheap though but it's undoable if it goes wrong, available from a company called temple4, who were beyond helpful to deal with, those custom high voltage blocks virtually never failed, can't even remember replacing one, it's possible someone here has one attached to a set with a flat tube.
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Last edited by greg_simons; 26th Jul 2023 at 10:44 pm. Reason: Repetition
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Old 27th Jul 2023, 10:16 am   #3
Philips210
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Default Re: Sony KV-1820UB / 2000UB help needed

It sounds like a challenging but interesting project. Greg's suggestion of silcoset 101 sounds good and I wonder if it's a similar type of potting that Thorn used in their jelly pot LOPTs?

Regards,
Symon
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