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Old 2nd Mar 2023, 11:52 pm   #1
alfatangowhisky
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Angry KB VC2 chassis LOPT primary getting hot and bothered!

Hi all

I generally only reach out when I am really stuggling... and I am.
This VC2 chassis is part of a KV002 that I am restoring. The cabinet refinishing is done, and the electronic side is really giving me the run-around.

Here is the case file (all aspects of the report relate to 405 setting; not much trialling has been done on 625 line):

1. The set has been stored inside the house next to a radiator for months and is as dry as bone

2. Most of the caps are original, being either Mullard mustards or the red Lemco look-alikes which seem to be also quite well respected. The only caps changed are any that looked a bit perished and the boost cap. This was replaced with a modern 3000v rated poly

3. The picture is always **overly wide** and the width control is set at minimum. All resistors (fixed or variable) related to this control have measured OK or have been replaced where necessary

4. HT starts out at the specified 235v but **drops** about a volt per minute (this is monitored on a DMM)

5. In conjunction with the above the LOPT primary starts to get very **warm** - after 5 minutes the wax is starting to soften

6. After about 6 or 7 minutes the picture has ballooned out and faded

7. I am monitoring the 30P19 (line output valve) current between top cap and the LOPT. This starts at around 125mA and after the fault condition as described above has taken hold, is at around 140mA

I have been poring over the circuit diagram and the chassis and cannot find anything else to chase other than some ceramic caps around the LOPT stage or the width stabilisation circuit (having changed all the resistors in this area already).

8. Boost is at or around 750v (by the tolerance of my eyes) and does not seem to drop noticeably


Can any knowledgable souls please help to point me in the right direction as once this set is all back together and working it will be a beauty. Thanks!
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Old 3rd Mar 2023, 12:15 am   #2
Graham G3ZVT
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

It's not what you'll want to hear, but I think the most likely explanation is that the EHT overwind is breaking down.
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Old 3rd Mar 2023, 7:47 am   #3
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

Hi ATW.

Your fault sounds very much as if the LOPTy is saturating due to trapped moisture within the overwind.

I know you mentioned that the chassis is ‘bone dry’ but the LOPT will still need drying out. One method that’s proven to work well is to pass a DC current through the overwind from a bench power supply for maybe 24 hours. You can do this in situ by connecting the DC supply between the Line output valve top cap, and the top cap of the EHT rectifier valve.

I should start at 50mA, and see if you can feel warmth in the overwind after 30 mins or so, but increase the current upwards if it still feels cool.

Hopefully, this should work. It’s best to not run the set with the transformer saturating and getting very hot, as this could damage the windings .

Fingers crossed here for some success!


Good luck.

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Last edited by thermionic; 3rd Mar 2023 at 8:06 am. Reason: Punctuation!
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Old 3rd Mar 2023, 9:54 am   #4
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

I forgot to mention that I have the same issue with a second LOPT installed in this set which came from a chassis where the cabinet was due to be Farrow&Ball'd to death. This was dry stored too.

I have done the overwind drying using DC before (see the Sobell 'field find' thread) and did this out of anticipation of the LOPT being well saturated, and have had other sets with damp overwind syndrome where the fault presented after 10 minutes or so 'normal' running, but in this case the fault is immediately present.

Nevertheless I will run DC through the overwind for a week and see what comes of it
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Old 3rd Mar 2023, 10:42 am   #5
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

Hi ATW,
Sadly, I think you can be 100% certain that it's the LOPT I suspect in your case that the primary is the cause of problems rather than the overwind. It's either already got shorted turns, or the insulation is breaking down after switch on.

The initial excess width would indicate low EHT. I've found with LOPT's that as they get hot, the EHT rapidly falls. The fact that the primary gets hot quickly would indicate that that's where the problem lies. Faulty overwinds tend to take longer to generate heat in my experience.

I don't think there is any such thing as 'dry storage', especially when you consider how much moisture there is inside a house. In the same way that it finds it's way into paper capacitors - even plastic cased ones, then it finds its way into LOPT windings. I doubt, in your case whether drying out the overwind will help, as I don't think that's where the problem lies.

There isn't much in a line output stage that stops it working or makes the transformer get hot quickly - other than the transformer itself. The fact that your second LOPT exhibits the same fault only proves that it's also faulty.

I've had more primary failures than overwind problems - usually insulation breakdown of the inner winding to the core - but taken over all, the majority of my sets have had or still have LOPT issues. It's something that I've come to accept as normal!

Let's hope it isn't the LOPT, but I'm not optimistic.
Cheers
Nick

Last edited by 1100 man; 3rd Mar 2023 at 10:44 am. Reason: typo's
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Old 3rd Mar 2023, 11:37 am   #6
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

I have had exactly the same problem with two sets fitted with the KB VC wired chassis. I managed to find a good LOPT for one, the other is still waiting. In my experience it is usually the overwind that fails but in the KB sets the primary seems to be a problem.
Can the primary be dried out? If not by passing current through it then by gently baking it? Would it be possible to rewind the primary?
Rich.
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Old 3rd Mar 2023, 2:02 pm   #7
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

Quote:
Originally Posted by slidertogrid View Post
I have had exactly the same problem with two sets fitted with the KB VC wired chassis. I managed to find a good LOPT for one, the other is still waiting. In my experience it is usually the overwind that fails but in the KB sets the primary seems to be a problem.
Can the primary be dried out? If not by passing current through it then by gently baking it? Would it be possible to rewind the primary?
Rich.
There are several taps in the primary so I don't see why not? I have 2 LOPTs to try it out on.

I can see why the primary would be susceptible to moisture as it is wax coated. The overwind is enrobed in pitch.

I'll let everyone know how I get on
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Old 6th Mar 2023, 10:25 am   #8
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

Disconnect the top cap from the EHT rectifier then run the set up from cold for 20-30 minutes. If the EHT overwind is warm then you either heave trapped moisture or shorted turns on the EHT overwind.
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Old 6th Mar 2023, 8:30 pm   #9
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

Hi Simon,

I am currently running some milliamps through the primary (out of the chassis); this will be deemed 'enough' at the end of the week so I will see if this has cured potential damp in the windings.
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Old 7th Mar 2023, 10:38 am   #10
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

Let us know how it goes.
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Old 11th Mar 2023, 4:43 pm   #11
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

Hi Simon

It went well!

200mA was run through it for a week, and now the voltage is more stable (drop from 225 DC to 220 DC over an hour rather than in 10 minutes (and never gets below 220), so there is still damp there but I am running for a couple of hours per day to 'work it out'.

Width starts out good now instead of already too wide.

Image also of where I ran the current through; basically all the way up the 'backbone' (between tags 1 and 5 in my diagram)
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Old 11th Mar 2023, 6:26 pm   #12
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

I had exactly the same problem on my VC2 despite the usual drying out process. Mike Barker rewound it for me.

As folk say, it's probably the primary rather than the overwind. But with nothing to lose, you could try removing the primary from the core, hooking up a tripler in its place and see what happens
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Old 11th Mar 2023, 6:52 pm   #13
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

Looks very good now!
Well done.

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Old 11th Mar 2023, 7:36 pm   #14
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

Quote:
Originally Posted by Niechcial,Steve View Post
As folk say, it's probably the primary rather than the overwind. But with nothing to lose, you could try removing the primary from the core, hooking up a tripler in its place and see what happens
Surely you mean remove the overwind and fit a tripler? Why not try the drying out process on the overwind ? if the primary had damp ingress it is lightly that the overwind has as well.
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Old 11th Mar 2023, 8:59 pm   #15
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

The only thing still bugging me now is the horizontal linearity (see right-hand side of the screen image). Position of the sleeve doesn't seem to make any difference.

Anybody know what else it could be?

Last edited by alfatangowhisky; 11th Mar 2023 at 8:59 pm. Reason: More info on fault
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Old 12th Mar 2023, 10:00 am   #16
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

Is the line linearity sleeve around the neck the right way around? Is the S correction cap ok?
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Old 12th Mar 2023, 12:21 pm   #17
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

Quote:
Originally Posted by alfatangowhisky View Post
The only thing still bugging me now is the horizontal linearity (see right-hand side of the screen image). Position of the sleeve doesn't seem to make any difference.
Actually, the linearity looks pretty good to me - certainly well within what I'd expect for a valve line output stage. Don't forget, this is a domestic TV, not a broadcast monitor, so perfect linearity wouldn't have been achievable from new!
After all, unless really bad, the customer would never notice anyway!

As far as I know, a valve line O/P stage produces an inherently non linear scan and relies on correction devices such as linearity coils & shorted turn sleeves to make it acceptable.

I'm not sure why this is, but I'm sure I read about it in a description of operation somewhere. I don't think transistor O/P stages suffer from the same problem.

Certainly, to a greater or lesser degree, valve TV's always exhibit gradually changing linearity from one side of the screen to the other.

I'm surprised (and impressed) that your drying out of the primary worked so well. It seemed already too far gone to recover, but goes to prove it's always worth trying. It also goes to show that even though a TV has been in a warm house, water can still be absorbed into the windings - after all, houses contain a very large amount of water vapour, it's just not immediately obvious.

Well done though - are you going to try the same procedure on the other LOPT?

Cheers
Nick
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Old 12th Mar 2023, 3:45 pm   #18
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

All OK now that I have the sleeve in the correct orientation (gap on the side, not the top or bottom) I have control of linearity and better control of width.

For reference, in case the sleeve gives anybody else the run-around, images are attached showing how it should be fitted, and a grab from a VC51 service manual which describes this, whereas the VC2 paperwork omits mentioning the sleeve altogether! Another photo shows shining a light through the sleeve to reveal the embedded copper foil, which helped me match up the alignment as per the VC51 info.

And yes, I will dry out the other LOPT (keep it heating for a few weeks and then seal it up and bag it as a spare)
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Old 12th Mar 2023, 6:49 pm   #19
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

That’s great news ATW!

What a relief that it dried out ok. Sometimes it might be necessary to melt out the wax and re seal with conformal coating, but yours seems fine.

Well done!


Cheers.

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Old 13th Mar 2023, 12:00 pm   #20
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Default Re: KB VC2 chassis LOPT primary getting hot and bothered!

That is great news. Its looking good. Now you just have to give it an occasional and good run to keep it going
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