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B&W TV Murphy V2015D

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PYE625
(@pye625)
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As a matter of interest and to confirm what I already know about the old overwind, I decided to cut it open and have a look. It is clear to see that the insulation has broken down and it has burnt out resulting in multiple shorted turns. I wonder if the encapsulation and plastics container restricts the heat escape and may contribute to the problem.

Anyway, if there was still a large call for these, I bet they could be made in China for ten a penny. We had the universal tripler, we need a company to make a universal overwind !

IMG 5918 50
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Topic starter Posted : 05/06/2021 12:41 pm
Katie Bush
(@katie-bush)
Illustrious V-Ratter Moderator

@pye625

More than heat escaping, I suspect it makes a perfect trap for condensation, and the perfect failure mode. Given that the interlayer PD will not be very high, the risk of an arc/flashover in the windings would be quite low, but add moisture to the equation, and you could now have the full-on EHT of the entire winding to leak across the moisture and fry the lacquer? Once it's gone that far, there would be no going back.

Just a thought.

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Posted : 05/06/2021 4:59 pm
PYE625
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Whatever the contributing reasons, I think the main one is the plain fact that the overwind's were not really up to the job even when the set's were fairly new. A bold statement? Well, judging by the track record of failures, I would say not. But of course, LOPT issues were not confined just to RBM.

So then.... what we need now is a LOPT rebuilding company alongside a CRT regun facility in the UK.

Any takers?  😋 

 

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Topic starter Posted : 05/06/2021 9:09 pm
Nuvistor
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@pye625

Its not as if LOPTX’s were new technology, granted final EHT voltages had risen but I don’t think there is any excuse for the demise of these transformers within their service life. Fifty years on I can accept but not within the first 10 years of service but fail they did.

 

 

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Posted : 05/06/2021 11:06 pm
PYE625
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A point worth noting is the fact that the overwinds from LOPT's for other RBM chassis could be used. In this case, an A774 LOPT overwind is suitable, albeit with differing lead lengths. A successful transplant was thus made.

 

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Topic starter Posted : 06/06/2021 12:03 pm
ntscuser liked
Doz
 Doz
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@nuvistor

 

I often thought this... there were exceptions. Did anyone ever see a G11 lopt fail? 

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Posted : 07/06/2021 4:25 pm
Cathovisor
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Posted by: @nuvistor

@pye625

Its not as if LOPTX’s were new technology, granted final EHT voltages had risen but I don’t think there is any excuse for the demise of these transformers within their service life. Fifty years on I can accept but not within the first 10 years of service but fail they did.

 

 

But you and I both know the reason for this, Frank - the attitude of British setmakers in that they all to a man viewed a penny spent as a ha'penny wasted.

 

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Posted : 07/06/2021 6:15 pm
Nuvistor
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@cathovisor

Yes that’s very true.

 

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Posted : 07/06/2021 6:22 pm
Red_to_Black
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Posted by: @doz

@nuvistor

 

I often thought this... there were exceptions. Did anyone ever see a G11 lopt fail? 

I have had one!!! fail, but not in shorted turns rather that the foil wound part went intermittent on one set.

Another two phenomenal transformers that spring to mind was the TX 10 chopper (EHT transformer in this set), and the TX 9 (hitachi made) lopt.

The TX 10 Lopt proper, a small match box sized transformer that was only was responsible for line scan did fail quite regularly due to a diode failing under load in that set (D831/ and D743 on the later plastic chassis mk3 version), I only ever had one TX 9 lopt fail, even that wasn't a failure of the lopt itself, it was caused by a dry joint which had been arcing for some time and severely burnt and carbonised the lopt between pins, this was on a PC1044 chopper mk3 version. 

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Posted : 07/06/2021 7:51 pm
PYE625
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I think RBM got it right with the A823 LOPT, I certainly have not heard of many (if any) that failed. Unless of course, you know different....

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Topic starter Posted : 07/06/2021 8:40 pm
Nuvistor
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@pye625

They had so much bad publicity about the CTV25 LOPTX that they had to do something. From memory it’s was a Quadrupler not a Tripler so a bit less stress.

 

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Posted : 07/06/2021 9:39 pm
Cathovisor
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@nuvistor

It was indeed a quadrupler, Frank - as far as I know, the only set to use one.

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Posted : 07/06/2021 9:50 pm
PYE625
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An additional fault came to light this evening. Once or twice, the picture dimmed a little and the height reduced. Now, one thing common that could affect the two at the same time is the boost HT voltage, however the width remained largely unaffected.

There is an error on the circuit of 780v being indicated as the boost voltage on the same line as a 600v boost. Impossible, so I think that the 780v is meant to be on the other side of 3R26. A measurement proved this to be the case. A reasonably stable 780v too.

However, the side meant to be 600v was a bit unstable, ranging from 450v to about 550v. My first thought went to 3C51, a 0.1uf mixed dilectric capacitor. Replacing this made no improvement. Looking at 3R26 aroused suspicion as there was a crack at one end in the metal ring. Removing this resistor, I measured about 260k as opposed to 180k which it is meant to be. It fell apart and was replaced. I now have a stable boost of 600v together with a stable height and brightness.

Here below is the point on the circuit where 780v should be (If you look carefully at the circuit, you will see what I mean about the error). I fitted a nice new 180k in the 3R26 position and all is well again.

IMG 5919 50
IMG 5920 50
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Topic starter Posted : 07/06/2021 9:54 pm
ntscuser, Lloyd, Katie Bush and 2 people liked
PYE625
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Now, the next fault took a bit more time. Not really me doing anything, except watching and pondering for a few days the above pictured circuit of the line stage.

On either 405 or 625 operation, the raster would twitch inwards but ONLY on the right side. It seemed to be worse on 625 and this certainly triggered it if it did not first appear on 405. It was a few mm and infrequent at first, but became more or less continual after some use of the set. The boost HT would drop by a few volts, about 5 or 6v, but it could be observed in accordance with the twitching.

LOPT troubles, PL504, PY88, boost capacitors etc etc.... but this would affect the whole raster I thought. So....what else is common to both standards and could affect the right linearity only? Well I temporarily shorted out the line linearity coil (3L7) just to eliminate it and also checked very carefully for dry joints or poor plug/socket connections. Nothing doing. So what could it be?

Hmm.. was thinking scan coils perhaps, dodgy leads etc... but what about our good friends the mixed dilectric capacitors? Looking at the circuit I could see that 3C24 was the only one common to BOTH standards. 3C21 and 3C22 are not in circuit on 625.

3C24 is (or is meant to be) a 0.1uf 600v. This one tested at about 0.16uf, but seemed not to be leaky however it was slow in charging up to 500v. A good 0.1uf capacitor is almost instant. Anyway, yesterday morning I replaced it and since then, the fault has cleared with not a twitch to be seen. The width has increased very slightly, but probably due to the correct capacitor value now in place.

Here, just above right of 3L7, is the replacement for 3C24. Fun and games.

IMG 5921 50

 

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Topic starter Posted : 13/06/2021 10:37 am
Nuvistor liked
PYE625
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It was interesting to note that upon using a heat gun to gently increase the temperature of the old capacitor, slight leakage did become apparent. The measured value also increased to nearly 0.2 uf. I did not make it too hot, it could still be held, but it was very roughly the kind of temperature found within a warm chassis.

I would have thought the above would cause a gradual reduction in width, not the intermittent cramping on the right of the picture. But I expect it was simply breaking down and causing the effect under conditions of line drive to the scan coils.

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Topic starter Posted : 13/06/2021 12:37 pm
Katie Bush
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Posted by: @pye625

Here, just above right of 3L7, is the replacement for 3C24. Fun and games.

My God, that little 'red coat' took some finding!

I sometimes think capacitors are about the most irksome of components, in as much as their innate ability to look alright, but sometimes pretend to be resistors, or to exhibit a different value to that printed on them, and to do all of this, quietly and invisibly, right under one's nose. I've also heard of them behaving like electromechanical transducers (microphony), and on odd occasions, reading as an inductive component.

I remember about 20 year ago, working with my dad on our tractor driven 20kVA generator. It had a capacitive excitation initiator circuit comprised of a single jumbo sized low value electrolytic cap. I said right then that I thought the capacitor was at fault. Granted, it would charge up, it would hold a charge and it would belt you one if you got your fingers across the terminals, but, in a discharged state it had a resistive value. We didn't have the means to measure capacitance at the time. So I poked about on the internet and asked a few questions on various forums and got shot down on every one. The most common line was "Capacitors can't change in value, it won't be that. Go and look at 'this, that, or the other'. I won't be the capacitor!".

After changing the rectifier bridge, having the Orex tranformer tested, examining every inch of wiring, flashing the field windings and testing with external power to the excitation circuit, we eventually replaced that excitation cap with a motor run cap of the same value and rating - Guess what happened next?

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Posted : 13/06/2021 3:22 pm
PYE625
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I had a look in my collection of bits but had nothing the same so used a modern component. That is the red one shown newly fitted in the above photo.

Here is the old capacitor....  0.1uf 600v if anyone has a spare of this type they want to part with?  😉 

IMG 5922 50
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Topic starter Posted : 13/06/2021 3:48 pm
PYE625
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I could have gone and replaced all the mixed dilectric and Callins electrolytic capacitors (a few on the signal panel) in one go but it is more interesting to actually use the set and address faults as they may or may not arise. The only caveat is that the set must never be left unattended whilst powered. But this probably goes for all older equipment. I certainly want to be around if the boost HT capacitor decided to go short. I've only got so many PY88's. 😋 

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Topic starter Posted : 13/06/2021 5:46 pm
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PYE625
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So far I think it is fairly safe to say that my repair to the dropper has been a success. Several hours worth of operation now with the set working well on either standard. The only thing I will say is that between standards, there is a slight difference in height with 625 being less than the 405 setting. But I don't think it is unique to this set, and is certainly obvious on other makes without a seperate 625 height preset equalizer.

 

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Topic starter Posted : 20/06/2021 11:38 am
Lloyd liked
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