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Old 18th Oct 2018, 10:00 pm   #1
1100 man
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Default Murphy V410 Oil filled LOPT surgery

Having decided that the heater winding is tracking down to ground within the LOPT can, the logical thing seemed to be to take it apart and fix it. Well the logical thing is probably to use a stick rectifier in place of the U26 but I can't resist finding out what's in the can!

The overwind could probably do with a drying out session anyway, as it would seem that even oil filled transformers suffer from moisture ingress.
So I donned the surgical gloves and removed some of the black filth covering the leads and transformer can so that the wires could be unsoldered. It didn't take long to remove it from the chassis.

I removed the filler plug from the top which looks like the end of a pop rivet, and inverted the can over a container. A few drops of quite pungent yellow fluid emerged. The rubber top of the can where the wires come out is still very supple and pressing this repeatedly caused the oil to come out in a stream. Eventually the thing was empty and quite a lot of oil was in the tray.
I've smelt that smell before but I can't think where.

So in the absence of a better plan, I'll go with John's method and cut the can about an inch from the top. There must be a fair bit of insulation (or oil filled space) between the windings and the can to prevent electrical leakage to the can. Hopefully, if I cut just through the surface of the can with a junior hacksaw, no damage will result.

I should get time over the weekend to have a go. I'm not sure yet what the best way to re seal the can is as it has to withstand potentially hot oil. I'll have to think about that.

The picture shows the cleaned up can ready for surgery. The filler hole can be seen at the top.
All the best
Nick
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Old 18th Oct 2018, 10:25 pm   #2
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Default Re: Murphy V410 Oil filled LOPT surgery

I'll watch this thread with interest as I may have to do the same with my V310 loptx.

I've attached a photo of an opened V310 loptx that I found a while back (it's not mine), pity I cant remember where! Hopefully it will help you decide where to cut, assuming the V410 and V310 shares the same part?

Regards
David
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Old 18th Oct 2018, 10:54 pm   #3
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Default Re: Murphy V410 Oil filled LOPT surgery

Hi Dave,
That's a really useful picture and looks to be the same as the V410 one. I see that one has been opened at the top by un- bending the lip. Cutting the can an inch down from the top looks to be quite safe- it's really handy to be able to see what's in there!
The heater winding looks pretty crusty in the picture on the right hand bobbin. I can't work out what the rectangular thing is between the two bobbins though. It's looks like it has the top cap connection for the 30P4 going to it.

I've been following your exploits with the V310 with interest. I also had a 30P4 die after 5 minutes for no good reason while I was doing the initial testing on the set. Suddenly the line whistle stopped and smoke poured off the temporary resistor that was feeding the screen grid!

Good luck with yours
Cheers
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Old 18th Oct 2018, 11:49 pm   #4
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Default Re: Murphy V410 Oil filled LOPT surgery

If you file the top lip at 45 degrees angle all the way around, the whole inside will pop out. It can be sealed up again using pu seam sealant.
Much easier to seal than cutting the can itself.
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Old 19th Oct 2018, 5:01 am   #5
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Default Re: Murphy V410 Oil filled LOPT surgery

By chance I got a few old magazines yesterday including 'Television', March 1972. There is a reader's letter from Mr John J. Widden of Aberdeen concerning replacing the U25 EHT rectifier in a Murphy V280C.

I have no idea how similar the line output transformers are, but he says :

'If the curved-in edge of the transformer can is bent straight with a pair of thin-nosed pliers the transformer assembly can be lifted out of the oil. When soldering the wire leads of the new valve it is important to avoid any solder spikes as corona can occur even in the oil. After repacing the transformer in its can it is only necessary to re-bend the edge of the can gently (to avoid fracturing the metal) with the pliers : a final seal can then be made around the edge with "bath sealant" which sets like plastic rubbber.'
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Old 19th Oct 2018, 8:11 am   #6
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Default Re: Murphy V410 Oil filled LOPT surgery

Some good ideas here! J.
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Old 19th Oct 2018, 8:18 am   #7
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Default Re: Murphy V410 Oil filled LOPT surgery

Could you use a glass jar with a screw top instead of the can?
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Old 19th Oct 2018, 12:19 pm   #8
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Default Re: Murphy V410 Oil filled LOPT surgery

It would need to be a plastic screw top to avoid insulation issues.
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Old 20th Oct 2018, 11:00 pm   #9
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Default Re: Murphy V410 Oil filled LOPT surgery

Well I'm in!! Actually, it was much easier than I thought it would be. Having considered the various options, I decided to try and un- bend the top lip. This was really easy with a screwdriver and then finished off with side cutters.

The whole thing was then easy to slide out of the can. Having previously drained all the oil out, it was really not that messy although I did wear gloves as I am still not sure what the oil actually is. It seemed sensible to keep contact with skin to a minimum!

The construction is similar to the V310 picture posted by Radio Dave, but the overwind and heater winding are different. In my one, the heater winding for the U26 is a single turn of wire round the base of the overwind. As can be seen from the pictures, there is a lot of carbonisation around this area. The brown supporting structure is resin bonded paper and the surface layer had gone black and crispy.

The thick yellow wire to the U26 base is actually two core double insulated cable. The end of this had also gone crispy so I cut that back. I removed the single turn heater winding which is just a piece of bare wire in a piece of systoflex. The ends of this were also cut back. The black areas were carefully scraped to remove the carbon and everything cleaned up nicely without anything disintegrating!

There is a layer of what seems to be carbonised oil on the inside of the can near to the EHT overwind. This will scrape off with a screwdriver- it's quite thick.

Before reassembly, I will leave the overwind 'cooking' for a couple of days- ie, hooked up to a power supply to hopefully drive out any moisture.

Thanks for the various suggestions for opening it up and then resealing it again. The jam jar idea would have been interesting!

So far so good!

Cheers
Nick
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Old 21st Oct 2018, 12:05 am   #10
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Default Re: Murphy V410 Oil filled LOPT surgery

I had a similar problem with my Bush A640 chassis fitted in Rediffusion cabinet.

There was charring on the overwind and paxoline substrate used to support the terminals onto which the windings are soldered. The insulation sheet beneath was "holed" where the arcing had occured.

The charring had originally started on the paxoline substrate and had carbonised the interleave insulation revealing the enameled winding.

After I removed the paxoline substrate, I used a sharp stanely blade to de-carbonize the obvious and then used switch cleaner and a tooth brush to get down into the windings. I dried the whole with a hair dryer and filled the void with high voltage insulation varnish.

I cleaned off the paxoline in much the same way.

I then used very thin paxoline sheet, used by Philips to insulate the V2000 power supply PC from its metal chassis, to replace the "holed" piece which covere the underside of the terminals. This was more difficult than I thought as it wouldn't cut easily. In the end I used very sharp scissors and stanely blade to punch out the required shape.

Before offering the thin insulation under the paxoline substrate, I again used HV varnish to protect the assembly.

After soldering the LOPT wiring loom back onto the terminals some more varnish and after a quick blow and dry, all was well.

I did wonder tropicalising the LOPT with varnish but have decided to store it and another on top of the CH boiler over the winter period. As the boiler is housed in a cupboard, I shall be placing a large notice warning anyone caught drying shoes etc will loose their right to eat at the family table!!

I hope my experience is helpful and wish you good luck.

PS One other thing, I notice HV transformer oil is for sale on the European Union edition of eBay.

Chris

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Old 21st Oct 2018, 9:52 am   #11
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Default Re: Murphy V410 Oil filled LOPT surgery

Hi Chris,
Thanks for sharing your experiences. Resin bonded paper insulation products whether Paxolin or SRBP circuit board have been a constant source of carbonisation and burn up's in TV's for years! I guess they are cheap and perform well enough though. It must have generated huge revenue for the TV repair industry over the years!

Shell 'Dialla B' mineral transformer oil is available on Ebay from the UK in small quantities.

However, that throws up a big question. In my reading of multiple threads on oil filled LOPT's, "never use mineral oil as it will not mix with the existing oil and will damage the materials of the transformer" has appeared several times.
Vegetable oil is recommended. Ex member 'MurphyV310' seems to have been very knowledgeable on the subject and I believe he worked for RBM for many years. It's his recommendation.

So what oil to use? I imagine vegetable oil would tend to 'go off' over time and probably will absorb water? The Dialla B might damage the transformer, but would be much better as it's designed for transformers.

I've just re-read this thread: http://www.vintage-radio.net/forum/s....php?p=1000220
Where Mikey405 refills his Philips LOPT with Dialla B. Is it still working I wonder? The thread also discusses the possibility of the oil containing PCB's and the hazards associated. The conclusion seems to have been that it might well be likely that it was PCB based as it dates from the late '50's. There are a few basic tests suggested that would indicate whether it was PCB or not so I will conduct these.

I think it's advisable to be aware of the potential hazards we face when dealing with this old equipment.

Cheers
Nick
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Old 21st Oct 2018, 9:10 pm   #12
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Default Re: Murphy V410 Oil filled LOPT surgery

The link in the above post contains an interesting article about PCB's and is worth reading.
It gives two simple tests to indicate the presence of PCB's in transformer oil.

1) Density. PCB's are denser that water whereas mineral oil is lighter. So a sample mixed with water will tend to sink if it contains a high concentration of PCB's.

2) Burning in a flame. If a sample is burnt on a copper wire in a gas flame, the chlorine content will turn the flame green. Mineral oil does not contain Chlorine so will not colour the flame.

I mixed a sample of my oil with water and shook it and then let it stand. The oil quickly floated to the top so that was a good sign.
I've not tried test 2 yet, but I did soak a piece of paper in the oil and set light to it. It burnt vigorously which would also suggest it did not contain high levels of PCB's.

However, as someone pointed out, even if it doesn't contain PCB's, it may be hazardous anyway so I think that the less that gets on my skin and into my lungs the better!

All the best
Nick
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Old 25th Oct 2018, 11:41 pm   #13
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Default Re: Murphy V410 Oil filled LOPT surgery

It turns out that running the LOPT in free space with no oil was not my brightest idea! What started as a fairly straightforward repair has now turned into a much more major exercise!

I wanted to see if there was any further leakage within the transformer and to see if the overwind was going to behave or get too hot.
I therefore rigged the naked LOPT on some longer wires and supported it so that the set could be powered.

The single turn heater winding terminates on two solder studs mounted on the thin paxolin sheet. There was a lot of hissing coming from this area and when the brightness was turned down the EHT increased to the point where a big spark jumped from these studs to the core.
As these studs had been in the charred area, I decided to omit them and solder and heatshrink the lead out wires directly to the winding. This was rather fiddly to do but solved the problem. No hissing & no sparks- all was nice and quiet.

The EHT was rather low at just 10Kv but then the HT was low at 170V due to the tired selenium rectifier. My fatal error was to give it the full 200V HT via a Silicon diode (& series resistor). A big fat arc appeared between the input wire to the overwind and the core. This carbonised a channel through the edge of the coil former and that was the end of that!

So I've had to partly dismantle the thing to separate the core halves and remove the overwind so I can repair it.

It's a pig of a thing to work on- all those bits of paxolin you can see in the pictures contain solder rivets where the lead out wires join the coil wires. The paxolin gets in the way and all the bits are held in by the core. Add to that the PVC lead out wires have turned very hard and the whole thing is oozing oil! Working with disposable gloves adds to the fun and all the tools get covered in oil too...
After an hour of messing about, I managed to get enough bits out of the way to be able to remove the overwind complete.

So I now have to repair the damage and then manage to get it all back together again! Arrgh

Lesson learned!

All the best
Nick
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Old 26th Oct 2018, 12:45 am   #14
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Default Re: Murphy V410 Oil filled LOPT surgery

That looks like a disaster.
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Old 26th Oct 2018, 9:03 am   #15
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Default Re: Murphy V410 Oil filled LOPT surgery

Quote:
Originally Posted by Refugee View Post
That looks like a disaster.
Well, hopefully not! I'm confident at this stage that I can repair it. I had a similar problem where the paxolin coil former on a dry LOPT had burnt in a similar way and become conductive. I made a new former for that which cured the problem.

In the picture, there is a small black channel that has been burnt in the paxolin- it's only 1mm wide. It should be possible to clean this up and then add a thin layer of insulation between the former and the core. It's all so oily that working on it is not easy!

Cheers
Nick
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Old 26th Oct 2018, 10:53 am   #16
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Default Re: Murphy V410 Oil filled LOPT surgery

My word, you do like a challenge don't you
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Old 26th Oct 2018, 1:08 pm   #17
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Default Re: Murphy V410 Oil filled LOPT surgery

It is a boosted challenge but as long as the arc did not got into the overwind still fixable.
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Old 26th Oct 2018, 4:50 pm   #18
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Default Re: Murphy V410 Oil filled LOPT surgery

Take your time Nick and a successful result of the surgery will feel all the more satisfying for it.
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