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B&W TV Murphy V410

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crustytv
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Posted by: @nuvistor

what oil can you use as a replacement? I checked online and only found transformer oil available in gallon or 25 litre quantities and it was expensive.

Trevor has pretty much written the book on Murphy's, covered the oil question on numerous occasions, posting both here and on UKVRRR. In addition, he wrote a fairly comprehensive article for the blog, covering all manner of Murphy hints & tips.

https://www.radios-tv.co.uk/vintage-electronics-blog-forum/servicing-the-murphys/

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Posted : 30/07/2021 11:48 am
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PYE625
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Indeed he has and I have read it several times. A very useful guide if I may say too.

One type of oil for consideration is DOT5 silicone brake fluid. It appears to be entirely suitable electrically and chemically. But then so is regular vegetable oil. Obviously any oil that damages rubber or neoprene is out of the question, mineral transformer oil for a start. I at least know the approximate quantity of oil that the can will hold, not dissimilar to my Murphy V280C LOPT as it is the same size.

The main problem is getting rid of the voltage leakage at the moment and I need to give it more time before I consider opening up the can for examination, should it be neccessary.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 30/07/2021 2:00 pm
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PYE625
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This evening fate stepped in. Whilst moving the LOPT around, the red wire suddenly became very loose. My hand has been forced and I had no option other than to open the can. Drat. But onwards and upwards.

This was extremely easy actually, simply using a small flat screwdriver to bend back the lip all around the edge and out popped the transformer. I could see immediately that the red wire had come away from a solder terminal and this was soon put right.

As I hoped for, there is no sign of any burning or damage to the windings or connections. The EHT rectifier heater winding solder terminals are covered with a varnish it would seem (last pic below). I may consider removing this arrangement and have a single turn of EHT cable around the overwind for better insulation.

Like I found with the LOPT in my V280C, there is definate leakage between connection terminals on the fibre support material.

So into the oven for a bake at 60 degrees and I hope this time, being fully in the warm open air, the thing will dry out.

IMG 6057 50
IMG 6054 50
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IMG 6056 50

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 30/07/2021 8:10 pm
PYE625
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I am finding that it is drying out and the leakage becoming much less. The EHT heater winding was quick to become over 2 giga-ohms, probably because there are only two terminals to consider. The main winding is about 1.5 giga-ohms to the grounded core, even when warm, so it is clear that the transformer needs to be in full circulation of warm dry air to get it to dry effectively.

The leakage resistances may seem high, but it has to be borne in mind that at voltages the transformer will operate at, it becomes a problem that must be solved. Ideally of course, infinate resistance is wanted but may never be achieved in reality.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 01/08/2021 10:45 am
Lloyd
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This thread has reminded me that I have a Murphy with an oil filled LOPT, a V250C. It still needs full restoration, but I have run the set in the past, and it’s been OK, but I guess it could pack up at any moment.

I wonder if it’s possible to wash off any old oil residue, either with fresh oil or white spirit? Certainly getting rid of the old oil looks to have been the way to go, hopefully baking it will finally get rid of any residual moisture now.

 Regards

Lloyd

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Posted : 01/08/2021 11:13 am
PYE625
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Hi Lloyd, I would do nothing to your transformer (other than an external clean) until it can be confirmed with an insulation tester that you have leakage. It may be perfectly fine and left intact. If it has not leaked oil, then even better. I would not be tempted to use any cleaning agent other than a dry cloth or cotton buds on the exterior neoprene top.

In my case, I have not used any rinsing or cleaning agent on the actual transformer assembly. If I do use DOT5 to refill, I will firstly use a quantity to soak the assembly maybe with some warming/cooling cycles, then discard and refill the can proper. This should rinse away much of the old oil.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 01/08/2021 11:22 am
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PYE625
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One thing I should perhaps mention when considering a leakage test on this, and probably other LOPT's too, is not to use a voltage that is much greater than the boost HT of the set.

The reason is that, certainly in this LOPT anyway, the winding that the boost HT is derived from starts near the core of the transformer. This means that it will not see a voltage as high as the outer windings on the primary. Thus, a slight leakage here is not so important as say the overwind insulation to the core would be.

To apply an excessive voltage could possibly damage the insulation between the primary and grounded core.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 01/08/2021 9:42 pm
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PYE625
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Using another insulation tester that will read up to just over 4 giga-ohms, I can confirm that at room temperature the leakage is above this limit on the EHT, overwind and main winding to the grounded core of the transformer. At 50 degrees centigrade, the EHT and overwind are still easily over and the main winding on the primary alone is about 2.5 giga-ohms to ground.

This to me is enough proof that the leakage is rapidly coming down with the transformer in open air. In fact, I doubt it will be much better. It is a massive improvement compared to when I first tested the LOPT. It is further proof of the NEED to remove such leakage due to moisture ingress before use and avoid damage or destruction of the insulation.

At 500v, the voltage roughly that of the boost HT on the winding closest to the primary core, I can't grumble at this value.....

IMG 6060 50

This meter came from ebay today and was just under £30. It is also very useful, having available a 1000v output, for testing of higher voltage capacitors for leakage.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 02/08/2021 8:03 pm
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PYE625
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Having installed the LOPT complete with new DOT5 oil back into the set on a temporary basis, I am very pleased with the results thus far. The oil would appear completely satisfactory after some hours of use this afternoon. The can of the LOPT is only warm and there is no longer any voltage leakage. The EHT is fine at just over 14KV, the HT of the set is a bit low due to the rectifier, but there is still plenty of width available. More so now the LOPT is working without the burden of the voltage leakage previously.

I need to find an effective soloution to re-sealing the can of the LOPT and then it can be installed without the tape around the top. One or two ideas are floating around here. I don't want to use silicone sealant as I did with my V280C LOPT.

IMG 6061 50
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To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 07/08/2021 4:48 pm
PYE625
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After some total of about 10 hours of irregular use, I am satisfied that the LOPT is working perfectly well with the new oil. I removed it just now to drain and inspect the internals. All is well, so in back went the oil and coil.

Poetry I think....

The oil and coil will not spoil, even though I did boil. Mr Murphy will not foil, now I won't go up the woil. (Wall).

OK, enough nonsense, I did attempt to seal the can using large diameter heat-shrink sleeving. This was of no use as I was unable to obtain a tight enough grip around the top rubber seal portion around the rim.

Next, I will try washers and a large (70mm) capacitor clamp with the lugs bent over the top to apply a good grip on the washer. More on this when it arrives, but a clamp with an inner flange would be ideal... Can't find one though. Wish I could make one.

Here below, you can see I have removed the sharp and slightly mangled upper lip of aluminium that folded over the top rubber seal. Then, heat-shrink sleeving that proved unsatisfactory. A method of applying even downward pressure onto a strong washer (last pic) around the seal should be ok. Furthermore, it will enable the can to be opened again with ease. Providing my plan will work of course !

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To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 10/08/2021 5:23 pm
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PYE625
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This picture may make it easier to visualise the idea of a circular clamp sitting in the groove of the can with lugs coming upwards and angled over in order to grip and pull downwards the washer to seal the rubber. The washer is a large shim washer of suitable size I bought from ebay.

IMG 6067 50

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 10/08/2021 5:54 pm
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Lloyd
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I’ve seen some interesting contraptions holding the lids on the paint cans at work, it’s like a ‘U’ section that is formed to the diameter of the paint can, with a sort of clip thing that pulls it tight, gripping the rolled edge of the top of the paint can and the edge of the lid. I’ll see if I can find one at work tomorrow and get a photo of it. It’ll be too big for a LOPT can, but I wonder if such things exist for the smaller cans of paint?

 Regards

Lloyd

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Posted : 10/08/2021 6:43 pm
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Lloyd
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Found one!

072846CB 8742 494A 81FF 833B211947CB
96F20144 D155 460B A004 67F4CCED4285
24BBC87E A6A2 4619 8A00 04DB5AC223D5

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Posted : 13/08/2021 7:24 pm
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PYE625
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If that could be obtained in 65mm Lloyd, it would be perfect ! I wonder what it's exact name is?

I have found with what I have available, that the best method was to use the large (65mm) capacitor clamp with the lugs shortened and bent over on the inside plus a clear perspex ring to act as a washer. Both items are available on ebay, the perspex ring being laser cut to the exact size upon placing an order. It is 5mm tall so thick enough not to bend too much when squeezed down by the clamp lugs.

I have shaken it, rolled it, held it upside down and the LOPT is not leaking. It is now back in the set and will have a good test with actual use.

So far then, it would seem like DOT5 silicone brake fluid oil is a perfect replacement to fill the LOPT can with. I am sure regular vegetable oil is OK too, but I am not sure as to whether it would go off after a time. I know it has a "best before" date, but this is for food use of course. Personally, I think DOT5 is a better choice.

IMG 6070 50
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To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 13/08/2021 7:40 pm
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PYE625
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This morning I was greeted by a new fault upon switch on. It took the form of distorted verticals and I thought perhaps it was hum introduced by lack of smoothing, maybe the main electrolytic had died? But there was no hum on sound. Furthermore, the hum bars could be "tuned" up and down the picture by adjusting the line hold control. In any case, the ripple level on the main HT line was normal so no problem here.

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So what could it be? Well as it only seemed to affect the line stage, I started looking at this section. I tried replacing the line stage valves one by one in case it was a heater to cathode leakage, in particular the line oscillator V13 a 6/30L2. Not this, nor the 30P4 line output or the U191 boost diode. So my next suspect, before getting the 'scope onto some line stage waveforms, was to simply bridge C99 with another capacitor as a quick check to eliminate this as I was suspecting excess ripple. To my delight, the fault cleared. Yes, C99 proved to be completely open circuit and so lack of HT smoothing on the supply to the line oscillator was the reason. Here below is C99, a 32uf shown upper left on the circuit. This capacitor had previously reformed very nicely and was a rather nice specimen of a Plessey capacitor. Shame, but there we are. I replaced it with a very similar one I had, but this one will be re-stuffed so I have a spare.

IMG 6081 50
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To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 14/08/2021 11:59 am
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PYE625
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Hot on the heels of this mornings fault, I thought it was high time to take a look at why the sound is quite low. Checking the voltages around V10, a 30FL1 used for sound output, nothing much seemed to be amiss. Changing the valve made little difference either.

However, 'scoping the signal at the anode of the triode of V10 it was quite healthy, but not much audio content was getting to the pentode grid. Similar at the top of the volume control too. After looking at the circuit and reading the circuit description reproduced below, it was soon discovered that MR3 (sound interference limiter diode) was pretty much useless as a diode and resembled a two meg resistor. After reading the description of operation in the circuit of MR3, this would explain why, as the set warmed up, the sound was louder and distorted with the occasional farting noise thrown in for good measure.

Screenshot 2021 08 14 175614
IMG 6079 50

The diode will see a reverse voltage of full HT when the set is first switched on so I tried a 1N4007 as a replacement. When tested, the sound was silent until the line stage started to supply the boost HT and then the sound came up very good and loud as would be expected for normal operation.

So, as the old diode was a series of selenium discs inside a small plastic tube, I discarded these and sealed the 1N4007 with black wax inside the small tube. This must be the first time I have re-stuffed a diode !

Here it is, centre of pic...

IMG 6078 50

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 14/08/2021 7:39 pm
Lloyd
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Ah, one of those diodes! I’ve seen them in a few of my sets too, mostly the Ekco’s, and so far I’ve not had a duff one, well not that I’ve noticed it being duff yet! Good to know a 1N4007 will do the job.

 Regards

Lloyd

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Posted : 15/08/2021 11:14 am
PYE625
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Hi Lloyd, they likely come in differing voltage specs, although sharing a similar style. Probably a general purpose rectifier, but always worth checking local voltage conditions before choosing a modern replacement if needed. A 1N4007 is usually a pretty safe bet though.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 15/08/2021 11:35 am
Nuvistor
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In sound limiters that’s are used in 405 line TV’s the 1N4007 would be fine, in other circuits careful choice is required due to the selenium forward resistance being 5 to 20k ohm depending on the diode. In some circuits that higher forward resistance as been factored into the circuit and a silicon diode can give unusual effects. 
Of course you can still use a silicon type, just add a suitable resistor.

Frank

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Posted : 15/08/2021 3:09 pm
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PYE625
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Electrically speaking, the set is more or less working as it should. A minor exception being the main metal rectifier (MR4) which is causing the HT to be about 20v down on what it should be. That is a fair bit, and I am sure that in some sets, I could not get away with it. The device is a Westinghouse LW17, but as it is not seriously defective, it will remain in use. If the width is set to maximum, the raster fills the screen and is satisfactory. No other adverse effects seem to result from the lower HT, even though of course the boost HT is reduced too.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Topic starter Posted : 15/08/2021 4:46 pm
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