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17th Jan 2023, 10:33 pm | #1 |
Hexode
Join Date: Jan 2007
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Murphy V310 LOPT rewind
The LOPT failed on a Murphy V310 while I was working on it. I am just a rank amateur when it comes to transformer winding, but a couple of Bush ones I did last year are holding up well so I thought I would have a go at the 310. The surgery worked so here are some of the details for anyone who fancies a go. It is only possible using basic kit to rewire the main coil. If the EHT overwinding has failed that is a specialist job. But that it is not necessarily a problem as you can replace it with a simple tripler as described below.
The main coil bobbin is fairly easy to rewind as the core comes apart quite easily, there is plenty of room on the bobbin for the turns required, and the wire is quite thick. The main complication is that the trannie lives in a can full of oil and will have to go back into the can with fresh oil to work. There is no need to cut or file the can to dismantle. Simply lift the lip of metal with a screwdriver all the way round, then straighten it fully with pliers (pic 1). The transformer then slides in and out easily. Pic 2 shows the contents- main coil is on the left limb of the core in the picture, EHT overwind and space for rectifier heater winding are on the right (heater winding already removed in this pic). In between the two coil bobbins is the dropper resistor for the rectifier heater. Beware the oil in the can may contain toxic PCBs so avoid excessive hand contact with it. Having got the transformer out the can, you will see how undoing two brass clips allows the two U-shaped pieces of the core to be pulled apart. Be careful not to lose the two plastic insulating discs that sit between them . After careful unsoldering of all the lead out wires, both bobbins can be lifted off the core. To rewind I used a simple £25 coil winder which has a mechanical counter (Pic 3). Pic 4 shows the main coil bobbin. Before I had got to this point I had used a simple ‘ringer’ to confirm the EHT winding was ok, and the fault was on the main coil somewhere. Every winding, on the main coil rang poorly, but that did not mean they were all faulty as a short on one winding is often enough to damp the whole transformer. I started where you have to, by pulling off the top winding on the bobbin using the winder to count the turns. This is the ‘hot’ winding which runs between valve top caps. I then used the ringer again. This time all coils rang well indicating the fault was on the winding removed. At this point I could have continued and taken off the rest of the windings and done a thorough job by re-winding the whole thing. However this would amount to about 3 times the amount of winding, and as I am slow at it I decided just to replace the failed winding. This is a decision I may later come to regret! Working from the inside out, this winding’s turns are 35-55-55-55-55-30. 285 turns in all over 6 layers each separated by Mylar tape (insulating tape will not substitute). I gauged the wire with a micrometer as 0.225 mm. I got fresh oil from a radio ham supplier- they do it in two litre bottles. You can test the main coil of the transformer outside of the can and oil but if you do, leave the EHT overwind off the core. I didn’t and running it in the air it arced everywhere and was damaged before I could switch off . The EHT overwind has no extra insulation on it and needs immersion in the oil to survive. Inside the can, all the transformer contacts are within a few mm of the metal can, hence the need for the oil which forms an insulating barrier. I built a 3 diode 3 cap tripler in the can in the space where the overwind had been and this works well. I guess instead of oil you could use other potting compounds, but these expensive, and once embedded you will never get the transformer out again if it fails again. The thing has been running stably for several hours now producing plenty of width, an EHT of 13 KV and a boost voltage of 440V. This compares with 14KV and 470V as given in the manual. I could probably get boost up by a tuning cap across the coil, but it works well enough. I’m guessing that the oil filled LOPTS on other Murphy models are probably very similar so similar rewinding may work. Last edited by Station X; 18th Jan 2023 at 10:34 am. Reason: Added some white space. |
18th Jan 2023, 12:01 am | #2 |
Octode
Join Date: May 2003
Location: Belper Derbyshire
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Re: Murphy V310 LOPT rewind
That’s a smart looking coil winder especially for £25! Where was it acquired from.
I have a couple of oil immersed LOPT’s . I think they are usually quite reliable as the can is hermetically sealed unless the oil is contaminated or leaks out. Christopher Capener
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18th Jan 2023, 8:13 am | #3 |
Hexode
Join Date: Jan 2007
Location: Beckenham, London, UK.
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Re: Murphy V310 LOPT rewind
That particular one seems no longer available for under £42 but here is what looks like a similar one for £24.99
https://www.ebay.co.uk/itm/282054552715 |
18th Jan 2023, 10:13 am | #4 | |
Pentode
Join Date: Sep 2017
Location: Ceredigion, Wales, UK.
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Re: Murphy V310 LOPT rewind
Quote:
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It’s all about the music……. |
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18th Jan 2023, 10:54 am | #5 |
Dekatron
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Location: Kilmarnock, Ayrshire, UK.
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Re: Murphy V310 LOPT rewind
Hi.
Great job indeed. The can incidentally is the same size as a Heinz tomato soup or beans tin which I've used successfully in LOPT rebuilds.
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18th Jan 2023, 1:16 pm | #6 |
Hexode
Join Date: Jan 2007
Location: Beckenham, London, UK.
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Re: Murphy V310 LOPT rewind
Thanks Trevor. Have you rewound other Murphy LOPTs?
The tube cover on the back of my V310 is missing. Any chance you could measure yours for me please so that I can fabricate a replacement? Thanks in anticipation |
18th Jan 2023, 1:19 pm | #7 |
Hexode
Join Date: Jan 2007
Location: Beckenham, London, UK.
Posts: 373
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Re: Murphy V310 LOPT rewind
I think the LOPT failure rate on Murphys is very high Rich. It seems to be something to do with the oil absorbing moisture over the time. Also the materials of the time of which they are made (including paper between the windings) , were not durable for prolonged soaking in oil.
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18th Jan 2023, 1:47 pm | #8 |
Dekatron
Join Date: Apr 2005
Location: North Wales, UK.
Posts: 6,882
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Re: Murphy V310 LOPT rewind
Steve - don't forget the plastic pudding bowl 'modification', though you may get some funny looks in the supermarket...
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18th Jan 2023, 3:37 pm | #9 |
Hexode
Join Date: Jan 2007
Location: Beckenham, London, UK.
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Re: Murphy V310 LOPT rewind
Blimey Glyn, what's that?
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18th Jan 2023, 3:52 pm | #10 |
Dekatron
Join Date: May 2003
Location: Edinburgh, UK.
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Re: Murphy V310 LOPT rewind
Very well done!
Peter |
18th Jan 2023, 6:21 pm | #11 | |
Pentode
Join Date: Sep 2017
Location: Ceredigion, Wales, UK.
Posts: 243
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Re: Murphy V310 LOPT rewind
Quote:
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It’s all about the music……. |
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18th Jan 2023, 6:24 pm | #12 |
Pentode
Join Date: Sep 2017
Location: Ceredigion, Wales, UK.
Posts: 243
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Re: Murphy V310 LOPT rewind
The plastic bowl that comes with supermarket treacle sponge pudding works well too!
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It’s all about the music……. |
18th Jan 2023, 6:40 pm | #13 |
Nonode
Join Date: Jun 2004
Location: Norwich, Norfolk, UK.
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Re: Murphy V310 LOPT rewind
There is no cover attached to the back of a V310, just a hole where the focus control passes through.
David
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18th Jan 2023, 8:10 pm | #14 |
Dekatron
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Location: Newcastle upon Tyne, Tyne & Wear, UK.
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Re: Murphy V310 LOPT rewind
Looks a good coil winder for the money and adequate for many jobs if a bit tedious on big rewinds
Ed |
19th Jan 2023, 1:21 pm | #15 |
Hexode
Join Date: Jan 2007
Location: Beckenham, London, UK.
Posts: 373
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Re: Murphy V310 LOPT rewind
Is there a little bump or something then David, because the tube does potrude beyond the back?
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19th Jan 2023, 1:29 pm | #16 |
Dekatron
Join Date: Sep 2003
Location: Near Swindon, North Wiltshire, UK.
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Re: Murphy V310 LOPT rewind
IIRC, on the V310, the focus control was actually the moulded brown cup shaped cover (phenolic material) that covered the CRT base connector and was screwed to the focus magnet assembly.The three fixing flanges of which often broke, due to rough handling.
Photo here:https://www.radiomuseum.org/r/murphy_v_310v31.html |
20th Jan 2023, 9:39 am | #17 |
Hexode
Join Date: Jan 2007
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Re: Murphy V310 LOPT rewind
Thanks for that Dave. Yes the focus flanges on mine are broken. So that whole thing on the back twists to control the focus?
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20th Jan 2023, 9:42 am | #18 |
Dekatron
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Location: Near Swindon, North Wiltshire, UK.
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Re: Murphy V310 LOPT rewind
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20th Jan 2023, 1:42 pm | #19 |
Hexode
Join Date: Jan 2011
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Re: Murphy V310 LOPT rewind
Hi Steve,
Please discribe your tripler with used values. How do you seal the can after servicing? Many thanks in advance! German Dalek
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21st Jan 2023, 8:56 pm | #20 |
Hexode
Join Date: Jan 2007
Location: Beckenham, London, UK.
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Re: Murphy V310 LOPT rewind
Greetings German Dalek! I used a 3 silicon diode, 2 ceramic capacitor tripler. Here is the source for the diodes: https://www.ebay.co.uk/itm/283196488...Bk9SR8bT5dy6YQ
I used 1000pf 6Kv capacitors because I had them in stock, but 500pf at 5 Kv or greater would be fine. The circuit is simply that as used in the Thorn 1500 and similar sets. I attach a photo. The input is from the feed to the top cap of the line output valve. I took the EHT out on a new piece of cable which I joined onto the existing EHT cable inside the insulated base cover of the now –not- required EHT rectifier valve. The ends of transformer windings are soldered to brass eyelets in the cardboard fixings on the transformer. Once you have removed the EHT overwind, rectifier heater winding and dropper resistor, that frees up enough brass eyelets to mount your diodes and capacitors. They fit in space where the overwind coil was. By judicious bending of leads I did just about get the tripler to work in air without arcing across, but it really needs to be in the oil where it’s fine. Getting the transformer in and out of the can is a bit fiddly with a slight risk of damage, so for test purposes you might like to run the whole assembly in a glass jam jar of oil first. I haven’t bothered to try and seal the can. I think it is generally reckoned hard to seal perfectly because the rubber by now has usually hardened and distorted. This means of course I can’t tip the set on its face to get the case back on. If I did want to seal it a bit I would just tap down the edge of the can lip again with a small hammer and a metal drift Good luck. If you try it I’d be interested to hear how you get on |