B&W TV Murphy V2015D
Any section was a common fault, had each fail at one time. Then again mains droppers in many TV models were a common fault.
The flywheel sync/interlace diodes were also a common failure, replace with individual silicon diodes (1N4148 ? Think that’s right) with series resistors to match the original forward resistance, 15k to 22k, not critical.
It's all a long time ago but it was the surge limiter 14 ohm section that seemed to fail most for me. Is that white material asbestos based around the tags?
Yes, and I believe LLJ mentions the 14r section going open in his article. But I guess any section is a likely candidate, as in many other set's. Asbestos looks to be the material around the dropper tags.
The interlace and flywheel sync diodes are in fact seperate items with resistors, rather than enclosed in a block on this set.
As for the want of the overwind and dropper, any further work with this set has been put on ice for now and I fear it could be for some considerable time. But at least I know that the set as a whole will be worth proceeding with if such items become available.
Hi Chris, the dropper is physically one item and the value's are 250 plus 14 ohms, and an electrically seperate 156 ohm.
(It is the 250 ohm section that is open, seen here below on the left).
Wonder what made me think 2.2 + 68? I was obviously wrong and this is now coming back to me.
The sad thing is, now I'm thinking back to the ads in the pages of "Television" magazine and even as late as the early 80s, I can remember ads for replacement droppers for these.
2.2 + 68 ohms rings a bell from Philips 210 chassis, possibly others. I used to wonder one of the causes of blisteringly hot droppers going open circuit was an oxidation interface between the resistance wire and the mild steel tag collars generally used, rather than the actual wire going O/C. They often looked as if they were simply twisted around a stamped tag under the unglazed coating.
2.2 + 68 ohms rings a bell from Philips 210 chassis, possibly others. were simply twisted around a stamped tag under the unglazed coating.
That's the one. I remember a Philips 210 putting up a mighty struggle in the line output department - turned out the very high value resistors taking the control grid of the PL504 positive (it had a "stabilised" line output stage) had gone o/c.
They often looked as if they were simply twisted around a stamped tag under the unglazed coating.
In this case, they are. And, it has given me an idea. This evening I removed the dropper from the chassis and scraped away the cement coating of the 250 ohm section. The resistance wire was open circuit along several points of the dropper and it was very easy to pull off in chunks. There are lugs at either end where the wire is wrapped around and I plan to rewind new resistance wire in place of the old. Crazy idea? Well, lets have a go...
I have resistance wire of the same thickness and tomorrow, I intend to wind the dropper. Then, if successful, I can coat it in heat-proof paint.
Then, if successful, I can coat it in heat-proof paint.
What paint are you planning to use? Just curious to know, and here's why. I've wondered about patching up the coating on droppers, where the coating has flaked of in sizeable pieces, and, as it happens, I have a can of "VHT" Paint which is intended for exhaust manifolds (It was left over from when my dad rebuilt a tractor engine in 2006). As luck would have it, it's matt black. It bakes hard when heated and ends up a nice powdery black, not unlike the colour of certain droppers (Thorn 1500 comes to mind).
Hi Marion, I had a casual look on ebay and have picked a spray paint for use on stoves and BBQ's. It is heat resistant up to 1100 degrees celsius, so should be ok.The stuff you have should be ideal, but to get an even surface, it might be worth touching up the gaps where it has flaked off your dropper with a small brush using a non spray paint.
This evening, I will attempt to wind the wire. It is 0.15mm thick and I have already cut off just over 260 ohms worth to allow a bit extra at the ends for securing to the tags on the dropper.
Last night I had a practise run and got almost to the end and stupidly let go. Yes, the wire immediately all sprung off again and I need to start from scratch. But it was useful to see that the correct resistance can be obtained with enough room on the dropper to accomodate the new wire.
Here below is a picture of the dropper almost ready to receive the new wire. Note the securing lugs where the wire must be looped and then they may be gently pressed down again to secure it. A small piece of the old wire is still attached and can be removed.
Now, it could all be a waste of time and go up in a puff of smoke when re-fitted. But I thought at least I will try.... You never know, it may work fine.
Well, I have done it. Completely by hand and it was a 'flippin nightmare. For nearly an hour I held this dropper whilst slowly turning it and keeping the wire taught with my other hand. I just need to carefully go around and even out the spaces between the turns of wire a little bit.
The turns are snug on the dropper and looped around the lugs at the terminals. Next step will be to apply the heat resistant paint when it arrives. I have tested it under load with a PSU with about 300mA passing and although warm, the wire seems ok. Importantly, the resistance is approximately correct too.
I have the 19 inch version with original legs. Performs as good as new.
I have the 19 inch version with original legs.
Would they be the V-shaped wire legs as per the Bush version?
Can't quite remember which way round, it was either ML or LM19 legs which clipped onto four metal plates fastened to the bottom of the set by the feet.
Now, I don't know if anyone else has attempted a repair on a dropper like this before.... surely they must have ? But in the grand scheme of things, it is not a too difficult task, less so if the section is of low resistance with thicker wire and fewer turns than I had with the 250 ohm section. The 14 ohm section would be a piece of cake for example. Obviously, you need calipers to measure the thickness of the old wire and obtain new stuff. I noticed a number of suppliers on ebay for resistance wire of various grades.
I can't see how my repair could be worse or better than an original, but of course the proof will be when I come to test it in the chassis. Certainly better than an open dropper at least ! 😊
Hats off to you Andrew for trying this out. How did you calculate (guestimate?) the length of wire to use?
Given that a dropper is a hollow tube, it wouldn't be too difficult to cobble up a winding jig from bits of scrap wood and dowelling, not that this is something you'll be doing every week. Or indeed ever again if it doesn't work!
How did you calculate (guestimate?) the length of wire to use?
I unwound the wire and measured the resistance as I went along at intervals until I got to just over 250 ohms. I then wound the amount onto an empty solder reel.
Given that a dropper is a hollow tube, it wouldn't be too difficult to cobble up a winding jig from bits of scrap wood and dowelling.
That is a good and sensible idea. I certainly won't be doing this by hand again. Far too fiddly and if you let go.... then you have one of them "slinky" springs that travel down the stairs !
If this repair is a success, then there maybe no excuse not to repair an open circuit dropper ever again.
But I must stress that it is only really an experiment at this stage.
I just wish the LOPT overwinds were as easy !