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B&W TV Rediffusion "Guildford" RT 194/19 Dual Standard

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WayneD
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Blimey! That's 856 turns 🤪 

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I need 980, but I'm going to stop for the evening. I predicted this would need 17 layers but as the whole thing tapers inwards it's been getting gradually less turns per layer.

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Topic starter Posted : 24/04/2022 9:29 pm
WayneD
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Unfortunately rewinding it didn't work. It started arcing to the bottom of the frame of the LOPT assembly. I could spend even longer trying to insulate everything but I've spent far too much time on this and enough is enough.

I've just removed the whole secondary and rewound the inner winding (22 turns) onto the core and left it at that. Apart from the lack of EHT the set is working.

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I'm sick of unsoldering, dismantling, reassembling and resoldering this LOPT. I'm going to break something else if I keep doing this. 😣 

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Topic starter Posted : 29/04/2022 9:30 am
PYE625
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Speaking for myself, I can manage to rewind a dropper resistor but LOPT's are a different ball game entirely. In future, I will leave this to someone who has done this before, know's the drill and has the correct tooling. I messed up my primary by thinking that I could do it myself.

Top marks for trying it though, there is nothing to loose.

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Posted : 29/04/2022 1:59 pm
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PYE625
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By the way, if you did a search on Ebay for "BRC THORN ITT", you will see an EHT doubler that may be of use should you require it.

But as Chris suggested earlier, Malc may be the kind gentleman to help here.

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Posted : 29/04/2022 5:41 pm
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WayneD
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@pye625 Cheers! I've just won it. 😎 

I've been studying how these work and how to add them into the A640 chassis. Kinda wish I'd just done this first but hey, it's all a learning experience. 

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Topic starter Posted : 29/04/2022 9:23 pm
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WayneD
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While I'm waiting for the EHT doubler from Ebay I thought I'd have a go at making a tripler out of some microwave oven diodes and suitable capacitors. 

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Potted them in an electrical socket box:

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Now, maths-wise you'd expect this to produce 21kV due to the 7kV input from the PL504 cap. However the amount of voltage drop across these microwave diodes could bring it down nearer to 19kV which is what it needs. Lots of experimentation is required here.

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Topic starter Posted : 04/05/2022 9:24 pm
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PYE625
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Interesting !  MK would be proud of you 👍 

Fitted to the wall, it would make a good burgler trap.... They go to switch on a light and ZAP !!!

Mind the threaded brass inserts though, they could attract a charge.

The 625 tuning winding will need to be fitted to the LOPT to achieve correct width on that system, but you should be able to achieve a reasonable 405 scan.

The width may be a little low, I found that with mine when I tried the Thorn 3-stick EHT unit. This could have been because the EHT was too high though, but I didn't measure it at the time.

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Posted : 04/05/2022 9:35 pm
WayneD
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Ever since Maplin closed I've been using these for project cases. They're really cheap too.

I put the 625 winding on (22 turns) before reinstalling it: 

20220427 212350

What I think would've been a better idea would be to remove the whole secondary winding, disconnected the inner 625 tuning winding, potted the whole secondary (leaving the hole so it can be put back onto the core) then rewind the 625 tuning winding away from the main secondary. But, hindsight is a wonderful thing...

I've got a 30kV meter on its way to me so I'll hopefully be able to see what's happening.

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Topic starter Posted : 04/05/2022 10:28 pm
WayneD
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So I thought I'd try out the tripler this evening.

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There's a very handy solder point where the PL504 cap connects to. The earth is connected to the chassis, the screw holding the top of the LOPT cage is perfect.

Switch on and.... 

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IT WORKS!!! 🤩 

Let's feed in a signal:

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I honestly can't believe this worked first time! 😀 

I've been watching (rather appropriately) Do Not Adjust Your Set for the past couple of hours and it's been fine apart from the occasional loss of line hold but that's an existing issue as I've only replaced half of the resistors and capacitors. The general performance is gradually improving. There was a bit of ripple at the beginning which has disappeared.

So:

4x Microwave Oven HVM12 diodes (12 denotes 12kV rating) 

4x 470pF 10kV capacitors 

Wilko socket box and blanking plate

Anode cap and lead salvaged from set's own LOPT

Positive and negative leads salvaged from a scrap mid 00s TV's focus and earth leads.

Total cost around £20

 

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Topic starter Posted : 05/05/2022 9:44 pm
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PYE625
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Now that is a fantastic result !! 👍

I wonder if a smaller version of your doubler could be made and potted into a container that may be bolted (nylon bolts) to where the EHT rectifier valve would have been?

Am thinking a tallish plastic container similar to the height and diameter of the valve and holder....

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Posted : 05/05/2022 9:54 pm
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Nuvistor
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Posted by: @pye625

Now that is a fantastic result !! 👍 

It certainly is. 👍 

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Posted : 05/05/2022 9:58 pm
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Jayceebee
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Well done, good to see another A640 brought back to life. I have a Murphy V1913U kindly donated by Chris which I must get around to looking at. Quick question, what did you use for potting?

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Posted : 05/05/2022 9:58 pm
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WayneD
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@jayceebee candle wax! Seriously! 

Wilko sell a large unscented candle for about £1.50. The insides are very powdery as it seems the manufacturing process melts the outside but the inside is still an unmelted fine powder: 

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Put it in a microwavable bowl and did everything in 1 minute blasts and keeping an eye on it at all times and I used welding gloves to remove it from the microwave, although oven gloves would do if you're very careful.

I did this in three layers, I put a layer of wax in the bottom of the box first. I let that set then I put the tripler in and poured another layer on that:

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And after that set, a final layer on top:

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This was only going to be temporary so I could figure out the voltage and unpot it as required but honestly it worked way better than I expected so I may just leave it as is for now.

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Topic starter Posted : 05/05/2022 10:53 pm
WayneD
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@pye625 yeah, probably using something a bit more professional than candle wax! The main thing seems to be keeping everything as separate and as flat as possible.

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Topic starter Posted : 05/05/2022 10:54 pm
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WayneD
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@pye625 this could be something to use, a Ray Tech Gel box. Screwfix sell them in different sizes. Obviously I can't post a direct link but they're easy enough to find.

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Topic starter Posted : 05/05/2022 11:04 pm
Jayceebee
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Your circuit is a bit different but hey it works. Why the earthed cap on the first diode, maybe you borrowed the circuit from a CTV design? The usual way it's done in Thorn mono sets is 3 diodes, 2 cap ladder for a doubler or 5 diodes, 4 caps for a tripler, no earthing. 

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Posted : 05/05/2022 11:12 pm
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WayneD
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@jayceebee bizarrely enough, I didn't look at any TV doubler or tripler circuits for this. The main thing I did was watch the EEV blog video tutorial on a Cockcroft-Walton Multiplier: 

Then I sketched out the circuit:

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I did flip it over so the wires would route better in the TV though. 

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Topic starter Posted : 05/05/2022 11:37 pm
slidertogrid
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@wayned That is very handy to know ! Thanks for posting. I have done the tripler trick with a GEC in the past but I like the idea of making a bespoke one that is easier to hide in situ! I would never have thought of using candle wax! 😎   

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Posted : 06/05/2022 12:13 pm
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WayneD
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@slidertogrid I suppose wax is what they used back in the day for capacitors and transformer insulation. The dielectric strength of paraffin wax is very impressive!

 

For reference I've done a couple of rough diagrams of the circuits. 

Original:

original EHT

 Modified:

MODIFIED EHT
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Topic starter Posted : 06/05/2022 3:27 pm
WayneD
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I videoed the whole process and uploaded it to YouTube. You get to hear me get excited when it powers up for the first time!

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Topic starter Posted : 06/05/2022 10:51 pm
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