Sobell 1000 DST at long last!!

[1100 man]

Hi Simon,
Your overwind looks identical to both of mine- except your cracks are in a different place

Where did you get the silcone EHT lead? That would be a handy thing to have for several projects. The two brown bits for the two top caps have gone rock hard on mine!

Not done any more on mine yet, mainly as the garage roof leaked over the winter, so my makeshift workbench is covered in woodlice & mould Am gradually making it usable again so hopefully will be able to play with it next week. My red R&TV books all have grey fur coats now!!

Cheers
Nick

[thermionic]

Originally Posted by 1100 man

....

Where did you get the silcone EHT lead? That would be a handy thing to have for several projects.......

Cheers
Nick

A few years ago now at a BVWS swap meet. I wish I bought more than I did, as I haven’t seen that stallholder there since!

I hope you have a better overwind than I have! After it’s initial 48 hour cooking, I re-fitted it and gave it a try................. unfortunately, the same sequence was repeated. I could hear some intermittent ‘ticking’ coming from the overwind, but although I could not see any corona discharge in a darkened workshop, I concluded that my overwind had probably seen better days, poor thing.☹️

When I dismantled the transformer to remove the overwind, I was surprised to see how much rust were on the bolts, it must have been pretty damp in there in the past, although, strangely, the sets chassis is free of corrosion.

I’ve lashed together a voltage multiplier with parts I’ve had to hand. The specified EHT is 16.5Kv, I’ve managed to get just over 14.5kV with a 150pF tuning capacitor from the top cap of the PL504 to deck. There is a compromise between EHT, width and correct boost Volts! Actually, I think I would be better off with the tuning capacitor being 120pF.

It works, and the set has been stable and has been entertaining me with old films in the workshop for the past couple of days without a hiccup. The slightly lower EHT does not appear to make any difference in the brightness. I intend to remake the voltage multiplier in a suitable plastic enclosure and fill it with potting compound for future reliability.

My fingers are crossed for your set!

Cheers. SimonT.

[Tazman1966]

I love the home made EHT multiplier. I have got a few Thorn 1400 and 1500 doublers/triplers knocking about if you need one...

[1100 man]

Hi Simon,

We are obviously both determined that whatever happens we're not going to shell out for a rewind!

I always think that voltage multipliers are a somewhat magical creation Does the value of the capacitors in the 'ladder' have any effect on the final voltage? I assume above a certain value that any further increase does not produce an increase in voltage?

What values did you use?

Do you not feel the tiniest bit guilty though? Think of that poor DY87, cast out into oblivion; it's cathode cold & desolate Never more to work along side it's good friend the overwind to achieve that Herculean task of producing 16.5Kv!

It doesn't seem right that all that can be replaced with a few tiny semiconductor diodes and some caps! It was all so hard in 1963!

Looking at your pictures, have you done something to the primary of the LOPT? It looks like it's got a red outer covering plus some red lead out wires!

Neat workaround though- especially when it's made into a potted module.

All the best
Nick

[thermionic]

Hi Nick.

I used 220pF discs, and they seem to work fine in this situation. I did look at what Thorn had used in their later 1500 chassis, and they used 470pF. I didn’t have any of these to hand!

As the picture is not ballooning at full brightness then I assume that the caps I’ve used are adequate, providing enough current without the EHT drooping.


I think what you see on my Lopty, is the winding which resided under the overwind, which was a swine to remove in my set. I’ve refitted this, and wrapped some tape around it and re sleeved the lead out wires. The original sleeving had hardened somewhat.


You’re right😲. My poor old DY is very upset, but I’ve promised him that when I come across a suitable replacement overwind, I’ll put him back to work. 😜

Cheers. SimonT.

[1100 man]

The next part of the plan is to wind a new sync winding on the outside of the primary. The outside is at much higher potential, so insulation will be necessary.

I'd got to a point, some while ago now, of stripping down the LOPT and removing the innermost two layers from the primary. I'd replaced one of these layers and got the transformer working fine but without a winding to provide a line pulse to the flywheel sync circuit. See post #34 for details.

So tonight the transformer was stripped down again. All the wires are just tacked on so it doesn't take long!

Two layers of clear plastic film (about 0.5mm thick) were wound on top of the primary to provide insulation and about 40 turns of insulated wire wound on top. The wire was stripped from some burglar alarm cable as that's all I had!

This lash up was tacked back together again and the transformer put back into the set.

With this new winding left disconnected, the set was powered up to make sure the transformer still worked. All was well with a boost voltage of 770V and a picture that could be made almost stationary with the line hold.

Connecting the new winding had a dramatic effect on the boost which went up to 820V but nothing else nasty happened. The picture could still not be locked and was just a mess!

Reversing the connections on my new winding fixed that and a perfectly locked picture was obtained, albeit with slightly bent verticals at the top.

The set boost control was wound down to get back to 770V, but over the course of 15mins it gradually crept back up again!

One end of the new winding is grounded and it was this that was causing the rise in boost. It's upsetting the operation of the transformer. Hardly surprising really, having an earthed winding wound over the high potential outer layer of the primary. That's obviously why they put it on the inside originally!

However, this was only an experiment just to see if it would work. My plan now is to wind a new winding with proper materials and place it on the inside nearest the core where it should be! I think there is just enough room to do this with a bit of care.

Considering that the tube showed zero emission on the tester, the picture is not that bad. The focus is not great at the edges and the highlights merge together as the brightness is advanced. But I'll worry about that when the LOPT is properly repaired.

All the best
Nick

[thermionic]

Originally Posted by Tazman1966

I love the home made EHT multiplier. I have got a few Thorn 1400 and 1500 doublers/triplers knocking about if you need one...

Well, thanks to Tazman1966 who very kindly sent me a NOS Thorn tripler, I’ve removed my homemade lash up and fitted this to the LOPT sub assembly.

Mounting the tripler was the problem. On the Thorn chassis, it clips onto the jelly pot transformer which supports it, and also connects it to the transformer by means of a stud. I certainly didn’t want to mount it directly to the metal chassis, so a rummage through the fixings box brought forth a couple of plastic stand offs which had a blind receptacle one end, and a round pin with a locking ‘tang’ the other; you know, the sort you find fixing PCBs to a chassis... so two holes were carefully drilled into the tripler to accept these, and the other ends fixed into 2 existing holes with short self tapping screws.

In order to compensate for the loss of the overwind, some tuning is required. I found that the sweet spot for this new arrangement is approximately 85pF from the anode of the PL504 to chassis, the 2 series connected capacitors can be seen mounted on the LOPT.

It’s all easily reversible if / when a suitable overwind turns up......! In fact I don’t feel too bad about this, as it’s apparent that STC had recently developed the stick rectifier for Thorn to use with their jelly pot transformers in the same year as this set was produced!

Tomorrow I’ll refit the assembly back into the set......

Cheers. SimonT.

[1100 man]

Hi Simon,
That's handy getting a nice new doubler. Fits on the LOPT assembly quite neatly. Mind you, it would have been quite fun to turn your own creation into a little potted module!

Of course, what we both need are Dazzlevision's universal replacement transformers as featured in another thread!

I've stripped mine down again in preparation for making a two layer inner winding. By keeping the inner former as small a diameter as possible, there should be enough room to fit both layers and still get the whole thing inside the existing primary coil.

Kapton tape should be arriving on Monday and should do a better job than the Sellotape currently employed!

Thanks for posting details of your one. Hopefully it will all work nicely and you can post some more pictures of the complete unit in situ and some of the front of the set displaying a picture!

All the best
Nick

[thermionic]

Hi Nick.

Yep, one of those universal LOPTYs would be great👍 However, in the meantime at least the set can be used and enjoyed. I’ve fitted it back in, and it works a dream. My EHT is now 16.5kV as near as damnit, and the boost voltage is within the limits specified for both 405 and 625 with the width about right!

Below, pictures of the lopty re installed, and the set displaying a 405 line still, and 625 line still.

Your job is much trickier than mine has been, and I wish you the best of luck with it. I’m sure you’ll end up with a rewarding conclusion.

Cheers. SimonT.

[Heatercathodeshort]

Originally Posted by thermionic

Well, thanks to Tazman1966 who very kindly sent me a NOS Thorn tripler.

Nice one Tas. John.

[The teleman]

It’s strange , this lockdown has bought out all these 1000/2000 series sets .
Previously I had done the GEC 2001dst without any lopt trouble but the next set I had on the bench was the McMichael 3001dst same chassis as the GEC only this time I have the same over wind fault as you guys seem to have . I do have a couple of old transformers I thought I could use the overwind from but the resistance is different the white overwind reads 117 ohms but the 2 black type only read 67ohms so not really any use so it looks like I will have to go down the same road as you guys and fit a tripler.
The sets pictured are 1st the McMichael 3001 DST
2nd the GEC 2001dst

[thermionic]

Hi Chris.

I think it would be worth trying the black overwind, you may well find it’s ok. The white overwinds seem to crack, indicating that all is not well within!

A look at Dazzlevisions post here, https://www.vintage-radio.net/forum/...d.php?t=165862 shows a universal replacement transformer that was produced for this range of sets. This also has a black overwind, so maybe yours is a later type?

Good luck with it!

Cheers. SimonT.

[Colourstar]

Originally Posted by The teleman

It’s strange , this lockdown has bought out all these 1000/2000 series sets .

The sets pictured are 1st the McMichael 3001 DST
2nd the GEC 2001dst

Those are lovely! Where did you find the McMichael? It looks in great condition. I've never seen one.

Steve

[1100 man]

Those are lovely! Where did you find the McMichael? It looks in great condition. I've never seen one

They are rather nice! Between all of us we seem to be coming up with the whole set!

I think all of these sets, and dual standards in general, exist in tiny numbers. The ones that survive do so by pure luck. By the time they were 15 years old we were well into colour and so most people wouldn't want to keep them 'just in case'.

I'm sure I saw the McMicheal version on Ebay last year, but it was miles away from me or I would have grabbed it. I've never seen another one. It's taken me enough years to find a 1000DS!

I think it would be worth trying the black overwind, you may well find it’s ok. The white overwinds seem to crack, indicating that all is not well within!

Yes, I would be inclined to try one of the different overwinds. The resistance seems to vary wildly between different makes of transformer, but after all they all deliver much the same voltage. (I know it's not quite that simple!) The gauge of wire they are wound with will make a huge difference to the resistance over so many turns.

I assume the white overwinds crack over time due to age rather than being faulty. I think if we can dry them out thoroughly (pass current for a week or so) and then impregnate them with insulating varnish, we might be able to save them from dying in use.

Simon,
That tripler fits in there a treat- I love it's little legs!! I'd be very happy with that as a modification in one of my sets.

So the question is now, are there any other folk out there with a set that uses this chassis? If so, please post some pictures of it!!

All the best
Nick

[Tazman1966]

That looks almost as if it was designed that way! I'm glad to have been able to help

[The teleman]

Well I tried the black overwind and it sort of works , absolutely fine on 625 slightly short of width on 405 but I think I might just get away with it with some careful adjustments
I also forgot to mention I have the GEC 2000 as well
The McMichael was purchased from a local guy only a couple of months ago , I think he gave up on the repair when it suddenly started blowing fuses .

[thermionic]

That’s great news, Chris!

Do keep an eye on the boost voltage though, you don’t want to give the LOPT a hard time!

It may be beneficial if you reduce the Set Boost preset to minimum, and adjust the linearity sleeve for maximum width / good linearity, and then re adjust the Set Boost control for the recommended boost Volts.

On 405, 750 - 770 Volts, and 625, a maximum of 800 Volts, I seem to remember.


Cheers. SimonT.

[1100 man]

Originally Posted by The teleman

Well I tried the black overwind and it sort of works , absolutely fine on 625 slightly short of width on 405 but I think I might just get away with it with some careful adjustments

That's good to know! I suspect it's probably got roughly the same number of turns but wound in thicker wire hence the lower resistance.

What did you do about the coil that's under the overwind? Simon managed to separate his one but I can imagine it was not easy!

I don't know how consistent the width is anyway between 405 & 625. I have found I tend to have too much width at the correct boost setting and have to reduce it by aggressive use of the linearity sleeve. Maybe careful adjustment of the value of each 'S' correction capacitor in series with the line coils might be used to set the width on each standard? I don't know how much the value alters the width, but there must be some reason they used very precise values?

All the best
Nick

[The teleman]

I have the boost voltage set at 750v on 405 but it seems higher about 820v on 625 this I think may be the cause of my width problem
As for the winding under the overwind it just came out without any problems which was a great relief as I was a bit concerned I would damage it removing the old overwind
I also have a fault that looks like line interference only on 405 I thought it was a tuner fault but this tested ok in another set
I’ll try and take some photos of the fault and post them later

[The teleman]

This is the sort of interference I’m getting , it looks like a line generated fault or a poor earth somewhere but I can’t seem to find anything wrong