BRC 2000 Tripler Rebuild

[crustytv]

Hi,

I'm planning on rebuilding a spare BRC 2000 tripler. It has been recommended that Dow Corning silicone be used. I've looked on the internet and found the Dow corning site where a PDF suggests the compound type I'm looking for is Dow Corning 736 or 732 Heat Resistant Sealant/ Multi-Purpose both used for electrical and electronic equipment.

Looking on E-bay there are many Dow corning products but I cannot locate 736, 732 is available though. Anyone have an opinion? Hoping Stan ( electrical ) sees this as he used Dow corning on his 3000 Tripler rebuild but was not sure of the number.

http://www.ebay.co.uk/sch/i.html?_from= ... cone&rt=nc

Dow Corning https://www.dowcorning.com/content/publ ... 0-3258.pdf

[Tony]

The nicest material to use IMO would be something like RTV101 - it'll pour like treacle and fill all the gaps plus as it's a 2-part will set all the way through without having to be built up in layers.

http://www.tiranti.co.uk/EdgeImpactShop/product.php?Product=1256

http://www.acc-silicones.com/content/products/silcoset-101.ashx

Not cheap though, it'll be over £30 with delivery.

[Electrical]

Hello Chris
The potting compound used was a two-part silicon rubber mixture, which was thrown out at work, as it was out of date. All I can remember was that it was made by Dow Corning.
Regards Stan.

[crustytv]

Finally decided on a product and have now purchased my potting compound for this tripler rebuild. I went for the one Tony recommended above, ACC Silcoset 101.

Next stage will be the rebuild, I will document it fully and publish it on the main sites "How To" Section.

[Valvebloke]

Good luck with the Silcoset. I have to say that we used to use their products as first choice when I was a high-voltage engineer at work and we did like them. When I saw your original post I was tempted to recommend one, but the downside of course is that they aren't cheap. The eternal truth applies though - 'You don't get what you don't pay for' - so I'm optimistic about this stuff.

VB

[crustytv]

Hi VB,

I'm pleased your past experience as a high voltage engineer affirms this compound as ideal for my intended application. You're quite right about quality comes at a cost, I considered that in making my choice as I want these tripler rebuilds to last.

[crustytv]

Open Tripler Surgery

This evening I extracted a working VDR from a failed tripler, this will be transplanted into the working one that had the Failed VDR.

I used extreme caution as my fear was to break the very fragile VDR, so took ages to ensure all the silicone was dug out before attempting the extraction. Thankfully it all went to plan.

VDR ready for transplant

[crustytv]

Please forgive the numpty question but I'm not sure and don't want to get this wrong. When replacing the failed selenium diodes I guess its not a straight swap old for new, to rebuild the ladder so what do I need to do to map the BY8410s in there?

I've no idea what the voltage rating of the old diodes were, there appears to be two distinct types in there. Red band and Yellow band

[valvekits]

Re-stuffing triplers is something I’m not familiar with as we didn’t have problems sourcing replacements back then. Judging by the physical size of the replacement BY8410 diodes I would have thought each section could take a couple of diodes in series to improve the peak inverse voltage rating?

Eddie

[Terrykc]

... I would have thought each section could take a couple of diodes in series to improve the peak inverse voltage rating?

I'm quite sure that 10kV is capable of jumping considerable further than the length of one of those tiny things as well!

[valvekits]

I'm quite sure that 10kV is capable of jumping considerable further than the length of one of those tiny things as well!

Agreed, but hopefully the potting compound is going to take care of that.
I recall you had to use your eyes ears and nose to track down some of those little chassis arcing problems, often just the way the EHT leads were dressed were enough to give you grief. Oh and the cans of green anti-corona stuff were a total waste of time, you know you were desperate when you started squirting that stuff about.

Eddie

[linesync]

Hi Chris , I rebuilt an EHT quadrupler for a dual standard bush colour set a few years ago using these little diodes.
I put heat shrink sleeve on them and fitted inside the old selenium sticks and it worked.
The only trouble was the rebuilt unit was producing to much EHT that could not be lowered any less by using the regulation control .
I`ve been told that the new diodes are more efficient than the old selenium ones and that's why its producing more EHT.
Not being able to find a replacement quadruper I gave up and have not switched the set on for a good year.
I`ll be very interested to see how you get on with this and perhaps I may learn something .

Robin

[crustytv]

Hi Robin,

That certainly worries me, how much over? could you not have reduced the number used, to off-set the over voltage? Certainly the 2000 runs happily at 22kV which is where I have mine currently set.

Instead of the little BY8410 I had considered the larger and more robust BY182 ( see photo) but the over voltage may still be an issue then.

I do have some NOS selenium sticks ( no idea of the values though) but only 3 so not enough.

[Tony]

I have plenty of the 3000-3500 sticks, type CSD118. They're about 85mm long, would they fit?.

[crustytv]

Hi Tony,

yes those would easily fit with room to spare as the originals are roughly 120mm long. I guess they would be much better being of a similar era, might have a similar efficiency, thereby keeping the voltage nearer spec.

Chris

[Tony]

I'll put a few in the post tomorrow though how long it'll take them to get there at this time of the year is anyones guess - hopefully before the silicone goes off.

[crustytv]

Cheers Tony, you're a star.

Let me know how to reimburse your costs.

Chris

[turretslug]

Presumably either the BY182s or Tony's selenium sticks are big enough to operate in free space, as it were, without the BY8410's need to be protected from its own tininess? What I'm getting at is, is it worth assembling/tack-soldering the tripler together and test-running it to prove the concept before committing a load of the expensive potting compound to something that proves to be less than optimum. Provided it's done in a dry place, the ozone shouldn't be too overwhelming...

And don't you love things that actually look like their circuit diagrams?

[linesync]

Hi Chris , Yes I`ve been told that if I take one diode out then that should do the job but that means pulling the quadrupler apart after I made a neat job rebuilding it and then the thought of not enough EHT if I went ahead removing the one diode would be very annoying so I just left it as is.
I didn't measure the EHT before the old quadrupler broke down but i now have 26KV which is to much for a 19" crt.
The rebuilt quadrupler will produce 30KV+ and the new little diodes do not arc over.

Robin

[crustytv]

Hi Tony,

Sticks arrived safe and sound a few mins ago. Thanks for them, I have two 2K triplers to rebuild so that's enough to sort both

I can't find any data on these, would you know what end is Anode/Cathode. I believe the double band denotes K but can't be certain.

[Tony]

I just tried one on a 100V current limited supply and the red band is the cathode.

They seem to have quite a high resistance, the short circuit current (with it connected across 100V) was about 2.5mA, the supply was set to limit at 4mA.

Edited to add: The forward voltage drop on the one I'm testing is about 80V

[crustytv]

Thanks for testing, as I said above I had a feeling the red band was K it would follow the convention of the originals. All that's left now is to rebuild and pot.

[crustytv]

Update

Old rectifier sticks removed, I think there was only one that failed as it stunk the most, the others seemed OK, still I replaced them all with the new ones supplied by Tony. Due to these new sticks being shorter, I had to make wire ends, wrap them around and solder so as to make them fit to the solder tags within the body. I knew the NOS box of RIFA's ( all cracked) would come in handy, they supplied the leads. Finally I fitted a recovered VDR and added the lead-outs for final anode and focus.

All that remains now is to mix & pour the potting compound, let set and test.

Old stick removed just leaving the caps

New sticks installed and lead-outs reinstated.

[crustytv]

There's no way I was going to be persuaded to run the tripler un-potted. I checked and doubled checked all connections and diode orientation. Secured the EHT and focus leads with tie wraps then wrapped in self amalgamating tape.

I then weighed and mixed the potting compound, quite nasty stuff by the looks of it, the curing agent classed as poison

When mixed it was thick, gloopy and fairly difficult to pour without some over-spill. Once set ( 48 hours) I will trim up with a sharp scalpel to make the edges tidier.

Next week, test time.

[sideband]

Did you try to pour the compound as smoothly as possible? The idea being to minimise air bubbles and gaps inside the tripler