Drying out an RBM Pitch Overwind.

Sounds interesting and I will be intrigued to learn of the results. Wish you would add some pics though, all your threads are very interesting ( Bush TV, CRT regen etc) but they would be greatly enhanced with visuals.

No criticism intended just an observation & personal preference

Are there any modern products that are more suitable for recoating the overwind ?

Hi Alastair, What colour is your 2015? I only ask as I have a 2015S in orange with the A793 chassis in it. and it has it's fair share of faults but it's not been on long enough yet to see whether the overwind packs up .

Cheers,Neil.

Are there any modern products that are more suitable for recoating the overwind ?

Epoxy maybe.

There are "potting" grades of two part versions of
Silicone
Polyester
Epoxy

Hi Alastair, What colour is your 2015? I only ask as I have a 2015S in orange with the A793 chassis in it. and it has it's fair share of faults but it's not been on long enough yet to see whether the overwind packs up .

Cheers,Neil.

Hi Neil,
My 2015D is near identical to that 1914 that sold on ebay for daft money yesterday!

Its a grey front with a wooden cabinet....

An update so far--

At 40V and 13W dissipation in the coil, the pitch softened somewhat but was still pretty resistant to removal after an hour.
--I thought,--what the hell, and upped the volts to 50, dissipation to 21W (Probably less since the coil and its resistance being now hot would be somewhat more)--Which if you consider the size of the coil and the mass of the ferrites etc, isnt a Huge amount.....

After about half hour, the pitch was sufficiently soft to remove without excessive force to reveal a fabric bandage-covered doughnut shaped coil. I observed that the pitch was bubbling slowly locally in places, but this got less and less over an hour or so.--I'm assuming that the moisture is vapourising to steam coming to the surface....

There is no burning/hot pitch/insulation smell, just a dusty 'old telly' smell from the coil when up close....

I'm not going to make the coil any hotter than this, I don't think it needs it, although I'll maintain this temp for a couple of hours...
--IF it dies, --well, nothing much lost, it was pretty bad/unusable anyway, and the set itself isnt in great condition!

The pitch is Horrible stuff. Ive taken ages to clean the stuff off my hands. Only thing that seems to shift it easily is 'Brake-Cleaner' I'll not be putting it back on.
I notice that the pitch when cold is very brittle, will chip and crack easily--maybe this is the cause of the issue in the first place!
I will probably coat the coil with a product I have here which motor-rewinders use, called 'Ultimeg' --Supposed to be resistant to moisture and H.V.

Hmmm...

http://www.aev.co.uk/aev_ultimeg_2000.php

They have
Epoxy
Polyester
Silicone
(as I suggested !)
also
Alkyd, Acrylic, Cellulose Acetate (last two as coatings/ masks).

So a family of "varnishes".

Strange combo

While still a little warm last night, I coated the coil with this-- (copied from the maker's website)

U 372
Alkyd
Motors / transformers
High build Alkyd air drying varnish. Available in
pigmented versions as anti-tracking enamels

This stuff is a clearish pretty viscous gluey goo about the same as thin honey and is pretty fast drying, having a xylene solvent base. This morning, I applied another coat. At least its nice and shiny now!

Hopefully this afternoon, I'll take a pic of the transformer and fit it to the set, see if it survived my abuse and if it was successful....

Success!

The set has been running now on 625 line for just over an hour, with no apparent width or EHT issues.

Before the drying, it wouldn't last 15 minutes on 405 lines, and much less on 625 for some reason...

So--to sumarise, this overwind was heated by 8W for 12 hours, then 12W for 1 hour, then 21W for 4 hours.
--On cooling, it was given two coats of that Ultimeg product above to re-seal against moisture absorbsion.

Realistically though--It was the last 4 hours that did the job I believe, as I saw the coil bubble just a little in places which passed after half-hour or so.

After the first hour at 21W I was able to easily remove most of the outer layer of pitch, revealing the pitch impregnated cotton-covered coil,-- that's when I saw the bubbling.

For the last 2 hours, I covered the entire transformer with an old cotton 'Tee' shirt as insulation to cosy it up all nice, this allowed the Primary to heat up as well, and a little pitch dripped from the coil....

I list the dissipation in Watts rather than the Voltage needed, as I'm sure that various overwinds will have different Cold DC Resistances....

Fantastic work Alastair

Thread duly made a sticky.

Ah--Thats appreciated--If it helps someone out--great!

Around Three Hours now--And counting!

The set does have a few issues--The height is insufficient, its down about an inch top and bottom with height cont at full setting--but good linearity and there's an odd effect with the contrast control. --At its full up position, we have what looks to be AGC overload with line tearing and frame slips. I haven't yet investigated either fault

At a normal contrast setting the picture is passable, but a bit 'tubey', although improving--Apart from the frame-fault, not bad considering.

As its sorta watchable--I'm just running it to see how the transformer holds out....

Now--Does anyone have a spare RBM TV125/8 etc overwind--thats faulty in the usual way, as I trashed one from a TV128 not knowing the possibilities of this cure!

there's an odd effect with the contrast control. --At its full up position, we have what looks to be AGC overload with line tearing and frame slips. I haven't yet investigated either fault

I had odd contrast issues on my TV125CU, it turned out to be VR21 ( agc Delay)

Trevor--

Yes, once the thick outer layer of pitch was off, a cloth tape wound 'toroidal style' was visible. Maybe there was something more solid under this-- I don't know I didn't remove it, only wiped it over while hot to remove the pitch drips, then cooled it off.

My TV128 had a greenish grey Plastic overwind, potted in cream coloured epoxy, but had exactly the same fault. I used a hammer and Mole-wrench on it!

Be really interesting to see if a Plastic type can be cured the same way,--but without removing any of its case.

The AGC delay adjustment could well be way off--This was the I.F. that I soaked in Hot Water and detergent!

Hi Trevor, I was, and still am interested in the TV128 but the distance is a bit of a problem now that diesel is as expensive as it is . if someone travelling to or from your area and could get it closer to somewhere it can be collected from easier then I would send you the money for it. but I realise it's asking a little too much for someone to go out of their way for me.

Cheers,Neil.

I think we can call this one a success!

The set ran for nearly 6 hours, with a small break to replace a couple of resistors in frame stage to cure the reduced height.

There were no width or EHT reg issues, the line stage performed faultlessly.

Really good news. Let's hope that this method works consistently well. Instinctively it seems better to heat something from the inside out rather than outside in if you're trying to drive off moisture.

I think this is probably why my recent Bush LOPT recovered itself by boiling off from within, Although Alastair's method is the better way to go, I was just lucky. Certainly offers some hope to owners with these troublesome LOPTS.

Agreed Jeffrey and sods law Alastair turned up with a solution after my troubles, which is why I said was very lucky it could have ended in tears. The LOPT gods were looking after me that day.

One question though Alastair, regarding supplying voltage to the overwind. Would my normal bench supply do? It's only 0 -30V but does have current limit or would I just feed it via a variac?

There is also the option of rectifying the output of a Variac.
If the resistance is low enough the bench power supply is very convenient as it already has meters.

I would stick to smoothed DC from (whatever) psu you have around....

Feeding with AC, you may not dissipate the full expected power in the coil due to the effects of the transformer's Inductance and the AC.

I used an old bench PSU 0-30V, 0-12A, and wired this in series with an old laptop PSU of 20V to get the final stages done at the required 50V. The meter resolution on my old PSU at low current is pretty poor, and was reading approx 250mA although should by calculation be a little over 400mA

Has anyone here got a Plastic and resin potted version of the Bush overwind showing the same fault want to try this out?
--Be good to see if it works with those too.....