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Drying out an RBM Pitch Overwind.

Pre War 240/405, Post War 405 Line, B&W Dual Standard, Colour Dual Standard, B&W S/S 625, Colour S/S 625 line, Hybrids. Standards converters & modulators, video recorders.
 
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Drying out an RBM Pitch Overwind.

Post by Alastair E » Sat Aug 17, 2013 3:57 pm

Since I read about a few cases of Bush overwind issues It got me thinking.

I have a Murphy 2015D set thats got the usual pitch-covered winding and displays the shrinking width and black-hole syndrome after about 15 mins.

I thought about running some current through the overwind coil to drive out the moisture from within.

My coil from this set reads 116 ohms.

The coil is still fitted to the transformer assembly and I'm connecting the supply to what would be the PL504 top-cap connection on the transformer, which is removed from the set, and the DY802 top-cap connection

First, I applied 20V and this would equate to around 3.5W dissipation. After about an hour, it was barely warm, so I increased the voltage to 30V, which is around 7.5W dissipation, and after an hour it was pretty hot to the touch, so left it overnight at this voltage.

The pitch made a slightly sticky mark on the paper the thing was resting on, but otherwise unchanged, and not bubbling or de-forming.

As the pitch covering on my transformer is rather bubbled in places, I plan to very carefully rise the voltage just to the point the pitch will just melt, so that I can then re-paint it back on after cooling the coil to re-seal it.
I have the voltage at 40V ( near 14W dissipation) currently, and I am monitoring the coil closely.

We see how it goes...... ttt:

 
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Re: Drying out an RBM Pitch Overwind.

Post by CTV » Sat Aug 17, 2013 4:31 pm

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 :thumb

 
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Re: Drying out an RBM Pitch Overwind.

Post by freya » Sat Aug 17, 2013 4:49 pm

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

 
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Re: Drying out an RBM Pitch Overwind.

Post by neil29 » Sat Aug 17, 2013 5:20 pm

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.

 
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Re: Drying out an RBM Pitch Overwind.

Post by Michael Watterson » Sat Aug 17, 2013 6:00 pm

freya wrote: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

 
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Re: Drying out an RBM Pitch Overwind.

Post by Alastair E » Sat Aug 17, 2013 7:40 pm

neil29 wrote: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.

 
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Re: Drying out an RBM Pitch Overwind.

Post by Michael Watterson » Sat Aug 17, 2013 10:41 pm

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".

 
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Re: Drying out an RBM Pitch Overwind.

Post by Michael Watterson » Sat Aug 17, 2013 11:12 pm

Strange combo :)

 
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Re: Drying out an RBM Pitch Overwind.

Post by Alastair E » Sun Aug 18, 2013 10:42 am

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! :thumb

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.... :qq1

 
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Re: Drying out an RBM Pitch Overwind.

Post by Alastair E » Sun Aug 18, 2013 6:19 pm

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....

 
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Re: Drying out an RBM Pitch Overwind.

Post by CTV » Sun Aug 18, 2013 7:06 pm

Fantastic work Alastair :aad

Thread duly made a sticky.

 
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Re: Drying out an RBM Pitch Overwind.

Post by Alastair E » Sun Aug 18, 2013 8:17 pm

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

Around Three Hours now--And counting! :thumb

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!

 
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Re: Drying out an RBM Pitch Overwind.

Post by CTV » Sun Aug 18, 2013 8:23 pm

Alastair E wrote: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)

 
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Re: Drying out an RBM Pitch Overwind.

Post by Alastair E » Sun Aug 18, 2013 8:34 pm

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! :aaj

 
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Re: Drying out an RBM Pitch Overwind.

Post by neil29 » Sun Aug 18, 2013 10:21 pm

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 :bbd . 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.

 
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Re: Drying out an RBM Pitch Overwind.

Post by Alastair E » Mon Aug 19, 2013 7:26 am

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. :aad

 
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Re: Drying out an RBM Pitch Overwind.

Post by CTV » Mon Aug 19, 2013 7:54 am

ppppenguin wrote: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.

 
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Re: Drying out an RBM Pitch Overwind.

Post by CTV » Mon Aug 19, 2013 8:14 am

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?

psu.jpg

 
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Re: Drying out an RBM Pitch Overwind.

Post by Refugee » Mon Aug 19, 2013 11:04 am

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.

 
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Re: Drying out an RBM Pitch Overwind.

Post by Alastair E » Mon Aug 19, 2013 5:54 pm

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 :aac

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..... :)

 
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Re: Drying out an RBM Pitch Overwind.

Post by Spot-Wobble » Sat Feb 01, 2014 10:31 pm

I am about to try the same on my PYE V4 LOPT.
It is currently producing around 10KV but drops down to 8 KV after a short while. It should be 14KV
The EHT drops and the width reduces after about 10 minutes.
I tried removing the pitch off one of the other faulty LOPTs overwind by heating it with a hair dryer.
It heated the pitch enough to remove it easily but without it melting to the point of going gooey.
Obviously need to heat the winding up afterwards to drive off any dampness, then re-coat it.

Andy

 
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Re: Drying out an RBM Pitch Overwind.

Post by freya » Sat Feb 01, 2014 11:09 pm

The lopt in my pye LV20 underwent a bake off just before Christmas, it was put in the fan oven on a wire rack with a tray below lined with foil.
The temperature was increased to the point where the pitch started to run out, after about 4-5 mins it stopped so using an old oven glove I rotated it 45 degrees to allow more to pour out from another pocket, then again until all surfaces had been down.
It does make a good stink that lasts for 2-3 days so if you can do it outside I recommend that.
After doing this the EHT is stable at 6.6 Kv (it was 4.4 Kv before) the focus wandering was also remedied by replacing the rectifier mounting paxoline board with a new fiberglass one.
Nothing was used to recoat the windings.

 
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Re: Drying out an RBM Pitch Overwind.

Post by marc » Sat Feb 01, 2014 11:18 pm

freya wrote:The lopt in my pye LV20 underwent a bake off
Nothing was used to recoat the windings.

Hi Freya,

Wouldn't that leave the LOPT vulnerable to damp ingress in the future ?
I did a Bush TV22 LOPT a while ago but I spayed it with Lacquer to protect it.

Marc.

 
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Re: Drying out an RBM Pitch Overwind.

Post by freya » Sat Feb 01, 2014 11:28 pm

No, even with the amount that came off there is still more that could not be removed from the overwind. The primary and secondary are both cloth taped anyway.


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