Melted mask.

[Heatercathodeshort]

I am in the process of overhauling/restoring quite a rare receiver from the late 1940's. If all goes well, and it looks like it will, a write up about it's adventures will follow.

It is suffering from a very unusual happening and I'm wondering if anyone has any ideas as to solving the problem.

Basically the receiver is in very good condition and was stored in an abandoned 1930's 'scout hut building' from 1960, you know the sort of thing, corrugated iron roof with wooden floor and a pot belly stove in the corner and oh yes, painted dark green. Very dry and cosy.

When I collected it I thought the EHT transformer had caught fire and the insides would just be a burnt out wreck, but I was wrong.

On closer examination it appears that the black probably thermoplastic mask had literally melted and I mean melted, turned to liquid and run down the face and sides of the tube ending in a pond of hard plastic in the base of the cabinet.

The Mullard MW31-7 tube reads 100%, of course it would I hear you shout.
The question is has anyone any ideas about removing it's black Dracula cloak? It looks very sinister.. At the moment I am soaking it in diesel, brushing it on once a day, the tube being placed in an old washing up bowl. It is having a very slight effect and may soften it over time to allow safe removal.

My thoughts are to leave it exposed to the hot sun for a day in an attempt to make it even softer. I expect baking hot summer of 1976 contributed to this mess. It must have become very heated to literally turn back into a liquid state.

Well guys any ideas? It's hard plastic not supple. It must have been a very early example of the use of plastic masks in television receivers.
John.

[Hartley118]

Probably just depolymerised like the drive belts and idler wheels that turn to goo. I’ve no idea what prompts those long strong plastic polymer molecules to return to their original short form, but I think you’re on the right track looking at organic solvents. I’ d be inclined to try something more active like petrol or cellulose thinners.

Martin

[Heatercathodeshort]

Thanks Martin. I must admit I had thought of that but they evaporate very quickly. I will do a few tests. John

[thermionic]

Hi John.

What a mess!! It may be worth trying a hot air gun on it to soften it enough to scrape most of it off, then try cellulose thinners to remove the smaller remains.


Good luck, and I’m now intrigued to see the chassis!

Cheers. SimonT.

[Radio Wrangler]

Sawn-off end of a 45 gallon barrel with something over the top to seal it and dump in a cup of thinners every now and then, letting the vapours soften it slowly. You might lose the outer dag of course, but that can be redone.

David

[high_vacuum_house]

Looks like even the implosion screen has cracked in two!!

Maybe try a solvent like brake and clutch cleaner to soften it. You might try a very sharp knife first to get rid of the larger and thickest areas. This would help get the solvent in to the inner layers of the old mask material and speed the cleaning up.

Christopher Capener

[McMurdo]

I have a couple of cans of cellulose thinners here made by 'Profi powerfix' and sold by Lidl or Aldi, I forget, but it was only a couple of quid or so for a litre. It will soften most plastics (I splashed some on my fluke meter a few weeks ago to my horror and it now has permanent streaks down its front).

You can also get it by the gallon from Brundles but it's closer to £15 per gallon which in my book makes the Lidl tins better value. Maybe you have a special friend at a car paint shop?

[mark pirate]

What a nasty mess
I had an RGD console set that suffered a similar mask meltdown, the actual set itself was way to far gone, but the CRT tested very good.

I used a hot air gun and a pallet knife to remove the majority of it (wearing gloves), then used a petrol soaked rag to clean off the remaining goo.
A very messy job, but very happy to have saved the CRT.

Mark

[Heatercathodeshort]

Thanks for all your suggestions. The hot air gun is probably the answer but impossible due to the local heating of the glass will probably implode the tube. A warm sunny day will at least warm the whole tube and be less of a risk.
I will see how the diesel goes and try some thinners at the weekend and let you know.
John.

[Restoration73]

The old-style Nitromors worked well, this type is still available mainly for the car trade;
https://www.spraygunsdirect.co.uk/in....RItqlpeZ.dpbs

The rubber sealing strip holding the safety glass in on my Decca DM45 has turned
to a similar goo and dripped down the glass !

[Heatercathodeshort]

Just a quick update. After a soaking in Diesel I have discovered what I thought was a plastic mask to be in fact black rubber.
The melted mess was as hard and as shiny as glass but now I have managed to slice lumps off and it is definitely rubber!

It will take a lot of patience and a few hot days to complete the task. It cannot be rushed for fear of it going pop! I'll let you know how it goes. John.

[Sideband]

Originally Posted by Radio Wrangler

Sawn-off end of a 45 gallon barrel

I thought you were going to suggest a sawn-off shotgun when I started to read it....!

[emeritus]

Sticky label remover spray will sometimes shift stuff that other solvents don't touch.

[Heatercathodeshort]

Good thought there E.T. I tried just about everything that came in a bottle but as usual it was patience and care that worked!

The problem was the original mask had literally turned to liquid and dripped downwards. It had then set incredibly hard forming a shell like surface that nothing would penetrate. It was the worst case of mask deterioration I have ever encountered.

It was a somewhat slow attack but this morning the final attempt was successful.
It is a very hot day and the tube was quite warm. For the last week I have been brushing on diesel in the hope that it might seep between the rubber and the glass but it didn't want to know.

Previously with the aid of a 'Stanley' knife I managed to make small slits in the shell like coating. Regular applications of diesel eventually softened the hard rubber and after repeating the procedure over a number of days, managed to slice off all the hard coating allowing the diesel to penetrate the underlying hard rubber. This was very carefully scraped away resulting in the MW31-14C tube being relieved of it's mantle.

Note. If you ever have to carry out this nasty operation, great care must be taken to prevent any stress to the glass envelope especially the point where the cone meets the face plate. This is the weakest part and is why much later rimband tubes have the projection band in this position.

I used the rays of the sun to warm up the entire tube. local heat will almost certainly implode the tube with disastrous results. I wore eye protection and covered the tube with a thick blanket exposing only a small area at a time. Just take great care!

The pictures show the tube almost rid of it's rubber rain coat, the finished tube [pic taken this morning] and the piles of rubber shavings.
This is madness!!
Oh well we will have to see how it performs when I finish off this rather rare receiver from 1948. Oh by the way. It has mains derived EHT. You have to be brave when bringing these old beasts back to life.. John.

[Electronpusher0]

Originally Posted by Heatercathodeshort

Note. If you ever have to carry out this nasty operation, great care must be taken to prevent any stress to the glass envelope especially the point where the cone meets the face plate. This is the weakest part and is why much later rimband tubes have the projection band in this position.

I used the rays of the sun to warm up the entire tube. local heat will almost certainly implode the tube with disastrous results. I wore eye protection and covered the tube with a thick blanket exposing only a small area at a time. Just take great care!

Well done! sounds rather like defusing an unexploded bomb.

Peter

[Panrock]

That tube was very lucky to find itself in the hands of an expert. The saga could have had a different ending.

Steve

[Heatercathodeshort]

It was very close Steve! J.

[thermionic]

Originally Posted by Heatercathodeshort

........ I wore eye protection and covered the tube with a thick blanket exposing only a small area at a time. Just take great care!......

Well, I’m surprised to read this...... coming from a Man who is known to bite the neck off a tube. ����


Seriously, well done John! It just goes to prove that there is no substitute for patience and elbow grease!

Looking forward now to the restoration of this secretive beast!

Cheers. SimonT.

[Heatercathodeshort]

It's a very shy beast Simon but hopefully the TV guys will find it interesting. J.

[Duke_Nukem]

Originally Posted by Heatercathodeshort

It's a very shy beast Simon but hopefully the TV guys will find it interesting. J.

Cant' wait ! It'll make a nice change to read about a Bush TV24 instead of the usual TV22

He-he, maybe start a sweepstake ? When I first saw the state of it I immediately thought Dynatron, but weren't they all afflicted with M*zd* CRTs? So lets see ... the first Peto Scott ?

TTFN,
Jon