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Old 28th Jan 2022, 2:30 am   #1
markfff
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Default Warped and yellowed dial cover

I found this little gem today and am rather pleased with myself. It's an FM tuner at least 70 years old and nearly pristine other than some minor scratches, and a warped and yellowed plastic cover over the dial. All the parts appear to be original, including the tubes. And it not only has the original knobs, but the felt pads underneath.

The problem is the plastic cover. I tried plastic cleaner and headlight restorer on a hidden edge and it did nothing. I'm not sure what's the best thing to do with it at this point, does anyone have suggestions?
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Old 28th Jan 2022, 3:08 am   #2
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Default Re: Warped and yellowed dial cover

Nice tuner, pre-dating any we have over here. I'm guessing the cover is celluloid, which would mean (a) that it's highly inflammable, so treat with care: (b) that there probably isn't much that can be done for it beyond gentle cleaning, I've used Brasso (a liquid metal polish) in the past and an old thread here refers to doing the same and living with the shrinkage. By cutting a block of wood of the right dimensions it would be simple enough to produce a replacement if there's an available clear plastic sheet that softens when warmed - if anyone here has discovered something suitable they'll let us know, though personally I confess to being partial to a somewhat yellowed, even warped, original window. Another old thread I landed on suggests that stocks of celluloid sheet itself could still be available, if at what might be a daunting price.

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Old 28th Jan 2022, 8:47 am   #3
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Default Re: Warped and yellowed dial cover

That cover looks absolutely fine to me with its yellowed patina, totally in keeping with the lovely vintage set that it is. I wouldn't replace it for no money, just carefully restore its smooth, shiny surface using Brasso. Job done, truly lovely little tuner!
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Old 28th Jan 2022, 10:20 am   #4
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Default Re: Warped and yellowed dial cover

The yellow is partly due to absorption of ultra-violet light from normal daylight.....worse if left in a bright room with natural sunlight. The UV seems to penetrate into the plastic and no amount of rubbing will completely clear it. Best thing is to clean it and then just enjoy the tuner.
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Old 28th Jan 2022, 10:54 am   #5
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Default Re: Warped and yellowed dial cover

Celluloid is composed of nitrocellulose and camphor. It’s highly inflammable and inherently unstable. It was widely use till the early 1950s for motion picture films, but highly inflammable and would spontaneously ignite at temperatures above 150F form hot projector beams, so was replaced with acetate. Used for table tennis balls till 2014, and musical instruments – accordions, and fountain pens.

https://en.wikipedia.org/wiki/Celluloid

It’s just possible that the window is celluloid, but that usually 'yellows' with age, which yours hasn't, so it looks to me like it's acetate which had largely superseded celluloid by 1950. I can understand why you might want to replace it - it doesn't look 'aged' - it looks like it's dirty and needs a good clean, but the discoloration will be caused by UV and will be right through the window - not just on the surface.

If you want the window to look ‘new’ and clear, I think the only option would be to create a new one using acetate of the same thickness (maybe 1 – 2mm?), to heat it, and use male and female moulds (US Eng 'mold') made from wood to press the heated sheet into shape. If the sheet is placed on a tray in an oven set to 125C (250F) for 15 – 20 minutes, it should be pliable enough to press it to shape using the two-part mould.

I've attached a rough sketch to show what I mean.

The aperture in the female mould would need to be larger than the length and height of the male mould to allow for the thickness of the acetate sheet.

Clear acetate sheet 1mm/2mm thick is widely available in small sizes such as A4.

There's lot of information on internet how to mould acrylic sheet using heat. Here for example:

https://www.wikihow.com/Easily-Mold-...Plastic-Sheets

I have two 1950 radios that I restored which - though different brands - are almost identical, both of which have discoloured 'blister' windows. I considered making replacements windows but as the dials are themselves cream/yellow coloured, the yellowing of the windows isn't so noticeable, so I left them in place.

Hope that helps a bit.
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Old 28th Jan 2022, 2:44 pm   #6
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Default Re: Warped and yellowed dial cover

Thanks all, I think I do have a can of Brasso around here somewhere. Besides the yellowing it also has a haze over the surface, so I'd like to get it cleaned up a bit but don't want to risk ruining it. Fortunately it the flange is not visible when installed, so I can use it for testing. I do like the idea of moulding a new one David, so I'll probably give it a try, if nothing else just to see if I can do it.
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Old 28th Jan 2022, 3:04 pm   #7
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Default Re: Warped and yellowed dial cover

Quote:
Originally Posted by markfff View Post
Thanks all, I think I do have a can of Brasso around here somewhere. Besides the yellowing it also has a haze over the surface, so I'd like to get it cleaned up a bit but don't want to risk ruining it. Fortunately it the flange is not visible when installed, so I can use it for testing. I do like the idea of moulding a new one David, so I'll probably give it a try, if nothing else just to see if I can do it.
To polish out the haze, first make a 'pad' out of a suitably sized cloth and position it underneath that raised section of the cover such that it supports it. That way you're less likely to accidentally damage it by pressing it in and fracturing it.
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Old 28th Jan 2022, 6:51 pm   #8
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Default Re: Warped and yellowed dial cover

Can't you just cut out a suitable replacement from a sheet of 21st-century polycarbonate/acrylic? [the stuff used for greenhouses is worth looking at]. I've done the same with yellowed/distorted celluloid dial-windows on vintage R1155 RAF bomber-radios, the result is rather good.
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Old 28th Jan 2022, 8:29 pm   #9
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Default Re: Warped and yellowed dial cover

That dial doesn't look too bad, I have dealt with worse in the past.
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Old 29th Jan 2022, 1:34 pm   #10
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Default Re: Warped and yellowed dial cover

I'd imagine that lens was originally vacuum-formed, there's a youtube video of how to vacuum-form sheet plastic at home.
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Old 29th Jan 2022, 2:24 pm   #11
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Default Re: Warped and yellowed dial cover

MDF is a great material for short run vacuum forming moulds, it is porous and therefore can "transmit" a vacuum.
 
Old 30th Jan 2022, 4:37 pm   #12
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Default Re: Warped and yellowed dial cover

I just ordered some thin acrylic sheets, so I'll give it a try. Thanks for all the help
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