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Old 20th Jun 2022, 8:48 pm   #1
G1RAO keith
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Default Advice needed with mains dropper

Hi all
Half of this dropped has lost its insulation flaking off due to age and heat,any idea what was used ? would shellac be ok to re cover it or am i likely to start a fire? it's ok left nude I just thought cosmetically it would look better.regards Keith
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Old 20th Jun 2022, 8:57 pm   #2
David G4EBT
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Default Re: Advice needed with mains dropper

Crikey no - not shellac.

If you want to spray it with anything, I'd suggest matt black stove paint, which is good to 600C - way above the temperature your dropper is likely to attain.

EG, something like this:

https://www.yorkshiretrading.com/pro...CABEgL1oPD_BwE

Ride at your own risk!
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Old 20th Jun 2022, 9:53 pm   #3
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Default Re: Advice needed with mains dropper

That looks very dodgy and may contain asbestos.
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Old 21st Jun 2022, 7:49 pm   #4
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Default Re: Advice needed with mains dropper

Car engine enamel or brake drum paint should do.
It looks like the original was some sort of paint, I doubt it was asbestos on that design of dropper

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Old 21st Jun 2022, 8:08 pm   #5
G1RAO keith
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Talking Re: Advice needed with mains dropper

Hi
Thanks for the replies. ok Dave no shelac I did wonder !no asbestos it's a plain old ceramic tube the stove enamel is the way to go then.the devil in me was kind of looking forward to the Roman candle \shellac shed warmer !! Regards Keith
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Old 21st Jun 2022, 9:00 pm   #6
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Default Re: Advice needed with mains dropper

I have used, to great success, TECHNICQLL HIGH TEMPERATURE SILICATE 1200°C. It is a grey paste in a 70ml tube it can be thinned down with water and painted on with a brush and air dried a couple of coats is normally enough.

Colin
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Old 22nd Jun 2022, 6:55 pm   #7
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Default Re: Advice needed with mains dropper

I would leave it nude; any sort of paint/coating will only reduce its ability to dissipate heat.

But equally I'd investigate why the original coating has been so comprehensively vapourised.

What's it from? Has something downstream of the dropper gone horribly low-resistance perhaps? Ot has it been fed with 250V when set to 120V ?
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Old 22nd Jun 2022, 11:37 pm   #8
Graham G3ZVT
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Default Re: Advice needed with mains dropper

What prevents the turns of nicrome wire from touching each other and reducing the resistance?

Presumably the paint.
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Old 23rd Jun 2022, 6:54 pm   #9
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Default Re: Advice needed with mains dropper

Quote:
Originally Posted by rambo1152 View Post
What prevents the turns of nicrome wire from touching each other and reducing the resistance?

Presumably the paint.
Or the oxide film that soon forms. Remember that the voltage between adjacent turns is low.

If making NEW high power resistors from new and shiny nichrome wire I have been known to pre-oxidise it by running it at a bright orange heat.
A vehicle battery or the mains supply may be used according to the voltage and current needed. Take care.
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Old 23rd Jun 2022, 9:05 pm   #10
G1RAO keith
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Default Re: Advice needed with mains dropper

Hi again
Interesting thoughts on the dropper its in my HMV 1115 1946 era so getting on a bit ,the end with the coating still on is the cooler end across the dial lamps I may just leave it as is ,strange though the 3amp fuse had blown in the plug and dial lamps o/c even though the mains tapping was set to the correct point no shorts have been found .Also the mains switch was high resistance now ok after a strip down and contacts cleaned and 're faced ,rather oddly the single pole is in the neutral lead ,The usual wax caps have been replaced and one 100 K ,the set is on the bench and working fine after osc and ae alignment.Keith
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Old 24th Jun 2022, 8:33 am   #11
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Default Re: Advice needed with mains dropper

It could be porcelain or asbestos looking at that dropper. Thankfully easy to tell though.
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Old 24th Jun 2022, 10:54 am   #12
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Default Re: Advice needed with mains dropper

Does it have to have insulation. As long as the resistance wire doesn’t sag I would leave it. Lots of my old Ekco radio have bare droppers Andy
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Old 24th Jun 2022, 11:01 am   #13
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Default Re: Advice needed with mains dropper

Got to say I agree with Andy.

Despite the loss of the coating it looks to be in good condition so if it works just leave it alone.

You can check the resistance to make sure there are no shorts.

Its part of the life of the set

Cheers

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Old 24th Jun 2022, 7:35 pm   #14
G1RAO keith
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Default Re: Advice needed with mains dropper

Hi
It's definitely a ceramic dropped ,although the set works there is a 2300pf cap (c19) on the circuit diagram that if it went s/c it would short out the top three sections which are the mains tapping,s so may be that would account for the burnt off paint it looks like a wax covered mica, as for the dial lamps after closed inspection I think they failed due to age no vaporized filament just a small disc in the centre,so c19 will be replaced as a precaution also all rubber wire has been replaced
and the wiring to the dropper replaced with silicon type.Not a pretty set with its green painted front but it's growing on me and saved from the skip.regards Keith.
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