Plessey Metalmite Capacitors - UK Vintage Radio Repair and Restoration Discussion Forum

joebog1 · 22nd Jun 2017, 11:32 pm

Just a quick question about Plessey Metalmite capacitors as per the photo.
I guess they are paper inside the pretty can. They have one end seal in red and the other in white, so again I am guessing the red end would be the inner foil ?
How reliable are they ?. These were made in 1971 and I have "bags" of them.
Bought from the RAAF auctions for 10 cents a bag of ten still sealed.
Regards
Joe

M0FYA Andy · 23rd Jun 2017, 9:34 am

I would suggest the only answer is to test a number for leakage at their rated voltage. If they are OK then they will probably stay reliable in use.
Andy

paulsherwin · 23rd Jun 2017, 9:55 am

1971 is very late for these. They are just conventional waxies but sealed inside an aluminium can. They were used in a lot of military and tropicalised kit, and you do see them in the odd domestic radio.

You are right about the red marking.

They do last much better than standard domestic waxies, but I would be wary of using NOS ones unless appearance was very important.

joebog1 · 24th Jun 2017, 12:44 am

Thanks everyone. I have tested them and they seem fine. I have used some in a Philips radio refurb to long term test them. I'll keep you posted.

The question I guess was to see if anybody had experience of them in domestic situations.

Regards
Joe

AC/HL · 24th Jun 2017, 1:46 pm

They would have been exposed to the atmosphere domestically, and possibly not as well made and tested initially, so we'd be wary, especially in critical positions. Their service life is just beginning!

Sideband · 24th Jun 2017, 10:04 pm

Quote:

Originally Posted by joebog1

I have tested them and they seem fine.

How?

You need to test them at their rated voltage to be sure and even then I'd be wary of using them in critical (ie audio coupling) positions.

Digital meter can give false readings if the capacitor is leaky.

joebog1 · 24th Jun 2017, 11:48 pm

I have a large toroid ( twas all I had) with a bridge rectifier and a couple of 100 uF caps that feed a 100k ( was the first resistor I found, so value is just that) resistor in series with the test cap. I have a variac on the primary of the transformer to set voltage. I use my HP 410B VTVM to measure the voltage across the resistor. The needle doesnt move even when first connected ( AFTER the cap charges of course).

Joe

paulsherwin · 25th Jun 2017, 12:05 am

If they have been stored in a sealed plastic bag for the last 40 years then they're likely to be OK, for a few years at least. The caps themselves are also hermetically sealed which gives them a much longer life than their domestic equivalents.

If I found Metalmite caps in a radio then I would change them, but they would have been exposed to the UK climate for maybe 60 years. Your caps are younger, have been stored in sealed bags, and have spent their lives in a hotter, drier climate.

suebutcher · 26th Jun 2017, 11:12 am

Metalmites were used as coupling capacitors in early Pye G63 stereograms. I'm repairing a 1961 G63, and they're all too leaky to leave in place. One of them has effectively become a 10k resistor, damaging an output transformer in the usual manner. But I'd use properly stored NOS Metalmites in something I was going to use myself, keeping in mind they'd need checking occasionally.

22nd Jun 2017, 11:32 pm	#1
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Plessey Metalmite Capacitors Just a quick question about Plessey Metalmite capacitors as per the photo. I guess they are paper inside the pretty can. They have one end seal in red and the other in white, so again I am guessing the red end would be the inner foil ? How reliable are they ?. These were made in 1971 and I have "bags" of them. Bought from the RAAF auctions for 10 cents a bag of ten still sealed. Regards Joe Attached Thumbnails

23rd Jun 2017, 9:34 am	#2
M0FYA Andy Nonode Join Date: May 2007 Location: Preston, Lancashire, UK. Posts: 2,511	Re: Plessey Metalmite Capacitors I would suggest the only answer is to test a number for leakage at their rated voltage. If they are OK then they will probably stay reliable in use. Andy

23rd Jun 2017, 9:55 am	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Plessey Metalmite Capacitors 1971 is very late for these. They are just conventional waxies but sealed inside an aluminium can. They were used in a lot of military and tropicalised kit, and you do see them in the odd domestic radio. You are right about the red marking. They do last much better than standard domestic waxies, but I would be wary of using NOS ones unless appearance was very important.

24th Jun 2017, 12:44 am	#4
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Plessey Metalmite Capacitors Thanks everyone. I have tested them and they seem fine. I have used some in a Philips radio refurb to long term test them. I'll keep you posted. The question I guess was to see if anybody had experience of them in domestic situations. Regards Joe

24th Jun 2017, 1:46 pm	#5
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Plessey Metalmite Capacitors They would have been exposed to the atmosphere domestically, and possibly not as well made and tested initially, so we'd be wary, especially in critical positions. Their service life is just beginning!

24th Jun 2017, 11:48 pm	#7
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Plessey Metalmite Capacitors I have a large toroid ( twas all I had) with a bridge rectifier and a couple of 100 uF caps that feed a 100k ( was the first resistor I found, so value is just that) resistor in series with the test cap. I have a variac on the primary of the transformer to set voltage. I use my HP 410B VTVM to measure the voltage across the resistor. The needle doesnt move even when first connected ( AFTER the cap charges of course). Joe

25th Jun 2017, 12:05 am	#8
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,947	Re: Plessey Metalmite Capacitors If they have been stored in a sealed plastic bag for the last 40 years then they're likely to be OK, for a few years at least. The caps themselves are also hermetically sealed which gives them a much longer life than their domestic equivalents. If I found Metalmite caps in a radio then I would change them, but they would have been exposed to the UK climate for maybe 60 years. Your caps are younger, have been stored in sealed bags, and have spent their lives in a hotter, drier climate.

26th Jun 2017, 11:12 am	#9
suebutcher Heptode Join Date: Jul 2012 Location: Daylesford, Victoria, Australia Posts: 675	Re: Plessey Metalmite Capacitors Metalmites were used as coupling capacitors in early Pye G63 stereograms. I'm repairing a 1961 G63, and they're all too leaky to leave in place. One of them has effectively become a 10k resistor, damaging an output transformer in the usual manner. But I'd use properly stored NOS Metalmites in something I was going to use myself, keeping in mind they'd need checking occasionally. __________________ The Waves That Rule Britannia