Flat paper caps in picture?

[RussellSher]

Hi all - Just wanted to confirm - the two flat caps in the picture - are these flats wax paper caps? I thought flat would be unusual (usually for mica)
The one on RHS is 200pf. The yellow one seemed to have printing in the wax?
From a PYE chassis (PE69RG - probably export model)
thanks
Russell

[David G4EBT]

They're both wax-dipped silver mica, and usually - though not exclusively - are in tuned circuits, so best left alone or the alignment can be upset. (200PF would be a very low value for a waxed paper cap). The problem with paper caps is that paper is hygroscopic - it absorbs moisture over time, but mica is not hygroscopic and nor is polyester, so don't suffer that problem.

Silver mica caps are usually terminated by a rivet at each end which secures the two sets of plates, sandwiched by the mica dielectric, generally with a Paxolin sheet on the upper and lower sides, then dipped in wax. Often, the 'wire' terminations aren't in fact wire, but thin flat strips. They're usually low capacitance values from say 5pF up to 250pF but physically larger ones can go up to 2,200pF. If you look at the cap on the right, you can clearly see the rivets, and if you look at the one on the left, you'll just see them beneath the wax. When used as padding capacitors in front end tuned circuits, they're often obscure non-standard values, custom made for the set in question. Silver mica are also hidden away in IFTs. They were used in circuits where frequency stability is called for as then tend not to drift in value. They're very reliable and really, should only be changed when they've been diagnosed as defective - not as a matter of course.

They were not exclusively waxed - they were often encapsulated in hard brown or black plastic of some kind and sometimes used the colour code using a series of indented dots. They often appeared extensively in military equipment.

The picture homes in on those two particular caps, but there are other caps in that picture just visible, which would be candidates for changing and may well be on the list to change anyway - the brown .01 'mouldseal' just in view at bottom right, and the two aluminium tubular caps at the top might possibly be suspect.

[ukcol]

There is a pretty comprehensive overview of different types of capacitor on Paul Stenning's excellent web site http://www.vintage-radio.com

The section on capacitors is here.

[paulsherwin]

I agree, both flat caps are probably wax dipped mica, which are usually OK. Anything smaller than 0.001uF (1000pF) is likely to be mica.

However, the brown round cap to the right looks suspiciously like a Hunts Mouldseal, a notorious cap type which is very likely to need changing.

[ms660]

And don't be fooled by the name Micamold if you ever come across them.

Lawrence.

[cmjones01]

The rivetted leads are the giveaway - those are 'Lemco' silver mica capacitors, and they're very unlikely to be faulty. They're usually used in fairly critical locations, tuned circuits and so on, so don't replace them unless you have definite proof that they're faulty.

I've got a fair number of Lemco silver mica capacitors in my component drawers, which came from a distributor's sample pack donkeys years ago. They exist in various varieties: no coating at all, wax dipped, red cement coating, and with the leads attached at 90 degrees or sticking straight out. I've never found a faulty one.

Chris

[bobskie]

I can see the rivets now, I was not aware of these wax dipped micas only the domino style and plastic dipped ones. I'll need to double check in the future!

[Craig Sawyers]

The brown tubular Hunts paper one also in the picture is a completely different matter. Replace on sight.