Mica capacitors should I change them?

[PeterPYE]

Hi
After a very warm welcome it's time for my first question

I've read the debate about changing these capacitors. They are the mica caps inside the IF transformers. The service manual (downloaded from here, thank you) says they are 100pf. I'm measuring 160 to 180 with my DMM, outside the 20% tolerance. So should I replace these or put them back? If I change them any suggestions for suitable replacement?
I'm expecting to have to do some trimming anyway (yet to learn how).

Thanks in advance.

[paulsherwin]

Mica caps are not 'change on sight' components, unlike waxies. They rarely fail, but your measured values are concerning. Personally I would leave them alone if the transformers can be peaked properly, but it's your call.

Oddly, mica caps in IFTs are a notorious point of failure in American radios, but rarely give trouble in British or European sets.

[HamishBoxer]

Get the set running first with careful methodical fault finding would be my answer.

Don't catch CCCD "Compulsive Capacitor Changing Disorder".

[cathoderay57]

The mica type that you have shown are virtually always OK. If, however, they were the pyjama button (PJ) type shown in Post #1 of this thread https://www.vintage-radio.net/forum/...d.php?t=150905 not so. You will see different opinions of these PJs in the thread but particularly in Ekco sets they are to be found lurking inside IFT cans and invariably I have found them to have drifted in value such that it was impossible to align the IFTs without replacing said capacitors. Cheers, Jerry

[crackle]

Hi Peter
I would be more inclined to doubt the reading on your DMM, which may be more accurate when measuring larger capacitors like electrolytics.
The long test leads may influence the measurement of very small capacitor values.
I would put them back carefully exactly as you found them.

Mike

[agardiner]

As others have said - get the set working first and if you can align the set properly then leave them be. Only change such components if it actually causes the set to not work.

[Nickthedentist]

My take on this is:

(1) Although they often look unpromising, those caps rarely fail, and they're invariably part of a tuned circuit which takes some skill, experience and equipment to set-up properly. So I leave well alone if I can, and that includes the way they're mounted, routing of leads, etc.

(2) OTOH, waxed paper caps (Hunts Mouldseals etc.) are almost always faulty to some extent. Where valved equipment is concerned, I change all the lot without even testing them individually, as performance of the set almost always dramatically improves and its future reliability should be greatly improved. Others here feel that this is a lazy approach for people who don't want to engage their brain, and I can understand their point of view, but it's always worked for me.

[ms660]

If the capacitors are out of tolerance and the IF is re-aligned with them still in circuit then be aware that the bandpass response of the IF stage could be altered due to the position of the primary and secondary slugs relative to one another compared to what they were originally, this is "implied" in several alignment instructions for various makes and models, the Bush DAC90a being one example.

Lawrence.

[Vintage_RC]

Originally Posted by crackle

Hi Peter
I would be more inclined to doubt the reading on your DMM, which may be more accurate when measuring larger capacitors like electrolytics.
The long test leads may influence the measurement of very small capacitor values.
I would put them back carefully exactly as you found them.

Mike

I think Mike is probably correct, I wouldn't trust the capacity range on a run of the mill DMM (assuming it has one) for anything much below 1000pF.

[mancunian69]

Originally Posted by HamishBoxer

Get the set running first with careful methodical fault finding would be my answer.

Don't catch CCCD "Compulsive Capacitor Changing Disorder".

CCCD LOL, I like that one!

[Sideband]

Originally Posted by Vintage_RC

I think Mike is probably correct, I wouldn't trust the capacity range on a run of the mill DMM (assuming it has one) for anything much below 1000pF.

Absolutely. My DMM is OK for higher value capacitors and that has been my experience of even the expensive Agilents we use at work. The only way to measure a low value capacitor is with an accurate bridge. DMM's have long leads which introduce a small amount of extra capacity which makes a big difference with low-value capacitors.

In the many years I have been restoring and repairing radio's I have only very occasionally had the need to change a capacitor in an IF can. Leave well alone until you have the set working. Chances are the set would not have needed alignment. Probably not the case now. Don't take this the wrong way....over enthusiasm with capacitor changing can cause un-necessary problems. You may now have given yourself a lot of extra work by upsetting the alignment when it wasn't necessary. Get it working first before suspecting alignment problems.

[Beardyman]

For what its worth I have had to replace some mica caps in a Defiant MSH902 due to silver migration. This manifested itself as a rustling in the audio, sometimes the occasional pop. That said the capacitors in question did measure spot on. That is the one & only time I've ever had to replace mica caps in all the sets I've restored. As others have said the capacitor function on a DMM is, at best, optimistic. Personally, I'd put them back in, change out the waxies, check for any wildly out of tolerance resistors & take it from there. Best of luck chap!

[Paul_RK]

Originally Posted by paulsherwin

Oddly, mica caps in IFTs are a notorious point of failure in American radios, but rarely give trouble in British or European sets.

Not sure if they're the ones you have in mind, but Micamold was a prominent American manufacturer whose flat lozenge-shaped capacitors in moulded casings often used a waxed paper dielectric with predictably poor long term reliability.

Paul

[PeterPYE]

Thanks everyone. Yes my DMM has a 10pf resolution in the 40nf range. So yes not ideal at those low values. Could I put them in some kind of circuit and test their performance?

[Sideband]

At best, DMM's are really only a guide for capacitor checking. The only way of testing them properly is an accurate bridge. Personally I would put them back as already suggested and get the set working first. In all the vintage radio's I've restored over the years I think only two have required any caps in the IF transformers.

Originally Posted by PeterPYE

Could I put them in some kind of circuit and test their performance?

Yes....the circuit they came out of....!

[Station X]

Hopefully I'll never find myself in this situation. If I did though I'd replace the capacitors with new silver mica types to avoid having to unsolder the old caps a second time if they are indeed faulty.

One of the new caps could be tested with the DMM to check the accuracy of its readings.

[stevehertz]

Originally Posted by Nickthedentist

.....

(2) OTOH, waxed paper caps (Hunts Mouldseals etc.) are almost always faulty to some extent. Where valved equipment is concerned, I change all the lot without even testing them individually, as performance of the set almost always dramatically improves and its future reliability should be greatly improved. Others here feel that this is a lazy approach for people who don't want to engage their brain, and I can understand their point of view, but it's always worked for me.

I've never really understood the notion that it's a lazy approach to not spend time taking measurements and figuring out how a circuit works in order to ascertain that the wax capacitor in front of your very eyes, the one that you know from time honoured, well documented experience that it's going to faulty, is in fact, faulty. Waxies and old leaky electrolytics, just change them, job done, move on to the real fault finding, not the faux variety.

[Craig Sawyers]

In general mica capacitors are reliable, even if they look awful.

That is not to say that they don't die. The Racal RA17 had, depending on the age, mica capacitors that failed. They looked just like modern types - epoxy dipped.

The problem in the RA17 is that they were used in the multistage loosely coupled 40MHz bandpass filter. If they needed to be replaced it was a real pain to realign and absolutely needed a wobbulator to sort it out.

But going back to the original statement - don't assume that they are dead unless you have compelling evidence that they have failed.

Craig

[Sideband]

Originally Posted by Nickthedentist

(2) OTOH, waxed paper caps (Hunts Mouldseals etc.) are almost always faulty to some extent. Where valved equipment is concerned, I change all the lot without even testing them individually, as performance of the set almost always dramatically improves and its future reliability should be greatly improved. Others here feel that this is a lazy approach for people who don't want to engage their brain, and I can understand their point of view, but it's always worked for me.

Another point on this is one of experience and possibly how long you have been restoring/repairing old radio's and TV's. Old hands at this KNOW that the waxies will be leaky and probably also the function of the particular capacitors fitted in the radio/TV. We KNOW from long experience that is isn't worth checking the waxies and just change the lot. Back in the 60's when I first started playing about with old radio's, I learnt very quickly about old wax capacitors, especially the ones connected across the mains.....! The first vintage radio I repaired back then was a Pilot Model 10 that had a full recap (still working with those same capacitors)

Less experienced or beginners might prefer to check the radio after each change so that they can assess what difference (if any) changing a particular capacitor has. I've never particularly liked that approach with radio's although with TV's I use a slightly different approach by changing a few essential capacitors first and then seeing if the line output transformer and/or tube are good before working my way through the rest of the waxies.

It's really a question of preference and what the individual seems comfortable with.

[Station X]

Originally Posted by Sideband

It's really a question of preference and what the individual seems comfortable with.

Indeed. The recap/don't recap discussion will go on forever.

Can we get back to giving advice relevant to the OP's specific question please.