Circuit for a simple mains-powered capacitor leakage tester.

[Radio Fixer]

Well mica caps are now becoming suspect particularly in US sets from the 50's. Being able to measure the leakage between primary and secondary reliably saves taking the IF apart and just replacing them. Just disconnect the wiring and measure. Also some Brit sets, Murphy I think, had open mica caps and silver grows whiskers so ... Even sealed micas now have reported failures, I guess the cases have leaks to the atmosphere. EMI mica caps are OK as they used a copper alloy for the plates.

It depends whether you just fix 'em up or do what I call a proper restore. With the former you may have to do some tricky fault finding. With the other you work through every component and when you are finished the set is probably going to work on switch on years into the future. I cant remember now but I had a bad mica cap in a Philips set that I found by systematically going through that probably would have been a B to find by fault finding. Not in a place where the set wouldn't work but where it might be intermittent or perform poorly.

Don't assume that all 80 year old micas are good.

Gary

[pwdrive]

And don't assume micamolds are micas.
How do you check for leakage in micas in an IF can without taking the can apart?

Lawrence.

[Skywave]

How do you check for leakage in micas in an IF can without taking the can apart?
Lawrence.

If one is the owner of such a device, a Q-meter will tell you if there is a problem in an IF can. Failing the ownership of such a piece of test gear, there is always the classic 'scope + sig. gen. + two 100 k-ohm resistors set-up, which I what I use, when the occasion demands it. You also get to discover / check the resonant freq., too.

Al. / Oct. 4, 2013 //

[pwdrive]

Al, yes I understand the Q measurement but my question was really directed as to measurement by conventional means to establish capacitor leakage measurement when the capacitor(s) in question are enclosed in an IF can as I can't see how that can be done without taking the IF can apart and disconnecting the capacitors that are connected across the inductors (parallel tuned circuit)
As I see it the only capacitor in an IF can that can be checked for leakage whilst still connected in the IF can is any capacitor that is used for pri to sec coupling ie: top capacitive coupling.

Lawrence.

[Radio Fixer]

Ah! its subtle. The capacitors, primary and secondary, are formed by depositing them on a piece of mica, one plate on each side. They have pick up contacts for the plates to the outside where the coils get soldered to them and to legs that go to the outside world. The mica is built into the base sandwiched between two plastic pieces and clamped by a large brass rivet. When new neither plates are near the rivet but over the decades the silver starts to leech and whisker. In my limited experience the growths reach the rivet before shorting the cap on the outside. So with a little disconnection of external wiring then measure between the tags for the primary and secondary coils and caps.

   

If leaky then disassembly of the structure and chucking of the mica is needed and new standard caps fitted on the legs inside the can.

There are various ways that the fault shows itself, the main one being "snap, crackle and pop" with no aerial and if a frame aerial if fitted then that disconnected or shorted out.

Gary

[tiv48]

I have seen this on American sets but did any British sets use them ?
Rob T

[Radio Fixer]

I have never come across any in IFs but possibly as stand alone caps riveted together with very little climatic sealing. EMI caps are completely open but fortunately there plates aren't silver.

   

Gary

[pwdrive]

Thanks for the explanation Gary, as I didn't realize that it was a specific type of IF transformer that you were originally refering to.

Lawrence.