Testing capacitors with a Megger

[Phil G4SPZ]

Just a quick message to alert members to some surprising results that I've encountered.

I regularly use a battery Megger model BM8 Mk 2 to check the leakage in capacitors before using them in repairs. Today I have 'binned' several NOS capacitors of types that I would never previously have suspected. The 'black' and 'brown' Hunts are fair game, of course, but modern Dubilier plastic encapsulated capacitors, and Erie dipped polyester types? I also thought that NOS RS types were above suspicion, but these measured well below 1 Megohm, in fact all the components pictured exhibited significant leakage resistance of 1 Meg or less on test at 250 volts DC. I have (or did have...) large stocks of NOS capacitors, and all examples of the types shown proved to be leaky on test, making these types automatically suspect from now on.

The BM8 is a very useful device indeed. It delivers almost exactly the stated DC test voltage (50, 100, 250, 500 or 1,000 volts) at a constant current of 0.7mA, both as measured on my Avo 8 Mk V. I would venture to suggest that testing the leakage resistance of NOS capacitors is more important than testing them for capacitance value. I therefore never fit a capacitor unless I have checked it on the Megger beforehand. The fact that most good, new components display infinite leakage resistance tends to make me throw out any capacitor that shows any leakage at all; who knows how long it will last in service?

One final pleasant surprise was a batch of 'green' Hunts NOS capacitors, which all exhibited no leakage whatsoever and still had the correct capacitance value!

[maninashed]

Its certainly the best way to test capacitors. The subject is covered fully here https://www.vintage-radio.net/forum/...ad.php?t=73265
Bill

[Lucien Nunes]

The fact that most good, new components display infinite leakage resistance tends to make me throw out any capacitor that shows any leakage at all

I use the same logic with insulation in electrical installations. It is often possible to achieve a very high value indeed, so when a system returns a test result in the pass band but much lower than average for its size, I want to know why before I pass it. Sometimes there is a valid reason, sometimes an incipient fault is revealed. However in radio circuits one can accept a significant amount of leakage in certain components and fairly wide variability of pass threshold seems appropriate.

May I take this opportunity to repeat my plaintive plea not to throw out stocks of NOS components simply because they are too far out of spec to use. I am trying to make a reconstruction of an electronics surplus shop and need lots of u/s but intact-looking parts to stock it with!

Lucien

[Phil G4SPZ]

Originally Posted by Lucien Nunes

May I take this opportunity to repeat my plaintive plea not to throw out stocks of NOS components simply because they are too far out of spec to use. I am trying to make a reconstruction of an electronics surplus shop and need lots of u/s but intact-looking parts to stock it with!

I also don't actually throw them in the bin, Lucien, so I'm sorry if I gave that impression... I take them to the Black Country Living Museum where I work as a volunteer in the 1939 wireless shop. If sufficiently old-looking, they help to form part of the period display. What's the date of your replica shop? I may have other bits that could help you.

Thanks for the link, Bill. I remembered posting to that thread, but my search failed to find it, possibly because of the quotation marks around 'Megger' in the original thread title!

[Billy T]

I don't throw them away either, they are fine in functions with parallel resistive paths and suitably low applied voltages. I wouldn't tempt fate by using them on full HT duty, but it is surprising how many NOS and preloved caps are still usable. Given the running time the average restored radio gets, most of those old caps will probably outlast me, and I'm quite sure that some are already a decade or two older!

Cheers

Billy