Quote:
Originally Posted by G6Tanuki
And I wind the voltage up to the "Rated working voltage" as marked on the capacitor's outside - even if it is actually being used in an application where it will be exposed to a much lower voltage. Quite often you will find seemingly-high-voltage capacitors used for things like decoupling a receiver AGC line or the detector circuit or the first-audio stage, where there will be at most only a few tens of volts involved. But these are high-impedance points where you may have several Megohms in series with the capacitor, which if its resistance is a Megohm it can seriously upset the biasing/time-constants of the circuit.
In practice, there are some capacitors I don't bother to test: any Waxies, the black Philips "Tar-bomb" type, and the metal-cased TCC Metalpack/Metalmite types [which despite their shiny aluminium outer case and rubber-sealed ends are just tarted-up Waxies].
Don't bother testing these... just go ahead and replace. Personally I like the appearance of nice shiny yellow MKT/MKP capacitors that show a proper rework of a piece of gear has been done properly.
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I concur with all of that.
I've retrospectively tested waxy caps, mouldseals and 'tar bombs' that I've replaced, and invariably they've been leaky when tested at their working voltage, and their capacitance out of spec. Given that they'll usually be at least six to seven decades old, we shouldn't be surprised.
Pic 1: 0.05uF Mouldseal capacitor on multi-meter 20 MegOhm range showing no leakage.
Pic 2: Tested at 250V on Robin insulation tester, shows 125K Ohms. (display shows 250K, but on that range, the displayed figure is halved).
Pic 3: Shows what the capacitor should be, and what it actually is. (When tested for capacitance on an MK328 meter, it measured 0.1uF).
Pic 4: Capacitors I replaced on a 1948 Murphy A124, tested after removal.
Pic 5. 'Tar bombs' replaced in a Stella 105U, which behaved more like low value resistors at their rated voltage.
I have a Robin 3131 insulation tester but also use a 'Victor VC60B', which is often sold under other brand names, and seems generally to be well regarded. Typically £35.00 on eBay and Ali-Express from China, or £45.00 on Amazon in UK. Useful for much more than just testing caps for leakage of course. (Often mis-described as 'Meggers').
A couple of YouTube video reviews of VC60B is use:
https://www.youtube.com/watch?v=q0aLf1TKwVo
https://www.youtube.com/watch?v=cBspKnKsk_A
Hope that's of interest.