When you are in the amp repair business on any level you soon progress from slinging a handful of orange drops at whatever strolls in the door to trying to do some real component testing. With non-electrolytic capacitors that usually comes down to figuring out ways of determining whether the leakage level has become unacceptable.
Why should this matter? Leakage in a signal capacitor gets the wrong kind of voltage in the wrong places, and it can shift the bias point of a tube enough to, in some cases, cause a meltdown. I was first introduced to this when rehabilitating a Fisher 500c integrated receiver amplifier a few years ago. One of the four 7591s was getting a sullen red glow in the plates after some time operating, and it was leakage of the blocking capacitor between the plates of the phase inverter/driver and the power tube in question.
Another reason is that in vintage guitar amps we don't want to replace anything that does not need replacement, lest we be accused of not knowing our trade, shotgunning parts because we don't understand the problem that presents, or worse, mining out vintage parts from our customer's amps and reselling them on fleabay-which has happened to my knowledge.
Well, sometimes it's a chore to fire up the Sprague TelOhmike, the special purpose direct reading capacitor leakage tester I built here a while ago, or something spendy like a Sencore LC3 if you should be so lucky.
Enter the IRT-2 insulation tester. Like many of its ilk it is a high resistance tester used by the refrigeration trades that can suss out high resistance shorts in entombed electrical equipment. It does this by a small power supply that can pump out up to 1 kv of ac at very low currents. It's quite enough to test the windings of transformers on the bench for high resistance shorts, but it can also be used, as is shown here, to test capacitors for unacceptable levels of leakage. As a practical matter, anything less than 200 megohms at 1 kv gets rejected here.
Pictured is a red paper Astron showing about 10 megohms resistance-far too low for my comfort and thus it makes a detour into the waste basket.
My IRT2 was obtained for five bucks at an auction of the estate of an old refrigeration and electronics man. A set of batteries and I was in business.
I'd advise against using a crank operated megger for this purpose-too many hands going in too many directions at once even though they do look cool. Good serviceable digital readout insulation testers are available for about fifty bucks shipped from the PRC, brand new with batteries at the usual place and I may get one if I can find a reason.