capacitance tester

[dewman]

any preferred source for a capacitance tester? I'd like to check the coupling caps in a 68 super reverb I am picking up soon. Thanks!

[epluribus]

I have an Extech true-RMS multimeter that I snagged on sale at Rat Shack not too long ago. About the most expert thing I can say about it is that it's very easy to use. (Although the "manual" stinks.) I'll be tuned into this discussion too, as I don't know how good or bad these things are at measuring capacitance--I get some really strange numbers sometimes. :rolleyes:

--Ray

[Wakarusa]

I suppose it depends on what characteristic you're looking to test.

To measure capacitance for coupling caps any Fluke with a capacitance scale will do. For the big electrolytics a DMM may not be able to get an accurate read.

If you're more interested in the performance of the capacitor then DC leakage, effective series resistance (ESR) and testing at full rated voltage are probably more important.

DC leakage is easy enough -- test the cap in the live circuit with a DMM. See if there's DC where there shouldn't be :) . For ESR or full voltage work more specialized gear and methods are required.

[epluribus]

Hey Todd.

Kinda thought DMMs were limited in what they can tell you. So far I've used one to look for leakage and to bench-test the posted value of caps, but beyond that I've had to resort to calculating numbers rather than directly measuring anything. Haven't figured out how to measure Cathode Current when there's a bypass cap involved, for example.

BTW, this cheapie Extech has scales high enough to handle electrolytics, but I can't vouch for the usefulness or the accuracy of simply probing the terminals out of the amp.

--Ray

[Wakarusa]

For quiescent cathode current you measure the voltage drop across the cathode resistor and apply Ohm's Law. The electrolytic in parallel should only contribute a negligible amount.

If you're worried about detailed measurements of cathode current with an AC signal applied (if, say, you wanted to measure screen current by measuring plate current less cathode current ;)) life gets more interesting. You'd have to know the frequency of the applied signal (preferably from a signal generator so the input is relatively pure), work out the capacitive reactance of the cathode bypass capacitor and then apply Ohm's Law using the AC component of the cathode voltage.

For testing large high voltage electrolytics often a DMM can't supply the current/voltage to get the electrolytic anywhere close to it's expected operating environment. For filter caps ESR is probably more important than being spot-on for capacitance value. Fortunately, there's a pretty simple little circuit you can whip up to test for this value in lieu of buying a specialized meter to measure it. On my bench I use a scratch built ESR tester and an old Sprague tester from the 50's for capacitance measurements. The advantage of the Sprague unit is that it tests the cap at working voltage (up to 1KV. Mind where you put your fingers). I think I'd argue that's overkill for a hobbyist though.