You may have seen this device in pictures of the workbench in The Dungeon.
If you don't know what it is, you may have said to yourself, "Self, what IS that grey box thing with knobs and a dial on it?"
It's a Heathkit IT-11 Capacitor Checker. It measures the capacitance of capacitors (imagine that) and does other tests. The main test I use it for is to test leakage of old caps.
Perhaps you have heard the phrase "leaky capacitors." That doesn't mean that they are physically leaking their innards (although that does happen). "Leakage" refers to a capacitor's inability to block DC voltage. As capacitors age, they become less able to do what they are supposed to do.
This is why we generally "recap" or replace old capacitors in electronic gear - e.g. radios and amplifiers.
Periodically I'll read "the capacitors look good..." You cannot tell the condition of an old cap by looking at it! (Unless it is physically damaged and leaking its innards).
Testing caps for leakage really is an academic exercise. Owning a cap checker/tester is not a necessity - it's really an excuse to have a cool looking piece of kit on your workbench. I use mine just a couple times a year, to be honest. When I first got it, I used it a lot. All it did was confirm everything I ever read about old capacitors.
Most any electrolytic cap over, say, 25 years old, is probably bad or is not doing its job as well as a new one. The infamous "wax" caps - paper caps dipped in wax to resist moisture - are ALL bad by now. Plastic caps are common in gear from the 1960s onward. They may be bad or not. Maybe you want to test them or maybe you want to replace them since they're cheap. You can test all the caps in your vintage XYZ radio, but I'd bet you lunch that the majority of caps in it are bad. Which I why I just replace 'em all.
If you have a vintage tester such as my Heathkit, you'll need to restore it before you do any testing. Which means recapping it! (One of the ironies of vintage test gear is that it too needs to be restored, just like an old amp or radio). I also converted mine to the more readily available and cheaper 1629 eye tube a number of years ago.
At any rate, after those caveats, we do have our old Heath, so let's test an old cap for the heck of it.
With the 'paper' type selected, we turn our attention to the right side of the panel.
There is a knob for the test voltage. We'll use that to choose the voltage for the cap under test. The range is from 3 to 600 volts.
Below that is the Bridge/Discharge/Leakage switch. We'll use "Discharge" when we first connect the cap, and before we disconnect it. "Leakage" is for the actual test.
The eye tube will tell us if the cap under test is leaky or not.
We'll come back to that momentarily.
In this case, it's a Sagamo .05 uF 450 volt cap out of a Valco guitar amp.
Looks pretty good for a cap that dates to about 1952, doesn't it?
The eye tube flicked closed for a second, then opened. This means at at 50 volts, the cap is ok.
But our cap is rated for 400 volts. What if we test it at a higher voltage?
With 150 volts applied, the eye closes and stays closed. (It does the same at any voltage higher than 150, if you were wondering).
The cap is leaky. I'm not leaving it in the amp.
Actually, I was 99% sure it was bad, but I tested it because I could.
So much for "it looks good," huh?
This is a Mallory 150, rated at 630 volts.
The eye is open, the cap is good. The eye hardly flicked closed at all - it just immediately opened.
After testing the cap, we move the switch to "Discharge" so the cap discharges the test voltage. On an electrolytic especially, a good cap can hold its charge for minutes, hours, or days.
Not fun to get zapped with 600 volts DC! So we play it safe.