Checking used capacitor

[chris ma]

How can I check if a capacitor is ok or not when it is out of the circuit? I have a few caps in a old Bogen mono valve amp that I would like to check.

Can I use two AAA 1.5V in series to charge one cap and measure it with a multi meter to see if it is holdng charge or it is shorted, blown or bad capacitor?
Thanks
Chris

[Netlist]

Hi Chris
The easiest way is buying a capacitor meter.
If you plan to acquire a multimeter make sure you buy one with integrated cap-meter. Fluke has a nice and very good range of DMM’s. Not cheap, but very good equipment.
Personally, I only measure capacitors for fun or just to see how bad they are. It's often faster to replace them then to waste time measuring them. (I'm talking about repairing TV's here).
Most of the time we pull out 2 to 10 caps in a TV set and a lot of problems are fixed in no time.
A real bad cap can be measured the way you propose, but then a normal DMM can do the job putting it at ohm-range. A good cap should read mega-ohms; a real bad one is zero ohm.

One thumb-rule: if in doubt...replace them. After all, small caps are very cheap.

/Hugo - doesn’t have black-gates pricelists

[SHiFTY]

Try the tubes part of this forum. It is often possible to "reform" old can capacitors if they are not too far gone.

[SY]

If you're talking about the filter caps, there IS a way to check them- but I wouldn't recommend doing so. Just replace them. There are false economies in this world, and coaxing life out of an old electrolytic to save ten or twenty bucks is one of them.

To check an old electrolytic, if you must, set up a DC supply near the rated working voltage. Attach it (power off) to the caps through a 47K/2W resistor (I'm assuming this is a tube amp). Make sure you have eye protection! Turn the power on and measure the voltage at the caps. It should reach the input voltage (more or less) within an hour or so- more for big caps, less for small ones. When you disconnect the supply, it should hold the voltage for a couple of minutes, assuming you're not draining it with the voltmeter and that there's no bleeder.

If it passes these tests, it will be safe to use for the time it takes for the new caps to arrive.

[chris ma]

It is a tube amp. I am planning to take one cap out at a time and test and/or replace one at a time. Certain one looks like a thick black paper cone like a roll of small change(coins) and there a a couple metal casing like those old 120mm film container with no markings...

I would like to check and test them out of circuit rather, I think it may safer for me using battery power.

[Bricolo]

since you're talking about bad caps,

I've got an electrolytic cap that has "exploded" because I wired it the wrong way. When I say "exploded", it made a big "paf" and jumped from it's cables from it was soldered. And the top of the cap (the metal top) is curved

But with a capmeter, it still has the good value.

Can I still use it (it's a PS cap in a gainclone)
Same question for the same cap that was wired in parallel but that hasn't exploded

[SY]

Bricolo, you've illustrated perfectly why cap meters alone are not sufficient.

[EC8010]

The health of an electrolytic in a valve amplifier can be easily guessed by the flatness of the (usually black) base from which the terminals exit. If there's the slightest hint of a bulge (isolated, or overall) replace the perisher before it causes more damage.

Alternatively, charged via a large resistor (perhaps 100k), an electrolytic capacitor should have low leakage after a few hours. leakage is proportional to capacitance, and capacitor manufacturers give guidelines, but any electrolytic leaking more than 250uA is likely to be suspect.

When old electrolytics explode, paper and foil goes everywhere.