"Jim Wilkins" wrote in message
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"PrecisionmachinisT" wrote in message

Good possibility it's a bad cap then.

You can check the cap with an older (analog) type ohm meter if you
like...
Set it to the lowest range, and touch both leads to the cap...the needle
should swing one direction and then return to zero fairly quickly, if you
then reverse the leads, it should swing the opposite direction then
return to zero...

The test may not work as well with a digital meter because they sample too
slowly. A more sensitive, but harder to interpret, test is to clip a
voltmeter across the cap, touch it briefly with a 9V battery and see how
slowly the reading decreases. I just tried it with an HF meter and 0.68
microfarad cap. With the cap the meter gives an intermediate reading
around 2-3V, without it the meter drops immediately to zero after the
battery is removed.

My good Keithley meter reads the HF meter's input resistance as 0.999
megohm on all the DC voltage ranges. Better meters might be 10 or 22 meg.
A 1 microfarad cap with a 1 megohm load will lose about 2/3 of its voltage
every second, i.e. 10V, 3V, 1V, 0.3V, 0.1V. That's not precisely
accurate, but neither are commercial-grade capacitors.
http://www.tpub.com/neets/book2/3d.htm


I try not to think too much about why it doesn't work with the digital
meters, just that it's a pretty reliable test for figuring out if the
capacitpr is open or shorted, which is one of the reasons I probably will
always keep them around.

Another thing I like them for is checkomg motor windings for
continuity--many of the digital meters lack the required accuracy in the low
ranges and so they don't work well if you're trying to differentiate between
a dead short and say 4 ohms or so.