The AM Forum

I would like to buy a good capacitance meter that can properly test caps at voltage, any recommendations?

Thanks

Rich

I have an inexpensive little ECG capacitance meter that I picked up from Antique Electronics Supply several years ago.  It is very accurate, and measures down to a few picofarads, but won't measure with voltage.  The instructions caution the user to make sure the capacitor is completely discharged before attempting to measure.  That can be a problem with large electrolytics and even oil-filled caps.  You can completely discharge  them, but after some time they may rebound with a substantial charge.  I usually keep them shorted for several minutes before attempting a measurement.

My Fluke DVM also allegedly measures capacitance, but the accuracy is so poor that the function is useless.  The V-O-M functions are very accurate, down to less that 1%, but capacitance readings may be off more 200% or more.

Sprague TO-4, 5 or 6.  There's a TO-5 on eBay currently.  These will check leakage at up to about 600v.  Mine is one of the most used gizmos in the house.

These will not measure value with as much precision as a modern digital instrument, but will certainly tell you whether or not to throw out the cap :-)

Grant/NQ5T

Meterman Model 37XR

A very handy portable unit that will measure any Cap or Inductor and has all the usual VOM features. Slips in your pocket to bring to festers.
Keith
WA1HZK

Get the TO 4,5 or 6 and add a inexpensive, accurate digital tester.  Actually the Sprague units are pretty good if the reference caps in them have been changed and the unit recalibrated.  It is close enough to work with anytime.

Of course, you can check insulation resistance and leakage up to the 600 volts Grant metioned.  You can't do that with the digital stuff. 

The best one I think is the one made by EICO, (mod# escapes me now). This is the one with a "magic eye" tube and can also do comparators for C and R. Also, you can perform 450 VDC electrolytic checks. Its a throw back but they can be picked up for a song at hamfests. Like a TIMEX, "They take a lickin' and keep on tickin' ", (ala Carmen Swazey)

I just picked up an EICO with the green eye tube for $2 at a yard sale.   They work very good in accuracy with low capacitances of the kind use in RF circuits.   Also good for leakage and voltage testing.   They also have a reference mode to compare a capacitor with a known one.    Much easier to use than my modern digital meter and more useful.

Keep an eye out for the military ZM-11 capacitance/inductance/resistance bridge.  It will measure a very wide range of capacitance (10pf to 1100uf) and also leakage using up to 500 volts as the test voltage.  It provides an indication of Q and dissipation factor using a built in 1 khz. oscillator.  It uses the rectified/filtered output from an RF oscillator to provide the  test voltage for capacitors so if the capacitor has extremely high leakage or a short the voltage will simply drop due to oscillator stopping due to the output load and there should be no damage to the instrument or an explosion from the capacitor.  It is also useful for reforming capacitors as you can slowly increase the voltage as the leakage current decreases (measured using 1, 5, or 25 mils full scale).

It will measure inductance and calculate transformer turns ratio and will function as a ohm meter reading up to 11 megaohms and as a leakage tester up to the 1000's of megaohms.  A very useful and compact instrument.  They do show up on ebay and at hamfests from time to time.

Rodger WQ9E

I believe the Eico that's been referred to is the Model 955. The Sprague TO-4, -5, -6, and -6A will run rings around the Eico as far as accuracy is concerned. I had both at one time, and A/B testing between the 955 and Sprague TO-6A showed the 955 had very poor accuracy as compared to the TO-6A. I finally dumped the 955 for a $1 at a local hamfest.
As  Jim and Grant pointed out, you can also do  leakage testing,  insulation testing,  and even cap re-forming. I  :) :) my Sprague.

Thanks to all for the advice, I have a good idea what to look for.

Rich

Thanks again for all the good advice.  I am the proud new owner of a Sprague TO-4.  I still need to learn this beastie though I know the HV works (Ouch).

Thanks again

Rich
W7SOE

73