ampage
Tube Amps / Music Electronics		 For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

 

capasitors


 :
7/13/2000 6:30 AM
mke white
 capasitors
how do you check a capasitor with a voltmeter?
  
7/13/2000 3:45 PM
Ken Gilbert
short answer:  
 
you don't.  
 
kg
  
7/13/2000 4:11 PM
Matt
 Re: Testing Capacitors
Caps have a working voltage and in guitar amps, the typical working voltage ratings are from 25V(bypass) to over 400 for filter caps to over 600 for signal caps.  
 
So the best test of any cap will be at the voltage it will be working at. A voltmeter will not test a cap, but an ohmmeter can give a basic indication of a cap. If your voltmeter has an ohmmeter, use that. This method is effective for very quick but not indicative of some modes of failure or problems in caps. This works pretty well for electrolytics since they have a detectable charge up time with this method, but not for signal caps.  
 
With the amp discharged, set the ohmmeter to 20M range, connect the + probe to the + connection of the cap and the - to the negative. You should see an increase in resistance tending to infinity. In circuit, it won't increase past a certain point, but the idea is that, by virtue of increasing resistance, it is charging, and can therefore hold a charge. It's best to take each cap out of circuit by disconnecting either the + or - connection. With some meters, you connect the - to +; just try both and see which one results in charging of cap. If neither, the cap is probably shot.  
 
Another quick test is the 9 V battery test. This one requires the cap be out of circuit. Measure the voltage of the battery. Then connect the battery to the cap, + to + and - to -. Now measure the voltage + to -. If it drops by more than .2V, the cap is leaky.  
 
If you're going to be serious about testing your own gear, then you really should go out and get a cap checker, like what Heathkit, Eico and Knight made. I have an Eico 950B and it's pure gold. Tests capacitance, impedance at 60Hz, inductance(using a standard inductor to compare to), and puts up to 500V on the device under test. Very cheap to purchase and will allow you to really test caps to see if they should stay in the amp.  
 
Matt
  

  Page 1 of 1