[R-390] Re: R-390 digest, Vol 1 #536 - 5 msgs

[Drew Papanek]

On Sat, 4 Jan 2003 22:54:20 -0800 Bill Smith wrote:

<snipped>
>I have found a simple test with a VOM works well.  Disconnect on end of a
>suspect cap, and measure the resistance with the highest ohms scale.  If 
>any
>residual resistance shows on the scale, the cap is leaky.  Disconnect the
>probe, then reattach.  If the cap shows a "kick" on the meter each time the
>probe is attached, it can't hold the charge of the vom battery, and is
>leaky.
<snip>


That test will often work, but sometimes capacitors do not show appreciable 
leakage (or breakdown) unless tested at higher voltage.  A more sensitive 
test is to use your VTVM or DVM set to the volts scale, in series with cap 
under test and a power supply of appropriate voltage (preferably the cap's 
rated voltage).  The meter serves as a sensitive microammeter with a 
built-in current limiting resistance.  To calculate a cap's leakage tested 
in this way, divide meter reading by meter's input resistance (typically 11 
meg).  A typical BBOD might show 100v reading with a 300v power supply.  
This works out to 9 uA leakage (with 200v across cap), and this cap would be 
removed post haste and sold on E-Pay to the highest bidding audiophool.  If 
you want to get really picky, adjust the supply voltage so that the 
difference between it and the metereading equals cap's rated voltage.

Heat the capacitor slightly (hairdryer) and watch leakage go through the 
roof!  Shows what happens as the radio gets hot.

Modern plastic dielectric caps such as the higly esteemed Orange Drop will 
show no discernible reading, just a fluctuation of a few tenths of a volt 
positive and negative (power supply noise).

Drew



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