76 What about testing other caps?
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
76 What about testing other caps?
This is also from Roy Morgan.
PAPER COUPLING CAPS:
Test interstage coupling caps (e.g. from an audio driver tube to the grid
of the output amp tube) by measuring the dc voltage at the grid (across
the grid resistor if it's not going to ground). Use a high-impedance
voltmeter like a VTVM or DMM. If it's above zero, you need a new cap!
The vast majority of paper caps from the 30's through the 60's are at
least moderately leaky now. Your tubes will thank you with long life for
replacing these caps. Ceramic caps have indefinite life expectancy, as do
good quality modern film caps.
You can do this kind of testing while you are re-forming the filter caps
in-circuit. The tubes are off, and will not be harmed by excessive plate
current while you find all those leaky paper caps. The voltages across
them will be higher than normal running conditions, because the driving
stage is not drawing any plate current.
SCREEN BYPASS CAPS:
With B+ applied and the tube pulled or set off, the voltage at the screen,
again measured with a high-impedance voltmeter, should be the full B+ or
value at the other end of the dropping resistor. If not, the cap is
leaking.
LOOSE CAPS:
Set your high-impedance voltmeter to a high-enough range and clip one end
of the cap to the DC probe and connect (carefully) the other end to a B+
supply corresponding to the rating of the cap. The meter will jump up
briefly and then settle down toward zero. Analog meters (VTVM's) are good
for this because you can watch the movement of the needle. Once the
reading settles, any indication much above zero indicates leakage. A
quick ohms-law estimate with the input resistance of you meter will give
you a value for the leakage. DVM's are often 10 megohms.
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