[Test-Equipment] Sprague TO-6A question

[k4pf at juno.com]

Hi, Ken

Glad to help!

Did you try measuring the insulation resistance
of the removed C-16 cap with the fixed Sprague TO-6?

An ESR meter isn't a good check 
for a small paper capacitor, because everything except
a shorted cap will show up as >99 Ohms.

A digital capacitor tester is also not a good test
to find leaky capacitors.  Try it with a good mica 
or ceramic capacitor and see how little the reading will change
with a parallel 1 Meg resistor added. 
My B+K model 830 digital capacitance meter is useless in this regard.

The Sprague tester does very well at spotting leaky paper capacitors:
You will see that the eye tube doesn't indicate a sharp null,
it is as if the null were diffuse, smeared around the dial
when doing a capacitance measurement on a leaky paper capacitor.

One hint: don't forget to renew the paper capacitors used
as the "standard" capacitors C-9 and C-10 in your Sprague TO-6A,
or even a perfect test capacitor may produce a diffuse null.
Polyethylene film caps would be a good choice for the standards,
and you can use your digital capacitance meter 
to select the standards for high accuracy before installation.

73,
Ed


> Ken Kaplan <krkaplan at cox.net> wrote

You got it! I should have looked at the schematic but I think I got a 
bit lazy. Actually I was in the process of recapping a Hallicrafters 
SX-100 & thought I'd test what I had just removed. Looks like I better 
recap the TO6 first. If the test equipment isn't any good, there's no 
point in digging into the radio.

Funny thing is I tested C16 after installing its replacement and it 
looked ok. I measured its value on a digital tester and it was within 
10%. I checked its ESR and found that it was > 99 ohms. 
So I don't know what to think.