Hi, Gang

I recently repaired a Sprague TO-4 capacitor tester.
In the process, I found that the Sprague can actually damage
electolytics under test, depending on their capacitance value.

The problem is that Sprague uses 85 VAC across the bridge elements
on the 0.1 to 50uF "C3" range (power transformer taps 8 to 10)
There is a fixed 2uF capacitor in one leg of the bridge.
So, if you are testing a 1uF capacitor, it will have 2/3
of 85 VAC across it, or about 60 VAC. This is not good,
especially if you are testing an electrolytic capacitor with a low
voltage rating. Testing a 40uF capacitor would be OK, it will only have
about 4 VAC across it.

Sprague uses 155 VAC across the bridge (transformer taps 6 to 10)
for the 45 to 2000 uF "C4" range,
with the same 2 uF capacitor in one leg. A 45 uF capacitor will have
about 7 VAC across it, using this range. Not too bad, but the test
shouldn't be prolonged.

The Sprague TO-4 is filled with "Black Beauty" paper capacitors, which
should be replaced automatically, due to their dismal reliability.
Interestingly, the original power supply electrolytics (ca. 1952)
tested fine. For safety's sake, change the power cord to a 3-wire
cord.

If your main potentiometer fails, you may have trouble finding
the large Clarostat 7.5K linear wirewound, since that value
is no longer manufactured. The pot carries military designation
RA30NASD752A. I replaced the pot with a 20K pot, military type
RA30NASD203A. (If the last letter is other than an "A",
the pot is not linear). Resistor R6 (150K) was changed to 400K
in order to maintain the calibration of the "C4" range, after the value
of the main pot was changed to 20K.

For future reference, here are the transformer winding voltages,
none of which are included in the Sprague TO-4 manual:
term. 6 to 8 70 VAC
8 to 10 85 VAC
1 to 3 440 VAC
1 to 5 665 VAC
1 to 7 715 VAC
1 to 9 850 VAC
(Measured with a 125V line voltage input).

73,
Ed Knobloch