IT-28 Cap Checker

[Fred Olsen]

FRED HOLNAGEL wrote:
 > I have one of these fine gadgets but no manual.
 > ... found that at 400 volts on the scale there was actually 500 volts
 > and at 450 on the scale there was 560 volts!
 > It can't be one of the switched resistors has changed value as the
 > overage is there at all settings.

Fred, the manual is on BAMA.  It's a partial but should be enough for
maintenance.  The full manual is actually there too, but is marked as
though it's for the IT-11.  I have no clue why, but the units are
essentially identical.

This problem is not typical of the circuit, which is really pretty close
for a kit or service-grade gear.  At last cal I took the following
readings from one of my IT-28s and they're typical of both my IT-11s and
IT-28s.  Taken with a 10 Meg DMM.

Line: 116
Range:  Actual:
      3     2.91
      6     5.74
     10    10.6
     15    15.7
     25    26.8
     50    48.9
    100   100.5
    150   150
    200   200
    250   250
    300   299
    350   348
    400   399
    450   449
    500   498
    600   611

Check the high B+ to be +600 DV at pin 3 of the 6AX4.  From there it
feeds directly to the leakage test divider switch.  If the error appears
to be consistent across all ranges I'd suggest taking a look at the
divider termination, a 560 ohm 1/2 W from the most-clockwise terminal of
the switch (as viewed from the rear) to ground at the terminal strip in
the center of the chassis.  Also check the integrity of that ground by
tightening the mounting hardware.
I'd suggest to anyone to ALWAYS check ALL grounds in ALL Heathkits for
tightness.  "I don't know how many times ..."

Best of luck with it,
Fred
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Outgoing checked by Norton AV

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