High voltage cap.

[wa5ddz]

Picked up a nice looking Heathkit im-32 vtvm at the hamfest this weekend.
It works very well on the ohm scale but nothing on the ac scale. Cleaned
the wafer switches and replaced the 6al5 tube, still nothing. I'll replace
C1 & C2 -- .05 caps. Now for the question. C3 is a .047 mfd at 1600v.
Why is the working voltage so high? The hightest voltage in the meter is only 120v. Is this high voltage necessary or did Heathkit happen to have a lot
of these on hand? Sure would like to get this meter working. Thanks.

[KA2DZT]

C3 may be a blocking cap for ac measurements.  The meter may run on only 120 volts but it can measure higher voltages.  It has been a long time since I looked inside a VTVM but most all of the common ones are about the same circuits.

The next higher voltage rating after the usual 600v caps was 1600 volts.

Does the meter work on DC voltages??

Fred

[nq5t]

This meter has both 1500VAC and 1500VDC ranges, which explains the 1600 volt blocking cap on the AC side.  Your problem on AC may not be that cap, but it wouldn't hurt to replace it anyway.  AES (tubesandmore.com) has 1600v caps at .047 uF.

[k7mdo]

Yes, it is an AC blocking cap but you could try a lower voltage one temporarily and then limit what you measure to lower voltages at the same time.  This would let you know if that is the problem.

I recently restored (fixed) an IM-11 and can tell you that often the kits were wired incorrectly from the get-go and so you may need to look for wiring errors too.

Tom

[wa5ddz]

Thanks for the information. The caps are on order and while I'm
waiting on them I'll start the correct wiring inspection. Thanks.
Bruce

[wa5ddz]

It's alive! Replaced the caps, used a 600v for test, in place of the 1600v
as Tom suggested. I'll put the 1600v in when it gets here. All
the scales work pretty good, the ac. is right on the mark. The calibration
is not quite right but it is very close. It is useable now. Thanks to all.
 Bruce
ps: maybe this carpet floor is affecting the calibration?