Heathkit Models IM-11 and IM-13 VTVM Video #2 - Calibration

Since the two meters appear to be functioning ok, calibration would be the next step. If the meters calibrate well, I'll declare them ready for use. If they don't, I'll hunt for the cause and try to make repairs. At best, these meters are +/- 5% so I don't expect perfection.

[seantonnesen]

Jim, I think you will have an easier time calibrating the meters if you follow the following order:
with the meter warmed up and in the upright position adjust the Zero Adj while switching between DC+ and DC- until no meter movement is seen then adjust the mechanical meter movement to Zero. After this is complete the DC Cal can be adjusted for a correct reading of the applied DC voltage.
Next switch to AC and adjust the AC BAL for no meter movement while switching between AC-DC+ and DC-. When this step is complete apply an AC voltage and adjust the AC CAL for a correct reading.

[mikehuedepohl]

Thank you for your video I am trying to calibrate a Heathkit IM -25 Meter and there is no where on the web that gives as good of a explanation as you do in this video I have now compleatly calibrated the IM25 with your help Thanks again Mike WD9GJK

[widecast]

Ac meters normally measure average voltage and display RMS. This is for a sine wave. If the wave form is substantially different in form factor then it won’t read correctly.

[widecast]

Wipe the face of the meter with a damp cloth the remove any static charge on the face plate. Then pick up the meter and rotate it vertically and if the needle moves up and down then the needle is bent and not balanced with the counter weight. Do this with the power off and it might help to short the meter terminals.

[harryshector]

Accuracy of meters is usually expressed in percentage plus or minus so many counts. That allows for the differences in the last digit. I suspect they are both within the specs, and therefore 'correct.'

[harryshector]

.707 is roughly correct. You can just use the peak voltage rather than P2P, because it eliminates division by two.

[harryshector]

You can fix the non-zero condition by adjusting the same element on the back side of the movement. You may have to disassemble the meter to get to it. This occurs due to changes in spring tension. You should also check the balance of the movement pointer by zeroing in the "Use" position, then holding the meter horizontal. If the zero changes, the pointer is not balanced. The balance can be fixed, but you need to know what you're doing. Being out of balance can contribute to linearity variances as the pointer moves upscale from zero.

[seantonnesen]

Jim try swapping out the two 12AU7 tubes between the two meters to see if the ZERO Adj moves to the other meter. One side of the duo-triode may be weak, Just a guest.

[widecast]

Jim, you have two ac millivolt meters in your shop. Why don’t you use one of those. They normally have a high range of 300 volts.

[harryshector]

You are using you digital instruments as the standard by which you are calibrating the instruments under test. A standard should normally be 10 times more accurate than the instrument under test, or better, if available. Test by setting the instrument under test to its cardinal points, and reading the difference from the standard.

[erin]

I’ve just sold all my analog meters.
. Digital is the way to go! Kits were nice when we were poor students struggling to get into biz.