[Heath] Heathkit VTVM model IM-13

[Ron]

The original circuit had a 10K pot for Ohms adjustment. This gave a
0 to 10K range. With the added 5.6K, the range became 5.6K to 15.6K.
The effect of the added resistor was a shift in the adjustment point.

I have built and repaired a large number of Heathkit VTVMs (this
portion of the circuit was common to most, if not all, models) and
cannot remember that such a mod was ever necessary. I'll venture a
guess that, due to component tolerances (say, meter too sensitive
or an out of balance 12AU7), your VTVM could not attain correct
Ohms adjustment and you added this resistor to correct the problem.

-- Ron




On 1/10/2013 6:55 PM, Dick KF4NS wrote:
> I restored this unit about 10 years ago and put it in the closet until now. The thing works great, even
> better than my nice IM-18.
>
> Now more to the point. I did something that most of us have probably done. During restoration I added a
> part and have no idea why.
> R13 (ohms adjust pot)  now has a 5.6K resistor on the wiper terminal. Wish I knew why I did it. Was there
> a mod somebody created?
>
> The pot has very little control from end to end but the meter needle is right on for infinity after a
> good warm-up. Let me know if this is a known mod and why.
>
> Thanks and 73, Dick KF4NS
> St Petersburg, FL 33714 USA
> Keep The Glow!
>
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