I bought this and I couldn’t get it calibrated. If I calibrated the voltage to 1.5 V at full scale and then switched to any other scale, the readings were very far off. Sometimes more than 10%.

Here is the schematic for the unit

When I measured the precision resistors inside the unit, they were not precise. Over the last fifty years I guess these drifted some. More than half of the resistors were outside their 1% tolerance.

These are the resistors causing trouble

To fix the problem I bought a bunch of metal film resistors that were as close the to correct values as I could. When I couldn’t get exact matches, I made my own by combining different values. All the resistors values. All resistors are now within a 0.5% tolerance.

Here is the unit with the new resistors and some of the ugly ones I made.

This process took a while. After I made all the high precision resistors, I clipped each of the old resistors out and replaced them one at a time.

Here are the old resistors. After checking them only about half of them were within the 1% tolerance they should have been. Some of the worst were off by as much as 10%.

After changing all the resistors, the 312 VTVM held its calibration beautifully. All of the full scale values stayed spot on.