Hi Philip,

If your "calibration resistor" is out of tolerance, too high or too low value, you have to correct it to the proper value.

But before you do any work on any of the components inside an AVO Mk III/IV VCM you must make sure that your meter is reading correct, that it has an internal resistance of 3250 Ohm and an FSD current of 30uA and also that the calibration line requires 27uA to be reached (90% of 30uA) and that the 1mA/V line requires 22.2uA (74% of 30uA). If your meter needs more current to reach FSD you can correct it either by having it re-magnetized or you can add external shunts and series resistors that makes it work up to some 37-38uA FSD current. The equivalent current and resistance with the 10K shunt inside the tester is 39.75uA and 2452.8 Ohm ( the calibration line is then at 35.775uA and the 1mA/V line is then at 29.145uA).

When you know that you have a fully working meter you can concentrate on the calibration resistor and adjust it so that the needle reaches the calibration line at the correct input voltage. You also need to check so that the transformers are working correctly with the correct output voltages before you proceed with the calibration/restoration.

In the AVO VCM you will probably have to clean the cut-out relay contacts as they can have a quite high contact resistance which means that the output voltage from the transformer drops quite a bit when you draw a lot of current from the Anode, this then affects the whole circuit making the measurements wrong. You can temporarily short the relay contacts when you are doing the calibration. In some cases I have had to replace the power switch too as that also had high contact resistance becoming warm when in use which also resulted in lower voltages, don't forget to clean the voltage adjustment switch on the front.

AVO selected resistors that worked with a certain meter, just like you match tubes they matched the resistors so that they would work with a certain meter with the transformers in the particular tester that the meter was mounted in. You can see this as the red line for the calibration spot is different on different meters. The line was drawn during the testing/calibration of the meter when it was outside of the tester, before being mounted in the tester, then the resistors were selected that made the needle reach the calibration line with the correct voltage from the transformers in that particular tester. This selection of calibration resistors means that one meter will not work in another tester unless you adjust the calibration resistor properly! So you can't move one meter from one tester to another tester without checking the calibration resistors. Of course the transformers need to be within tolerance for this to work, and they were tested according to a test specification for this to work out properly. A meter can only be moved to another tester if it has the same currents at the 1mA/V line, calibration line and full scale deflection point, the other points on the scale are not equally important as all meters are a little bit non-linear anyway.

So by just swapping one meter from one tester to another tester means that you in most cases will have wrong results as they mostly don't match up.

Then when you know that you have the correct voltages from the transformers, the correct meter readings and the correct calibration resistor you can continue to check all of the other resistors for the other ranges.

It is important to do the calibration process at the correct mains voltage according to the calibration procedure in the AVO calibration document, and if possible do it with a pure sine-wave mains voltage. If you have a look at the mains voltage with an oscilloscope (don't do it unless you have an isolating transformer for your oscilloscope or you will probably short your scope as they are usually grounded) you will see that you don't have a pure sine-wave mains voltage, it is usually seen as the tip being a straight line or a jagged line. This is due to all of the other equipments connected to the mains voltage which either disturb the signal or as in some cases communicate by amplitude modulation which adds or subtracts a voltage from the mains voltage. So performing a calibration at one place might mean that the tester doesn't perform equally at another place as the mains voltage varies from place to place. You can have a look at what the mains voltage looked like in my old apartment in the document I mentioned earlier in this thread, there the tips looks like slopes.

I perform calibrations with a pure sine-wave inverter with a variac so that I know that I have a pure sine-wave and also the correct voltage. This might seem like overkill but it is the only way of making sure that I know that the voltages during the calibration and the restoration are always the same.

I hope this was the answer that you were looking for, otherwise please ask again!

Sorry for hijacking this thread Loek, but I hope that you appreciate the answer anyway!

Regards,

Martin