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Calibration of the early Mark Meters

Video of the calibration Procedure

BATTERY REPLACEMENT

To get to the insides of the meter, there are three screws on the right hand side of the meter, holding the end to the case. One is at the top, one the bottom edge, and one is on the back of the meter in the right hand end panel. These screw holes are usually filled with wood putty, but upon careful inspection, can be easily found. Dig out the wood putty, and remove the screws. The end will come off, and the meter will slide from the case. On the earlier meters, you may have to disconnect the TA jack. You will see which type of meter you have when the end is removed and the disassembly procedure will be obvious.

There are 3 adjustments on the early Mark meters, both British and American. These are labeled as the male setting of the tone arm, the infinity setting and the balance setting. This is accomplished by moving the adjustable potentiometer carefully until the needle is on "Set".

From the top, on the model with the internal TA counter, the locations are here:

The batteries can be replaced with any 1 1/2 volt rechargeable batteries. They are better if obtained in a unit,  like these shown, but any will work.

The red is positive, Black is negative, and the white or blue is the 1 1/2 volt tap for the voltage path.

 To adjust the meter, once the batteries have been replaced, this procedure is followed.

First do a standard calibration of the meter, place the TA on 2.0, and female load resistor across the can leads. Using the trim knob, bring the needle to "Set".

Balance. Move the sensitivity knob from 32 to 0. If the needle moves, adjust the ta a little higher and try it again. If the needle moves further away from set, then lower the ta, and try it again. Keep doing this until you find the location of the Tone Arm where the needle does not move at all when the sensitivity knob is rotated.

Once this point is found, adjust the trimmer adjustment until the TA is at 2.0 and the needle is at "set".

The purpose is for the needle not to move at all from the set line when the sens knob is rotated from 0 to 32.

Adjust the balance adjustment (B) so the needle does not move when the sens knob is rotated. This will then usually not have to be changed much once the center point is found at 2.0.

Then you calibrate the M (Male) setting.

Male: Place a 12,500 ohm resistor across the can leads. Put the Tone arm on exactly the center of the 3.0 mark on the meter face.

Using the M adjustment screw, again place the needle on "set".

Put the female resistor back on, TA at 2.0 and then adjust the trim knob to standard trim for 2.0.

Now, place infinite resistance (air) across the can leads. Move the Tone arm To 6.5. Adjust the "I" Infinity trimmer until the needle is at "set".

Place the 5000 ohm female resistance across the cans again, and do a standard trim.

Move the TA to 3.0, place a M resistor across the cans, and readjust the "M" trimmer until the needle is again at "set".

Repeat the above steps.

You may have to cycle through these steps several times making smaller and smaller adjustments each time. Each of the settings depends on the other to some degree, and are adjusted in unison.

Once the B (balance) is properly adjusted, it will usually need very little change during the rest of the calibration procedure for the F, M, and Infinite settings.

When the needle does not move at all, trimmed at 2.0, and the sensitivity knob is rotated from 0-32, and both M resistance is correct at 3.0, and Infinite resistance is correct at 6.5, the meter is calibrated properly.