AVO 8 mk 6 calibration

[PJL]

I picked up an AVO 8 Mk 6 as it was really cheap and complete with case, manual and leads (but only alligator clips and no probes unfortunately). It all works but is reading maybe 3% low. Do I just remember to add 3% or is it possible to fix this?

[pmmunro]

PJL,

The circuit diagram for the Model 8 Mk 6 is attached for reference.

Does the meter read low to the same extent on all ranges? A full assessment of the accuracy on all ranges will give you a guide for a course of action. Usually these meters stay in calibration if nothing is damaged but a component fault is unlikely to affect all ranges - that would suggest a possible loss of magnetism. It is also worth checking that all switch contacts and the cut-out contacts are clean. (Do not use a spray cleaner as there is a risk of overspray condensing on the hairsprings of the moving coil). Is the pointer straight?

There is no magnetic shunt adjustment as on instruments up to Model 8 Mark IV.

As the whole instrument works around the characteristics of the moving coil, it is important to first check the sensitivity of the 50 microamp (DC) range. The circuit diagram shows a resistor (R27), in series with the moving coil, which is marked as approximately 1K6 ohms, Adjust On Calibration.

The other critical figures given on the circuit diagram are 100 mV at 37.5 microamps. across the movement and a resistance of 2667 ohms. As R27 is inside the arrows enclosing these figures, it would appear to be intended as part of the 2667 ohms. The correct values are critical to getting the correct readings on all direct voltage and direct current ranges.

For AC ranges the adjustments are, R28 (for alternating current ranges), R6 (10V AC), R7 (for alternating voltages above 30) R8 (3V AC) are also marked AOC.

I don't have a service manual for the Model 8 Mk 6, but the order of adjustment, if needed, R27, R28, R8, R6 and R7 is logical.

The circuit of the Model 8 Mk V is electrically very similar and the same component reference numbers are used so the two attached fault finding pages from the Mk V service manual may be of some help.

PMM

[PJL]

Thanks moderators for correcting my stupid error and moving this!

PMM: It came with the Operating Manual, do you want a copy of the circuit from it? I will check the input resistance and sensitivity on the 50uA range,