AVO Multirange No 1 High Sensitivity Meter

[sparkymike]

Well yet another AVO has surfaced from the loft. I had thought that I had three put away for a rainy day after a few boot sales, but this one now makes a total of six.
On first examination, it looks very pristine, neat and tidy under the face and the two small bakelite boards either side of the movement are intact, unlike my 8 mk.3 on which I had to fit paxolin splints underneath them to do an effective repair. Now this meter looks very similar to the 8 mk.3.
Is it the same circuit in general. I ask this question as there is a discoloured resistor on the lower long coil/resistance board that I would think has been overheated. Due to the lack of colours, I am unable to replace this component.
It is sited 43mm. from the end of the board with a small wire-wound resistor to its left.
This meter has a metal case so presumably made for heavy duty use, i.e. the services ?
If the Belgian man is still collecting serial numbers, I can provide him with all of my collection and photos, if he sends me a PM with his email address.
Mike.

[sparkymike]

I have taken some photos of the internals of the meter. Remarkable condition for its age. The unknown resistor is arrowed in photo 001 .
The transformer is encapsuled in resin. Was this normal with this model.?
Mike.

[pmmunro]

Mike,

The Test Meter High Resistance No. 1 is a military specification meter based on the Model 8SX Mark III but with ranges corresponding to a Model 9 Mark II, except that the Model 9 Mark II has international symbol markings rather than English language markings. The Test Set No. 1 has a NATO Stock Number and was used by the British and Commonwealth armed forces and possibly the forces of other NATO members.

The 'S' suffix indicates that the meter is screened, (by the outer steel case) and the 'X' that it is a "Panclimatic" version with vulnerable components encapsulated or otherwise modified to resist tropical environments.

The carbon resistors are trimming resistors which supplement the main, usually wire wound, multiplier resistors and add a small percentage to the main resistor's value to give the required figure for accurate calibration. For this reason, the carbon resistor values are specific to each meter.

The circuit diagram and the parts list for the bottom board are attached. This should allow you to identify the multiplier section which the wire coil and the carbon resistor make up and allow you to measure their combined value.

Sadly Steve Chellingworth who began the survey died in February 2011 but the survey is still running and we would be glad to have your contribution.