[pmmunro]

On the Model 8 Marks I to IV, the ohms and ohms x 100 pots. are sealed types, Morganite from memory. Unless you know a technique for resealing these, cleaning is probably not practical as they would have to be cut open. However, the most troublesome pot. is often the ohms div by 100 and this is an open wirewound track. To clean it the movement has to be taken off the front panel.

With Model Eights, you have to be careful to avoid any sideways force on the spiked studs which make contact with the battery compartment 'fingers' as the studs are mounted on the bakelite plates which support the adjustment pots. and undue force will break the mouldings which do not seem to be amenable to gluing. I have found that a good repair can be made by bonding two thicknesses of 0.125 " pcb material together and drilling and cutting to the template of the original. Cut off the moulded feet for reuse as spacers.

A non-magnetic screwdirver (brass or a suitable grade of stainless steel) is recommended for unscrewing the two 2BA screws which retain the movement to the front panel but a piece of plastic sheet of the right size can be used to prevent contact between a steel screwdriver and the magnet. Knocking the magnet can reduce the flux and require re-magnetising. DO NOT on any account loosen the magnet clamp nuts.

The pot. track wire can be cleaned with isopropyl alcohol and a cotton bud. It is probably best to clean the wiper arm and spindle in the same way and to lubricate the spindle bush with a film of petroleum jelly at the same time.

When the movement is refixed to the front panel, the cut-out will have to be readjusted as the clearance between the leaf spring on the movement and the wire pushrod will have changed. The cut-out table can be rotated a few degrees when the two clamp screws are released and should be set to trip on an overload of around 12 times and NOT to trip on an overload of 4 times. (The exact figures vary with different versions of the meter and service manuals but 12 times is a reasonable figure).

While working on the meter, cleaning the cut-out button shaft is a good idea as it is often sticky due to dirt. However, work on the cut-out mechanism is quite involved.

I hesitate to give too much information in case I give the impression that work on these meters is easier than it really is. A combination of skills similar to clock repairing and instrument building and the correct data and equipment are all needed in addition to general electronic skills.