Selectest Multimeter

Super 50  Mk2

serial no. 8428 MRF

Selectest Super 50  mk2 multi range test set was one of the electronics engineer's favourite multimeters, it is on a par with the much more popular Avometer model 8 which is almost exactly the same size. The scale on this instrument lacks the dB scale of the Avometer but is somewhat larger. In 1972 it had a list price of £34.15 just a few pence less than the Avometer model 8 Mk4 or the model 9.

interior with scale plate removed

The meter uses the same type of batteries as the ubiquitous Avometer model 8
 

The instruction plate is held in position with four screws and hides the battery compartment.

I was given this test meter by Bernard Griffin in 2016 who told me that it came from a school physics lab but had been condemned on the grounds of health and safety. Surely it would be perfectly safe for kids on LV experiments? When received it was in need of a clean and the glass was loose and I also noticed that the terminals seemed to be insecurely fixed. The meter was working but I noticed that the 10 volt DC range was inaccurate and indicated a higher voltage than it should despite reading correctly on the current ranges. Removal of the instruction plate revealed the battery compartment and a corroded battery contact. The meter front cover is held in place by four very long screws at the corners of the back panel. The two range selector knobs and the ohms adjuster need be pulled off before the front casing can be removed. Be careful not to lose the cutout and reverse moving coil buttons.

Once the meter was apart I found that the moulded bridge on which the terminals and adjuster potentiometer was held in place by two clip washers in the battery compartment, these appeared to have lost their grip so I resorted to a couple of drops of super glue at  the points where the bridge met the  rear casing. The glass was removed, cleaned and glued back in place and the meter scale gently cleaned.

I measured the resistance between the + and - terminals and found that the resistance was as expected on the 50 µA and 2.5V ranges but was 177kΩ rather than the expected 200kΩ on the 10V range. This meant that further disassembly would be needed as the meter would have to be removed to get access to the printed circuit board. Both are held in place by four screwed hexagonal pillars on the underside which can be undone using a slotted screwdriver after removing the protective wax insulation. The meter movement can then be disconnected and removed from the board by unscrewing two of the pillars. The resistors are clearly marked with their values and I soon found that the one marked as being 150kΩ was in fact 127kΩ I replaced  this with a slightly high 150KΩ 5% in parallel with one of 4.7MΩ.  The arrow on the picture below indicates the position of the replacements. After this modification I measured the the current required to give full scale deflection on the 50 microamp range and found it to be reading 97.5% low  and similarly across the scale. I adjusted the magnetic shunt (arrowed in the picture below) to adjust for full scale.

I am indebted to Roger Walker who kindly sent me a copy of the instruction manual.

View with meter movement removed and underside of circuit board

The following taken from a 1972/3 catalogue gives a good indication of the capabilities and specification of the instrument.