Taylor 88b reading low

[Barnmead]

I have been given a Taylor 88b multimeter. Like the AVO model 8 it has a 37.5 microamp movement with a universal shunt to reduce the sensitivity to 50 microamps. When I first tried it with a 9 volt PP3 it indicated about 12volts rather than 9 so I suspected that the universal shunt might have an open circuit resistor. The meter worked correctly on the current ranges but a tad low (4.8 instead of 5 mid scale) though the DC voltage ranges were adrift. After opening it up and finding an instruction manual at

http://raremanualdepository.blogspot.com/

I found that the volt ranges are partially separate from the shunt and that the error must have been caused by a bad contact on one of the switches as the voltage ranges now miraculously work more or less correctly.

Unlike the AVO the meter movement is totally enclosed so calibration and adjustment of the sensitivity is via variable slider resistors of which there are four. I am reluctant to meddle as I guess they must be tweaked in the correct order.

Any ideas please?