Bosch MMD 302 an ancient multimeter painted with Green color

[Kiriakos GR]

rotnei,

All such calibration pots are famous for instability ( especially at handheld products due vibrations).

Modern multimeter design 2005 and later they have as standard, calibration functions due software ( Goodbye Pots ).   

Test and Trial of few pots stating of high value ( 100K ), this is a good way to start your repair.
Few years back I was had an personal archive of ancient (low cost) multimeter schematics, as assistance for repairs, later on I did realize that restoration of ancient meters, this is bad for our brain cells.
Modern tools are so much faster with new functions, these making them 80% more productive and compatible to modern applications.

This Bosch meter internals (components) are interesting, this meter using high quality parts, and this translates to improved electrical safety for the user, and also better accuracy in Law of Ohm measurements.   

I wish you good luck at repairing it.