Hi there,
I was able to repair the tach amplifier on my '82 by carefully peeling the potting compound off the board and resoldering all the connections, also had to resolder the pins where the wires go from the board. Mercedes used terrible solder and it seems to fail on almost everything that they soldered together. The cruise control amplifier board had the same problem, also the heater control board.
Regards,
Richard Wooldridge
'82 300D/4.3L V6 - No engine problems since installing the astrovan V6 two years ago, get 28 mpg on the freeway, 20+ around town, almost twice the power now!