I have had this rev counter since around 1986, its been unused for over ten years and the internal battery was dead and the power plug corroded in place.
There are more modern instruments but the Scitsu has a nice big easy to read display so it was time to dismantle it and see what it contained.
The Scitsu contains a Ni-Cad battery, cadmium is posionous!!
To get the casing apart it was warmed in an oven at 100 degrees for an hour or so to soften the glue, after that it came apart very easily. I didn't
get a picture of the inside before it was dismantled but it contained the meter, a small circuit board and the battery. The battery was badly corroded and was disposed
I will be running the XBR with a total loss ignition system so unlike previously, I will have a battery that could be used to power the rev counter.
The decision was made to remove the circuitry, replace the connector and then drive the meter directly from my ignition system using a PWM output.
The Scitsu circuit is very simple, it uses a 555 timer IC as a monostable. Every time there is a spark a pulse is generated, the mechanical damping of
the meter mechanism integrates the output signal and gives a stable reading. Its very simple and very effective.