[Early_S_Man]

Kevin,

A Zener diode is a Voltage regulator with precise characteristics when installed reverse-biased in a circuit.

In my unit a transistor was installed at the D1 loacation, but others that have emailed me have had a small glass 400 Milliwatt Zener diode mounted with the colored band Cathode end going into the 'E' hole and the other end going into the 'B' hole on the board. When a diode is installed, nothing goes in the 'C' stenciled hole location, which is electrically isolated from the rest of the board! Just to reiterate ... on my schematic, the Zener Anode end is grounded at the 'B' terminal and the Cathode end (with the band) goes to the 150 Ohm 2 Watt resistor connected to the 'E' terminal!

It isn't too unusual to find the emitter-base junction of a silicon transistor used as a Zener diode, but since an exact replacement for that Siemens transistor can't be readily found, it is much easier to use an 8.2 Volt Zener diode like a 1N756A that is still available from many sources.

Ron,

Did you get my 'calibrator' circuit built? If you connect the output leads from the contacts between ground and the Black/Violet wire pulled from the distributor ... with the ignition on, the tach should read 1200 rpm or 2400 rpm, and the presence of the +12 Volts at the Microswitch should tell you if the Speed Switch is working.