View single post by jls
 Posted: 12-15-2006 10:45 am
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Joined: 12-15-2006
Posts: 1
I have just fixed the tacho for my 1970 MG Midget which is the same as yours.  The fault was with the component you have omitted from your circuit which was a 2.5uF 16V electrolytic connected between pins 1 and 2 of the transistor (+ve end to pin1 which is the + rail of the circuit).  The original component was an axial type which went from the pin2 (base?) connection to the +ve rail, with the +ve connection looped under the edge of the board.

The fault on mine was that the capacitor had gone short circuit (old age).  I replaced it with a 2.2uF 63V aluminium electrolytic in a radial package which fitted the holes available in the PCB nicely.

I tested and calibrated it with a pulse generator.  The 50 ohm pot is for calibration.  The repaired instrument seemed to be quite accurate and surprisingly insensitive to pulse width and to ignition circuit current.  I haven't fathomed how it works or exactly what the feedback winding of meter current to the transformer winding does.