View single post by jls | |||||||||||||
Posted: 12-15-2006 09:45 am |
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jls
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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.
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