Smith's Tachometer Information 9 - 2008 R. Kwas
This info is presented for reference only...and unconfirmed by me. It has been a long time since I've been inside a tach, and the last time I had one open, was long before electronic cameras were commonplace. The next time I have one apart, I intend to confirm details, and that the circuit within is as shown below.
Notes: Notice the inductive link only, of the input (primary of T1). No galvanic connection between input and internal circuit gives two distinct advantages: It assures that no failure of tach can dissable vehicle IGN, and that the same circuit (with some simple reconfiguration) can be used with "positive earth" vehicles (typical configuration of Britisch vehilces of the era).
The slider on the calibration potentionmeter (R4) goes intermittent and/or open. To permanently fix this, measure terminals of variable resistor, and replace with soldered-in fixed resistors. Recheck calibration after repair.
I recommend replacing C3 as a matter of course, and apparently Mr. Olson has some experience with other components failing.
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Additional Links:
Link to Brickboard Thread of subject of tach: http://www.brickboard.com/RWD/index.htm?id=1190301
...and a Link from that Thread to an informative site mentioned: http://www.classictiger.com/techtips/motach.html
Link to UK Volvo Forum Thread: http://www.volvoforums.org.uk/showthread.php?t=48795
