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CM;
It sounds like a classic "thermal intermittent"...fancy term from the electronics industry, which doesn't help much in fixing it...but if a number of you are experiencing the similar problem, it might indeed be a fault of the circuit design (some transistor does flacky things when that hot)...but at least now you have a high-tech name for it...
ORRRR, the calibration pot is mechanically intermittent and opening just when hot, sending the reading high...this is a well-known symptom and effect of the early tach (found in the carburated cars)...perhaps a similar thing is happening with the later tach also (I've never experienced in the '73ES)...I was lead to believe the circuit used is it is common to all Smiths tach (I have not been able to confirm this!) ...I had a circuit diagram of the tach which I will try to locate and post...
...I wouldn't say the tach "is .... on the way out." just yet...
Cheers
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