Way off topic...tachometer circuit..

[orange_glow_fan]

I hope I can be forgiven for this.. ;) I had a TIA in February and my head still isn't working quite right..

Going to have to work on my OLD (1962) GM tachometer. It will only go up to 3,000rrpms,  then stop. It really doesn't 'look' like the meter movement sticks at that point, though I guess  it could..

I understand the circuit operation up to a point. It's pretty simple. I'm not sure of the part I circled. Do C2,  CR2and CR3 make up a voltage doubler? Also what is VR1 for. They suggest using two 9vdc zener diodes stacked cathode to cathode as a replacement.

[peter bunge]

It's a charge pump. VR1 fixes the pulse amplitude so the tach is not sensitive to battery voltage. C2 determines the charge per pulse and the meter averages the current because it cannot follow the pulses fast enough. R6 calibrates it.

I suggest putting a 'scope on it and follow the pulses from the input. Are the pulses on the collector clean and of the same duration and amplitude, and do they keep increasing in frequency and follow the input rpm?

Check it on the bench, not in the car. You can calibrate it against a pulse generator when it is fixed.

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[Mark Moulding]

I think bunge.pip described it pretty accurately.  (Charge pump / voltage doubler - tomato / tomahto)  In this case the doubler actually generates a voltage that's negative with respect to ground, which is why the meter polarity is "backwards".  All the circuitry to the left of the module is just wave shaping to clean up the pulses from the distributer, and VR1 is the voltage regulator.  Actually, it seems to me that a regular Zener should work there - I wouldn't think you'd need back-to-back devices...

I may just have to breadboard this up and play with it a bit.  If I were to design this from scratch, I think I'd use a 555 in a monostable configuration, triggered via a small capacitor from the the coil input.  Again, the meter needle mass would act as a low-pass filter for a train of fixed-width pulses whose frequency was related to engine speed (3/rev for a 6-cylinder?).

~~
Mark Moulding

[orange_glow_fan]

Thanks guys, that pretty much lines up, with what I was thinking...Sadly, since it seems to be frequency based,  I don't see a failure mode that would duplicate the problem I'm having other than the meter itself. 

It's odd as the problem first showed up when the sense wire, connected to the coil, came loose at the coil  and  made the tach operation very intermittent After fixing the wire I was left the meter stopping at ~3,000rpms. Before the wire came loose the tach worked fine. It's a PITA to remove the dash to pull the tach..

[Instrument Resources of America]

Automobiles are NOT the cleanest environment in town. Is it possible that dirt has entered the movement?  Is it a highly sealed movement? I sort of doubt that being an in expensive auto product. Does it have a removable lamp on the meter where dirt could get in?    Ira.

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