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

On Thursday, August 12, 2021 at 7:04:53 AM UTC-7 bung...@gmail.com wrote:

> 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.
>
> On Thu, Aug 12, 2021 at 9:17 AM 'orange_glow_fan' via neonixie-l <
> neoni...@googlegroups.com> wrote:
>
>> 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.
>>
>> [image: tach.sch1.5.png]
>>
>>
>>
>>
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