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 < neonixie-l@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] > > > > > -- > You received this message because you are subscribed to the Google Groups > "neonixie-l" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to neonixie-l+unsubscr...@googlegroups.com. > To view this discussion on the web, visit > https://groups.google.com/d/msgid/neonixie-l/9f464b84-4da8-4f97-a8bd-83b35b519fe1n%40googlegroups.com > <https://groups.google.com/d/msgid/neonixie-l/9f464b84-4da8-4f97-a8bd-83b35b519fe1n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/CAFxqQoL-gmLteUWzfVHkWkO1QHyLOi56pNK1XyUkMRBLr3Y4LA%40mail.gmail.com.
