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.
On 8/16/2021 3:53 PM, 'orange_glow_fan' via neonixie-l wrote:
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..
On Friday, August 13, 2021 at 4:29:08 PM UTC-4 Mark Moulding wrote:
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.
tach.sch1.5.png
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