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The D.C. voltage measurement is a good way to test.
You should see the SIN and COS voltages swing from 1V to 9V, or so.
(Not at the same time.)
The tach should be as linear as it was before you fixed.
The critical parts are the ones in the tachometer charge pump.
(Datasheet, Figure 7) R1, R2, C1, C2
And of course the IC.
Looks like the only one you changed is C2.
I think its purpose is to reduce gain at high frequency.
Its 20% tolerance anyway.
Good work!
What I found was that using the 5.1V reference as a common point, that both sides (sin and cos) would swing between nearly zero to approx 5+ volts. When sin was rising, cos was falling. They tended to cross at approx 4 volts, for whatever thats worth.
Now if I measured using ground as my common, then I believe it was sin that varied between lets say 1 and 9+ volts but cosine just sat there at 10.2 volts.
Again I'm not sure exactly what it is that I should be seeing based upon my generators input, but I was encouraged by the fact that the shifting/opposing voltages appeared at both outputs as long as I used the 5.1V reference point as the common. And as the tach movement get's its common from there, I "THINK" what I saw means it's all good. We shall see.
Very nice, just got mine to work, thank you for details. Mine had a bad transistor inside chip pin 9 and 10. An external transistor, NPN universal fixed mine as in picture. Grounded on pin 7.
Symptom: Needle moved and stayed at zero RPM, engine running or not. Test signal from amplified sound card in PC, freeware signal generator SW.
Did a temperature photo after an hour powered with test signal, Chip (U1), D3, R4 pretty hot. Everything else cold.
Originally Posted by Kilroy1024
Cosine should move as well as sine.
The fact that it doesn't indicates a problem with the chip.
Unfortunately, the chip is obsolete and hard to get.
Here's some pics of my test of a tach.
Its not a vette tach, but it uses the same chip.
it shows the Sin and Cos voltages referenced to ground.
2 volts/division, ground is on the bottom.
06-24-2006, 02:17 AM
We shall see. 
