For completeness, the meter works. The scale is from 0 to 10K RPMs and the transfer function is RPM = 20K * mA.

 

I simply couldn't figure out how C3 could make current flow through the meter and have been staring at it for a long time. I went back to the board to double check the schematics that I had created and I found an error. I drew the schematics with the Ge diode D3 backwards. I have corrected the schematics:



This makes a lot more sense to me. When the points close, C1 and C2 charge. When the points open, Q1 turns off. D2 clamps the collector voltage to about 12.64V. C1/C2 discharge through the meter, R5/R6 & D4 until C3 is charged to about 12.64V. This moves the needle. After the points close again, C3 is discharged via D3.

I still don't have it working yet. I am down to replacing the caps, but am waiting till I get my hands on a 0.15uF cap.

I sure appreciate the thoughts you all shared with me. I will let you know what I learn as I go.

Mark

 

To close out this thread, I was able to get the tachometer working again. I managed to get my hands on a couple more tachometers with the same circuit and saw that there was a ground wire to C1and C2. When I reconnected the ground wire, the tachometer started working. Now I have to figure out why the other two tachometers have failed. Should be a lot easier now that I have a working unit to study. Here are the final schematics:

 

To further close out this thread, I finally started trying to repair the second tachometer. It is a Model 97 as opposed to the first which was a Model 797. The only differences are in the values of R4, R6 and C3. In the Model 97, R4 is 580 ohms, R6 is 330 ohms and C3 is 0.2uF, 100V. Another interesting thing I noticed is that the transistor marking is much clearer on the Model 97 unit, and I had the GE part number wrong. It is actually a 4JX1C1224. I found an NTE dross reference table that says that an NTE102 is an equivalent part.

I found that D2 was open, so I replaced it, but the tachometer is still not functional. I looked on the scope and it is clear that the transistor's E-B junction is open. I have ordered a few NTE 102s and will try replacing that to repair this tachometer.

Here's the latest schematic for the Model 797:



For completeness, here is the latest schematic for the Model 97:

 

For posterity, after working on this off and on for a few months, I was able to finally figure out that I still didn't have the schematics correct for this circuit. D2 was acting like a silicon diode, but what was really in the circuit was a failed 6.2V Zener diode. This actually makes more sense to me because the Zener diode ensures that the cap is always charged to the same level regardless of the battery voltage level.

I was able to repair several of these tachometers all of which had failed Zener diodes. I used 1N4735s to replace the failed Zener diodes. In one unit, I also had to replace a failed transistor. I figured out that an NTE 102 is a good replacement transistor as well.

Here are the latest (hopefully final) schematics for this tachometer: