Hi,
This was an awesome post on the tach amp.
How the Tach Amp works
I can only take a wild guess at what is wrong. I was thinking the threshold was a static value but apparently it isn’t. Capacitor C4 in the amp is some kind of a peak detector circuit that adjusts the threshold based on the frequency of the pulses coming in. You’ll apparently get less energy on C4 at lower rpms because the pickup fires less often. Maybe C4 isn’t working correctly and it is bleeding off power too fast at low rpms. Also there may be a short across C4 causing a higher bleed rate than the 1.2Mohm resistor puts across it.
Tantalum capacitors are usually pretty durable but maybe something has gone wrong after 30 years.
The other possibility is the tach. The schematic suggests you can hook up a signal generator to the tach and sweep the frequency to see if it works. Alternatively you can use an oscilloscope to check the signal between your tach amp and tach to see what’s going on.
Or you can try swapping parts if you don’t have the instruments. Here’s a thought, people here, and me, have had good luck resoldering the solder bumps in the tach amp. Maybe you have a weak connection in series with C4 causing the peak voltage to read low.
Good luck.