So I've had to fire the guy that resoldered my amp. Turns out he was over confident in his abilities and couldn't be bothered to check for solder bridges.
The output was shorted to pin 14 which caused it to sit at ~2.5 volts instead letting the pull up bring it to VCC. Made the output cycle between 2.5 and 0 which the tach correctly ignored...
With that corrected, the tach now works. I'm going to hold off on potting it for now and will look into compounds that aren't such a pain to remove if it needs repair in the future.
I did measure both the voltage across the pickup and the current. Voltage was ~2.3 and current was about 27mA. The amp drew about 50mA at rest without the tach connected as displayed on my bench supply.
Michael