So, synchroscopes are phase detectors, and only work when you're fairly close to the correct phase. They don't actually report frequency per se.

People have built mains frequency counters: https://fowkc.wordpress.com/2013/12/10/mains-frequency-displ... (via https://hackaday.com/2013/12/12/mains-frequency-display/ )

The core problem is getting an accurate enough reference. That project uses an RTC IC; you might want to try building something that uses a PLL on a GPS pulse-per-second output.

The second problem is cleanly spotting where the zero-crossing is. You can take a mains feed from a current transformer or a regular transformer and low-pass filter it appropriately. It is then easy enough to use an op-amp to spot the zero-crossing.

Thanks for the links. Indeed if the synchroscope needle is turning clockwise I know the grid frequency is greater than 60 and if counter clockwise then less than 60 and exactly 60 no movement of the needle at all.

To me there is something about watching the inertia of the entire system change and it controlling itself to stop frequency excursions and then AGC or other human interventions to restore the balance.

Thanks for the ideas and links.

For one thing, it is a lot easier to get the wall socket voltage and current down than it is to get the signal generator up to wall socket level.