[old_tv_nut]

The -4 volts on the oscillator grid could be suspicious. It corrresponds to the high plate voltage. It says that the phase detector output voltage is low but the oscillator/hold coil is tuned to offset it.

Since this is closed loop operation, it is difficult to say if there's a foult or where it is.
Start by measuring the oscillator grid voltage while turning the hold control from unlock on one end to unlock on the other, and see if -2 volts grid occurs somewhere within the locked range. If the grid voltage is always low throughout the locked range, then I would check the phase detector circuit. Maybe the first thing is to replace the tube, then look for bad components.