[PETERg0rsq]

The prom "reads" the divider lines from the counter to the divide by N and sets the limits by pulsing the "tune up" or "tune down" lines, which simulate the operation of the spin-wheel, until the out of range frequency comes back into range.

Each range and channel spacing setting will have a different "lookup table" in the PROM

You can eliminate the PROM by disconnecting the two lines from it to the digital board.

I have reverse engineered the PROM code, but first it is best to "read" the address lines going to the PROM and check the counter circuits are working correctly. Sometimes a divider chip fails and doesn't pass on the carry bit to the next divider, so you might need to decode all the dividers, which is easier than it sounds if you have a logic analyser.

Start with the basics, make sure you have the correct pulses coming into the counter chain and check the pulses propagate through the dividers correctly (the division factor of each chip is indicated in the manual), and can then be read at the PROM

Then check the blanking circuits (examples of how it works in the service manual) and finally check the divide by N section is working