Do not use 4 AA batteries, max is 3 cells. 6V is over the maximum rating for IC27 which is powered by CMOS Battery voltage. It's absolute maximum supply voltage is 5.25V. You should check IC27 to see if it is still functioning properly.

Did you measure the voltage on the 5101 with the game off?

When you power up the game and it starts displays off, are you seeing the LEDs come on and turn off? Use your meter (or logic probe) to see what the blanking signal (TP4) is doing in this case? Is it HI (5V) or LOW (0V)?

Did you have to install a socket for the 5101 or was there a socket there already? If you installed a socket, check your work.

Check the status of the Memory Protect signal (TP3). With the coin door open, it should be LO and with it closed it should be HI.

What about TP3 with the coin door open?

If the socket is good and you are getting 4-5 v at pin 22 of the 5101 with the power off, I’d try another 5101.