I concur with Andy regarding the OA95.
The OA95 diode's job is to supply RAM power from the battery when the main power is off.
The ZD1 circuit's job is to detect the power being turned off and put the RAMs into a safe state so that other parts of the circuit don't cause spurious writes of bad data as they get powered down.
Even if your battery is good, and D4 is working correctly, and the RAMs stay powered up (above 2V needed for data retention), if the Zener circuit isn't working then a write of random data could happen as the power is turned off. This would appear as RAM corruption when you next turn on.
I had this exact problem on my DX7 which I tracked down after changing the battery, D4, and several RAM chips, with help from this forum!
For interest, if you didn't already know:
A standard diode only allows current to pass in one direction, from anode to cathode, with a small voltage drop. This forward direction is the direction of the the triangle in its symbol. When the voltage at the cathode is higher than the anode (reverse biased) the current is blocked.
A Zener diode is a special type of diode that is designed to 'break down' at a particular reverse bias voltage and will allow current to pass from the cathode to the anode. In the case of ZD1 this voltage is close to 6.2V.
The location of ZD1 in the DX7 PSU allows it to detect if the voltage before the voltage regulator has dropped and ultimately turn off the chip selects on the RAMs before the output of the regulator (with lots of power supply capacitors) drops.
I hope this makes sense!
Ant.
