As Ken said, there are a couple of other chips which could cause problems in this circuit.
This is from
http://www.pinrepair.com/sys37/index1.htm#battery....there are two other chips involved in the memory protect circuit on sys6/7. On system6 that's IC27 (4071 CMOS) and IC12 (7808). But frankly it is very rare that either of these chips fail.If you've got someone to help with solder work, you could try replacing these. It's also a good idea to install a socket if you do decide to change them.
