*If* there is a problem with the RAM and it is not the RAM itself, investigate its selection circuitry and memory protect. If you want, you can use my system 11 testrom. Send me a PM with your email address if you are interested. It will make diagnosing a bit more easy. Still you need some knowledge in electronics and a logic probe or oscilloscope. And an Eprom programmer to burn the testrom.

For the selection circuitry, Q40, U35, U14 and U34 are involved. For the read/write - memory protection, U36, U31, U34 and U35 are to be investigated.

Marco