Rohde &Schwarz APN 62 keyboard problem

[Simon Spiers]

Hi all. I have just bought a R&R APN 62 LF sig gen in non working condition.
The unit powers up but I have no control from any keyboard button other than the rotary encoder. The unit is stuck on the termination adjustment  which seems to work fine but that's all I can do.
The generator is outputting a waveform so it looks to me like the CPU is working fine.
I have looked at the only chip that has any logic on it regarding keyboard entries and that is a NEC 71055 parallel interface unit.
Logic can be seen on pins 1 thro 4 and 39 and 40.
There seems to be no change to any other pins on the chip.
I have changed this IC thinking it may be the problem but it's not.
No manuals are available  unfortunately.
Any ideas pointers or help would be greatfully appreciated.