PC85 Core Board on 1248 Rev I Base board?

[Ernest]

Thanks Someone and Ernest.
Someone, clearly I have not read your post with 3 jumpers. I will endeavour to find it.
Ernest, I am using the same PSU as always. 
Someone and Ernest, I notice the main board board is using red header sockets. I might swap them out for headers that are a better match for my core board.
Ernest, The !PORT A signal. Is that the one on pin 9 of X4 on the main board? Because it seems to be connected to pin 10 on IC34. I couldn't see it anywhere else, but I haven't poured over the schematic either.

Cheers!

John

Yes and No.
Yes !PORT A is pin 9 of X4
No it should go to Pins 2 & 8 of IC34 on the Coreboard, but yes it comes from Pin 10 of IC34 on the Mainboard (how confusing) .This is the trigger with either read or write to latch the address for ROM Pak selection and NET ROM (Portion).
To remove one more variable, try removing TELCOM from the NET ROM location and see if the MENU comes back to normal operation.

I have had a bit of trouble with the RED headers some are fine some are a headache and best replaced.

Also Make sure that your Mainboard is fitted with a Z80 rated at 4 Mhz some of the early ones were on 2 Mhz and then modded to 3.75 by changing amoung other things, the 12 Mhz crystal to 13.5 Mhz however some CPU's and PIO's could mostly work but not always so you need a Z80A and same for the PIO.

Ernest.