WARNING: You do this project at your own risk!
First of all open your GX-4000 (2 screws) and search for the ASIC. It's the IC with 160 pins (yes, the IC With the really small pins :-) ).
Set them on the existing 2 RAM-chips (IC10,IC11) and connect all Pins except pin 16. This pin must be left free because it's the Chip-selection for the RAM. Now you have to connect the pins 16 from the upper RAMs together. At one pin, you also connect the 33ohm resistor. The end of the resistor will be connected with pin 91 of the ASIC.
Now search for IC8 (it's a 74HC4051N). Here you have to connect a resistor 10kohms at pin 9,10 and 11. The other side of the resistors will be fit together and then connected to pin 8 of the chip.
Now connect pin 119 from the ASIC with that side from C17, which is not connected with the ASIC. There you also have to connect a resistor 2k2ohms. The other side of the resistor has to be connected to ground.
Now connect a resistor 2k2 to the ground side of LK4 and one to the ground side of LK1. The other side of the resistors will be connected to pin 127 and pin 129 of the ASIC.
Now cut the pin 159 from the ASIC to the PCB. Instead of the old connection, connect the pin to the last resistor 2k2. the other end of the resistor will be connected with 5V on the PCB (5V will be found directly in front of pin 159 - it's the hole in front of pin 159).
The keyboard-pins will be found at the ASIC from pin 71 to pin 80. Pin 80 is keyboard row 0, pin 79 is row 1, ... , pin 7l is row 9.
The lines for the keyboard will be found at the Soundchip (AY-3-8912 or IC14).
Sorry, but I can't give you a good instruction how to connect the pins with your CPC-keyboard because I know more than 6 different keyboard-pinlayouts which will be used by AMSTRAD. The easiest way to find the right pins is to look at your old CPC and connect the same soundchip-pins with the keyboard as on the old CPC.
For the rows look at the 74LS145 on your old CPC PCB. There you can find the right pins.
Connect the the same way to the ASIC like they were connected to the 74LS145.
So, if you are ready with the GX-4000 up to this point you can make your first test. Connect the GX-4000 to your Monitor or TV and insert the CPC- Plus cartridge (not the original GX-4000 cartridge!). Then switch on the GX- 4000. On the screen should the the selection menu (F1/F2) appear. Now you can choose BASIC and the normal BASIC-screen should appear. Here you can test your keyboard.
Most errors were made at connecting the keyboard!
If all is right you can go on and make your Printer-port (if you need one) at the Expansion-port. I don't tell you how you have to connect the pins to the double-sides PCB'S because you can find the connections in every book about the CPC (e.g. your manual).
All pins will be found at the ASIC:
|146||Data Bit 0|
|147||Data Bit l|
|148||Data Bit 2|
|149||Data Bit 3|
|151||Data Bit 4|
|152||Data Bit 5|
|153||Data Bit 6|
|154||Data Bit 7|
signals from the Z80:
signals from the ASIC:
Now you can connect every expansion to the GX-4000.
But if you want to connect your old DDI-1, there is a little thing, which you have to correct. The DDI-1 has an internal ROM and the Cartridge from the Plus also contains the AMSDOS. So you have to "kill" the selection-system from your old DDI-l.
Open the DDI-1 Controller and remove the AMSDOS-Rom Then you have to cut pin 43 from the connector to the PCB. Now your DDI -1 will work correctly with your CPC-Plus or GX-4000.