The Timex halves of the two EPROMs are completely standard TS2068 ROM images. The Spectrum half of the HOME EPROM is a very slightly modified version of the Spectrum ROM, modified to reset port 0xF4 and 0xFF on startup. The Spectrum half of the EX EPROM is a very slightly modified version of the Timex EX ROM, modified to jump to the START-NEW location in the Spectrum ROM on startup instead of the Timex HOME ROM location.
These modifications are necessary to ensure that the computer starts up every time reliably when in Spectrum mode. Unfortunately it is not possible to boot reliably on a TS2068 from a 100% exact Spectrum ROM.
The motivation for putting a Spectrum 48K ROM in a Timex Sinclair 2068 is to be able to run Spectrum 48K software. Whereas there are probably less than 100 Timex Sinclair 2068 programs, there are more than 10,000 Spectrum 48K programs. The Spectrum 48K ROM is also somewhat faster than the Timex Sinclair 2068 ROM and has fewer bugs. It therefore provides a good platform for exploring the TS2068's extra video modes.
To perform the modification, the components connected to the channel 2 / 3 switch need to be removed and the switch re-used as a ROM select switch. Take the TS2068 apart by undoing the seven case screws on the underside of the machine and unplugging the keyboard membrane from its header on the PCB. There are two sizes of case screw so preserve where each screw went. Then undo the three screws holding the PCB onto the bottom half of the case and take out the PCB. Take the cover off the RF modulator shield and examine the inside.
Remove C46, R57 and R56 by desoldering them from the PCB and removing them with pliers. These three components are connected to the channel 2 side of the switch. and are in a line in the top left corner of the shield to the right of the solder pads for the switch itself. Leave C47 in - this will provide a useful anchor point to connect +5V to on the underside of the PCB (the solder side). Remove R28 and R24. These are the next two components down after C47. Finally, remove R58. This is the component to the left of the bottom pin of the channel switch.
Solder a link wire in the pads where C46 used to be. This connects the channel 2 side of the switch to GND. I used black plastic covered wire to indicate that this wire is at ground potential. On the underside of the PCB, solder another link wire between the side of C47 connected to the switch and a +5V point. This connects the channel 3 side of the switch to +5V. Looking at the underside of the PCB, it is the leg of C47 on the right. A suitable +5V point to use is the top leg of Q2 - this has a thick trace snaking away from it to the left and down.
Now the switch provides +5V or GND, the next step is to take this signal to the ROM sockets and reconfigure them for EPROMs instead of mask ROMs.
On the standard machine, jumpers W1 and W2 are fitted in the centre of the PCB. This connects MREQ# to CS# on the HOME ROM and ROMCS# to pin 27 of the HOME ROM. The mask ROM in the standard machine has an output enable pin at this location. The ROM is decoded by three signals, MREQ# connected to CS#, ROMCS# connected to OE1# and RD# connected to OE0#.
In the modified machine, a 27256 EPROM is used for the HOME ROM. Pin 27 becomes A14 which is connected to the channel 2 / 3 switch. ROMCS# is connected to CS# (pin 20) and MREQ# is connected to OE# (pin 22).
Remove jumpers W1 and W2 and solder a link wire between the bottom left pad and the top right pad. This connects ROMCS# on the bottom left to CS# (pin 20 of the HOME ROM) on the top right.
Solder a wire from the pole of the channel 2 / 3 switch to the bottom right jumper pad. This connects the new ROM select signal to A14 of the new HOME EPROM (pin 27).
In the modified machine, a 27128 EPROM is used for the EX ROM. Solder a wire from pin 27 of the HOME ROM socket to pin 26 of the EX ROM socket. This connects the new ROM select signal to A13 of the new EX EPROM.
Solder a wire from pin 28 of the HOME ROM socket to pin 1 of the same socket. Solder a wire from pin 28 of the EX ROM socket to pin 1 of the same socket and to pin 27 of that socket. These wires carry +5V from pin 28 (Vcc) to pin 1 (Vpp) of the EPROMs and also to pin 27 of the EX EPROM (PGM#).
Next, cut the track above and to the right of the top right jumper pad. In the standard machine this carries RD# to pin 22 of the HOME ROM. Solder a wire from the top left jumper pad to pin 22 of the HOME ROM socket. This connects MREQ# to OE# of the new EPROM.
Now the machine is ready for the two EPROMs. Remove the original Timex mask ROMs from the two ROM sockets and install a 27256 EPROM programmed with the file "Switchable TS2068 27256.rom" into the HOME ROM socket. Install a 27128 EPROM programmed with the file "Switchable TS2068 27128.rom" into the EX ROM socket.
Test the machine at this point by connecting a power supply and a composite video display to the PCB and turn the power switch on. It should boot up in one of the two modes, Spectrum 48K or Timex Sinclair 2068, depending on which position the channel 2 / 3 switch is in. Spectrum 48K mode is the channel 2 position and Timex Sinclair 2068 mode is the channel 3 position. You can mark the case to this effect if you like.
Assuming all is well, put the PCB back in the lower half of the case, put the PCB screws back in, reconnect the keyboard and put the case screws back in. Enjoy your switchable Timex Sinclair 2068 in its original mode and in its new Spectrum 48K compatible mode.