Replacing Model 1 ROMs With A Single 27256 EPROM
Introduction
The Model 1 Level 2 ROM is 12k bytes and is divided into three ROMs (4k each) or two ROMs (8k and 4k).  The ROMs are often installed using a small daughterboard that connects to one of the mainboard ROM sockets using a 24 conductor ribbon cable plus a a handful of additional wires.  These instructions describe a procedure to replace the three or two ROMs and daughterboard with a single 27256 EPROM.  This can be done to either upgrade a Level 1 machine to Level 2, update a Level 2 machine to a later Level 2 ROM, install a custom ROM such as the FreHD AutoBoot or one with lowercase patches, or to just eliminate the daughterboard.
Although a 27128 (16k x 8) is large enough for the 12k needed a 27256 (32k x 8) is used here to simplify the installation.  The mainboard ROM sockets are 24 pin DIPs and the 27128 and 27256 are in 28 pin DIP packages.  The 28 pin EPROM is installed in the 24 pin socket with pins 1, 2, 27, and 28 overhanging the socket.  Pin 24 of the socket (5V) will conctact pin 26 of the EPROM (A13) - by using a 27256 A13 can be 'sacrificed' for the 5V connection and A14 used instead as the additional address connecton that is needed - otherwise for a 27128 pin 26 would have to be be bent out so as to not contact the socket and 5V would have to be wired separately to the EPROM - it is simpler to just use a 27256 and use its A13 for the 5V connection.
Becasue A14 is used instead of A13 the EPROM contents must be scrambled to match the connections.  However, in the procedure described here the A11 and A12 connections are swapped to the EPROM which require the contents to be scrambled anyway, so the scrambling for A13/A14 just affects the scrambling order and doesn't necessitate an additional step.
EPROM Programming
Start by obtaining the desired 12k binary ROM image file - the file size should be exactly 12288 bytes.
To facilitate the scrambling the 12k ROM file first needs to be split into six separate 2k files.  This can be done using a utility of your choice.  In Windows this can be done using the CPART utility available from here:
https://www.kadifeli.com/fedon/index.php?util#cpart
Assuming your ROM file is named level2.rom, use the following command to split the file:
cpart.exe -s level2.rom rom 2048
This will create six files named rom.001, rom.002, ..., rom.006 - each of which should be exactly 2048 bytes.
The 27256 is 32k bytes so the full scrambled image will be 32k, but only 12k is acually ever used.  The contents of the unused portions doesn't matter since it is never used so it can be filled with anything.  An erased EPROM contains all 1's (0FFh) so filling the unused portions with FF's is a reasonable choice - to do this you need a 2k file of all FF's which you can create or download here <insert link> - which should be exactly 2048 bytes.  Assuming that file is named ff2k.bin the scrambled file can be created with the Windows copy command:
copy /b ff2k.bin+ff2k.bin+ff2k.bin+ff2k.bin+rom.001+rom.003+rom.002+rom.004+ff2k.bin+ff2k.bin+ff2k.bin+ff2k.bin+rom.005+ff2k.bin+rom.006+ff2k.bin 27256.bin
This will create a file named 27256.bin which should be exactly 32768 bytes.  This file can then be programmed into your 27256 EPROM.
Alternatively, if the file ff2k.bin isn't conveniently available you can instead just fill the unused EPROM with an additional copy of the level2.rom:
copy /b rom.001+rom.003+rom.002+rom.004+rom.001+rom.003+rom.002+rom.004+rom.005+rom.005+rom.006+rom.006+rom.005+rom.005+rom.006+rom.006 27256.bin
EPROM Installation

1.	Prepare an EPROM that has been ‘scrambled’ to match the connections utilized here, as described above.
2.	Bend Pin 20 of the EPROM to stick out so it does not connect to Pin 18 of the socket.
3.	Jumper Pin 20 (CE) of the EPROM to Pin 22 (OE) of the EPROM, as the original ROM has no OE.
4.	Jumper Pin 26 (A13) of the EPROM to both Pin 28 (Vcc) of the EPROM and Pin 1 (Vpp)of the EPROM so that Pins 28 and 1 (which will both be unsocketed) will get 5V from Pin 26.
5.	Solder a fly wire to Pin 2 of the EPROM (A12) – Shown in Green.
6.	Solder a fly wire to Pin 27 of the EPROM (A14) – Shown in Orange.
7.	Insert the Pin 2 fly wire (shown in Green) into Pin 18 of the socket (which is the pin that Pin 20 of the EPROM would be in if it wasn’t pulled out in step 1). This connects EPROM A12 to socket A11.  It is recommended to attach a pin cut from an old DIP IC and attach it to the end of the fly wire so that it inserts securely into the socket.
8.	Insert the EPROM into the socket so that the bottom of the EPROM aligns with the bottom of the socket. This leaves pins 1, 2, 28, and 27 hanging off without being socketed.  Make sure that the fly wire insertert in socket Pin 18 in step 7 does not contact EPROM Pin 20 which was bent out in step 2.
9.	Attach the other end of the Pin 27 fly wire (shown in Orange) to Pin 3 of Z35 (for 16k RAM machine) or to Pin 3 of Z21 (for 4k RAM machine), so you are connecting EPROM A14 to the Mainboard A13 signal.
Prior to installation the prepared EPROM should look smilar to this:

Partially installed.  Note the fly wire from EPROM Pin 2 inserted into Socket Pin 18:

Fully installed:

Note the EPROM window should be covered with an opaque label or tape before final reassembly.
X3/Z3 DIP Shunt
The DIP shunt is labeled X3 on the schematic but Z3 on the Mainboard silkscreen.  Make sure the connections from Pins 10-7, 9-8, 11-6, and 16-1 are all shunted across, as shown in red below.

If any of these connections are broken then reconnect by replacing the shunt or by carefully bridging the open connections with a drop of solder.  Note the the metal shunts may be oxidized and the plastic body will melt easily with too much heat so clean the metal first and use flux.
The other connections (2-15, 3-14, 4-13, 5-12) are for RAM so leave those unchanged.
Power On
If all went well the machine should boot.  Here is an example of a ROM with lowercase patches was installed:

Replacing Model 1 ROMs With A Single 27512 EPROM
The procedure is exacly the same as for a 27256 EPROM except a 64k programming file is needed.  Follow the instructions to create the 27256.bin file, then:
copy /b 27256.bin+27256.bin 27512.bin

Replacing Model 1 ROMs With A Single 27512 EPROM With Two Different ROM Images
The procedure is similar to that for the 27256 EPROM.  However two scrambled images must first be prepared - call them 27256A.bin and 37256B.bin.  Combine these into a singe 64k programming file:
copy /b 27256A.bin+27256B.bin 27512.bin
Instead of jumpering EPROM Pin 1 to 5V connect it using a 1/8th watt 10k resistor.

The A or B image can be selected by either leaving Pin1 open (for B image) or shorting to ground (for A image).  For example, you might do this if you want to sometimes use the stock ROM and sometimes use the FreHDAutoBoot ROM.