The Amiga 3000 ROM bank switcher is a simple board which allow the installation of 2x 2MB ROM/EPROM/EEPROM in the Amiga 3000 Kickstart ROM sockets U180 and U181. The Amiga Kickstart ROM socket typically only supports a 512KB or 1MB ROM. This board provides jumpers which allows the other space of the ROM to be used as alternative Kickstart images. Please be aware that this switcher DOES NOT WORK in early Amiga 3000 motherboards (6.x) which require the ROM Tower.
DO NOT INSTALL THIS BOARD IN U182/U183. It does not work there, and could damage your Amiga and/or your ROMs.
You may use two of the Amiga ROM Bank Switcher boards, one in each socket of the ROM Tower. But ONLY if you have a Commodore ROM Tower. If you do not have the ROM Tower, or you would like to replace it, please check out the Amiga 3000 ROM Tower Bank Switcher. It replaces the ROM Tower entirely and allows for dual 2MB ROMs to be switched for early Amiga 3000 boards.
The A18 jumper supports three positions:
Some Amigas (A1200 for example) allow for 1MB ROMs instead of 512K. I don't know whether this is the case for the Amiga 3000, as there are already two ROMs and I'm pretty sure the maximum addressible space is 1MB. If you want to try using 2MB Kickstart ROM images, this board can accommodate that by placing the A18 jumper on A18/Bus. As far as I know, no Amiga which supports 4MB Kickstart ROM images, so you should probably not install a jumper at A19/Bus.
Instead of a switch or switches on the exterior of the computer, another option is to just use two-pin jumpers, placing one each on the A18 and A19 pins. This won't make changing the desired ROM image very easy, but you may only want to use a single ROM image anyway.
The Gerbers are available in Github here: https://github.com/cdhooper/amiga_rombankswitcher
The board on the left has been populated with 2x of 20-pin male-male bottom headers for the U180 socket. The board on the right has those headers soldered down.
The next step will be to install the two SMD resistors on the top of the board. Unfortunately there's no photo for that step.
Install the U180 socket and solder it down.
Install the bottom headers for U181 and solder them down.
Next, install the socket for U181.
and solder it down.
Finally, install the jumper headers. As you see here, I used two-pin headers, and installed them in the Low/A18 and Low/A19 positions. You could choose to install 3-pin headers if desired. You will want to use 3-pin headers if you plan to use Romy with 2MB or 4MB Kickstart ROM images.
You are done with assembly.
Verify that all header pins on the bottom of the Amiga 3000 ROM Bank Switcher are straight. This is very important, as you will not be able to see all the pins as you insert the board into your Amiga. Align the ROM Bank switcher with the Amiga kickstart sockets, ensuring the notch in the Amiga's sockets match pin 1 of the switcher. Use the photo on the left as a reference for the direction of the board.
Press very slowly, ensuring all pins which are visible around the edge of the board go properly into the Kickstart ROM sockets. I've found that using a magnifier helps a great deal with this task.
After the sockets are installed, you can install your two Kickstart ROMs/EPROMs/EEPROMs. The U180 ROM might be marked as "Low" or "ROM-1". The U181 ROM might also be marked as "High" or "ROM-0".
When done, you should minimally test that there isn't a dead short between GND and 5V with the switcher installed. The easiest way to do this is to check for continuity between pin 22 of either socket and GND of the Amiga (any metal contact from the chassis). If you see only a few Ohms, there is a problem. Pin 22 is located in the top right corner of the chips in the photo above (Pin 1: bottom left corner, Pin 21: bottom right corner, Pin 42: top left corner).
Next, you may want to do a test power-on of your Amiga to verify the ROMs were built and installed correctly. If you've removed your power supply from the frame, I suggest doing a power-on test where you can still access the ROM sockets. For the first run, plug in your power supply, hold your finger on the ROMs, and press the power on button. If your Amiga doesn't power on, immediately turn it off. If the ROMs get hot, immediately turn it off. If either happens, check that the sockets are installed correctly (for example, not off by a pin, installed on the correct side of the board). If the Amiga does power on and you don't detect any problems, verify that the power LED toggles from a dim state to a fully powered state. If it does, then you know that Kickstart has run at least far enough that you can confidently reassemble your Amiga.
Installing jumpers on the two headers will select the lowest address 512K block (each) of the two ROMs. Removing both jumpers will select the highest address 512k block of the two ROMs. Since 512K is selected in each ROM, this provides access to a 1MB image. If you've installed an enhancement such as Romy (by SpeedGeek), you will get access to a full 1MB image. Otherwise, you will be limited by Gary to a 512KB image. If you have Romy and wish to select between 2MB images, you should place a jumper on the A18/Bus position. The Low/A19 jumper can be used to select which 2MB image. If you would like a single 4MB image, then also set the A19/Bus jumper.
Enjoy!