32kb Sharp Cartridge

Some old Sharp/EPCOM cartridges are 16kb, but I wanted to modify them to run Hot-Logo, which is a 32kb cartridge. This should be easy, just replace the original EPROM with a bigger one, right?

Nope! These Sharp cartridges lack the A14 track on the connector! It can't handle 32kb by itself.

However we can still add a 32kb EPROM if we get creative. Since we don't have A14, we can use /CS1 to switch between the upper and lower 16kb of the ROM. Those are the modifications required, starting from an Editor de Música cartridge:

First you have to remove the old 27128 EPROM and replace it with a 27256, previously programmed to have the Hot-Logo ROM. You can also clear the board of old flux using some acetone. Then, cut the board on the position shown in the picture below. Pin 27 is /P on a 27128, and it was correctly tied up to pin 28, which is Vcc. On a 27256 however, this is A14 instead, so we need to cut the connection.

Next you need to remove the solder on the jumper shown in red below, disconnecting /CS2 from the old /OE, as this isn't a 16kb BASIC cartridge anymore. The new /OE can be the same as /CE now, and we can solder the pins 22 and 20 of the EPROM together to ensure that. I just added a jumper on the holes shown in blue, and soldered it to the EPROM from below.

Finally solder a wire from /CS1, which is pin 1 on the connector, to A14, which is pin 27 on the EPROM. Ensure the jumper on /CS1 is *not* shorted, as it would connect /CS1 to /OE instead.

That's it, with these changes the Hot-Logo cartridge works as expected!