When your games go on the fritz - Suteneko's repair & mod thread.

[Suteneko]

SEGA Master System Cartridge ROM Replacement:

@anon8946355 asked on our Discord if anyone could program EPROM as:

I want to de-solder the Mask ROM on some SMS games and replace them with an EPROM.

For example:
De-solder Enduro Racer and replace it with the Japanese ROM.

So I started to look into this, but unfortunately perhaps as an anti piracy or more likely a price cutting measure SEGA opted to include a Memory Mapper inside the actual Mask ROM of the majority of SMS titles.

Without a Memory Mapper the SEGA Master System can only access 48KB which really isn’t enough for a lot of the games that would benefit from a Mask ROM swap to apply fixes or modifications.

However, a few titles did consistently use an external Memory Mappers and these therefore can have their Mask ROM replaced with an EPROM with capacities higher than 48K of which there are three known variants:

315-5208 - 1Mbit Maximum
315-5235 - 4Mbit Maximum with Battery Backup
315-5365 - 4Mbit Maximum

Here is an example of two different 2MBit SMS cartridges:

The one on the left (Rainbow Islands) has a MMC embedded inside the Mask Rom while the one on the right has an external 315-5235 MMC (Rastan).

If you want to read up more about this check out the Paging Chips document over at SMS Power.

Has a test I decided to try to put the SMS Power Phantasy Star re-translation 2.00 onto my original Phantasy Star cartridge:

Opening up my cartridge you can see in has an external 315-5235 MMC:

Using my De-soldering Gun I de-soldered the Mask Rom:

Mask ROM removed:

Phantasy Star uses a 4Mbit Mask Rom so I needed to find a pin compatible 4Mbit EPROM that matches the pin out below:

I choose a 27c040:

Using Lunar IPS I patch the Japanese Phantasy Star ROM (CRC:6605D36A):

Which results in a patched ROM with a CRC of A04CF71A.
I then write the patched ROM file to a 27c040 using my programmer:

Then I soldered in the 27C040:

Since this will be inside a sealed cartridge there is no need to cover the EPROM window:

Because the Phantasy Star re-translation contains FM audio I decided to test this in my MegaDrive as I don’t have a FM enabled SMS, but I do own a DBElectronics SMS convertor that includes FM Sound:

and it runs perfectly! Even my original saves load up fine:

Sadly a lot of games that would benefit from patches (such as Rainbow Island which has a glitch that crashes the game so it can not be properly completed) do not use an external MMC, so you would have to find an appropriate donor cart which is not something I like to see being done.

As for the original Phantasy Star Mask ROM, I inserted it in the ESD safe tubing that the EPROM originally came in and Blu Tacked it inside the case: