Probably the worst fault that you can have on an MTX is the "black screen of death", i.e., powering on the MTX to be greeted by a black screen and constant tone from the sound chip. The tone is generated by the sound chip (obviously) before it gets initialised early on in the ROM boot process. The fault can be caused by many issues, such as ROM, RAM or other logic chips, but far the most common seems to be a failed RAM chip.
A few years ago, Mark posted a method of disabling the on board RAM so that system RAM can be supplied by an external RAM card . . .
http://primrosebank.net/computers/mtx/r ... repair.htm
At the time, Mark used this technique to allow him to use one of Andy's replacement RAM boards . . . .
http://www.nyangau.org/memcard/memcard.htm
But there is another way of supplying the RAM . . . . .
Normally, MFX supplies additional RAM to take either a MTX500 (32k) or MTX512 (64k) up to 512k of RAM. However, with a replacement GAL fitted to MFX and the on board RAM disabled as above, or removed, MFX can supply the base RAM too. I will shortly be posting the GAL code (created by Martin) to the MTX Firmware page for use by MFX users as needed.
http://primrosebank.net/computers/mtx/p ... mfx_fw.htm