Re: Dead DX7 Mark I - Repair questions -- Help!

Where are you located?

You need to look at the schematic and parts list. 

The stock ROMs, including the SER, are of type 27128A or 27C128. Your 27C256 could be kludged in but I don't recommend it.

The SRAM is 5116 or 5118. The best thing is just to search for Yamaha DX7 SRAM probably.

There is no v1.9 the last factory was 1.8.  The SER was made by Yamaha Canada. It's good, but if you are a novice then the stock is better.

There are several other expansions that will give you 0-127 on the MIDI out. None of the standalone EPROMs will at this time. I made versions of the standalone EPROMs that turn MIDI Active Sensing off but haven't done the MIDI out correction (yet). It's only needed if you use the DX7 as a MIDI controller.

Sys Ex is not so bad if you have good instructions and a known good computer setup. I will leave that description to others.