Toshiba EX100 Tech help needed

I am having an issue with a (no longer supported) Toshiba EX-100 PLC. The problem started with an external wiring short that shut down the PLC.

The short was repaired, fuses replaced, and the power supply modules were checked.

The hand held controler displayed a TCOM error so the CPU module was replaced with an NOS unit and the old RAM chip was placed into the new CPU module.

The HHC then displayed the error I/O Y0003. We switched modules 3 and 4 then the HHC displayed the error I/O Y0002 so we replaced the defective module with a NOS unit.

The HHC then displayed the error code; I/O YW0003 (01), C07! I/O NO SYNC. According to the user guide this means the self diagnostic thinks the module is not sucurely seated. We replaced the receptical(back plane) with a NOS unit.

The HHC now displays the same error as before.

Any thoughts?

Sounds like maybe the contacts on the modules or backplane are dirty or damaged. Inspect them closely with a good light and magnifier. You also said you moved the RAM chip to the new CPU unit, maybe it is defective. Did you also move the EEPROM chip to move the current program? I am including some links to manuals.
Links to manuals:
http://www.toshiba.com/ind/data/tag_files/EX100%20Handy%20Programmer%20Manual_519.pdf 5 megs size
http://www.toshiba.com/ind/data/tag_files/EX100 Users Manual_513.pdf 13 megs size
http://www.toshiba.com/ind/data/tag_files/M20M40EX100 Users Manual_1566.pdf 301k size

Here is a link to the Downloads page. To use, Click on Product column, Scroll down to PLCs, Left Click, then in the Download Type column, Scroll down to Manuals, Left Click, Apply filters. Find a file you are interested in, Right Click on the floppy disk icon to download, DO NOT Left Click.
Link:
http://www.toshiba.com/ind/downloads_main.jsp

Edit:
The No SYNC error means the I/O module did not respond. By the way, what does the "NOS" abbreviation mean?

NOS is the acronym for New Old Stock. The parts replaced were purchased/inspected from PLCCenter.com.

It was the EEPROM that was replaced,, not the RAM chip. My Bad.

The contacts were inspected and appeared to be ok.

Modules 3 and 4 were swaped after the new back plane was installed. The error code remained with the 4th slot.

I would first remove the EEPROM for safekeeping of the program. Next I would try the PLC without the EEPROM. Next, without the EEPROM, "Write" a program with just the END instruction on one rung to the PLC. Remove all the I/O modules, leaving just the CPU in the backplane and test in that configuration. If the problem still exists, I would suspect the NOS was not completely tested at the repair company and also has a problem. Unless they were in some type of sealed container or envelope, maybe the NOS could possibly be defective or exchanged without your knowledge.