Random MVS MV-1C Backup RAM Error Help 00D00000 5555 5545

[Josh128]

Hello everyone.  I have an MV-1C that I consolized recently.  It has the odd problem of giving the backup RAM error below about half the time I boot the system.  Note that the error is almost always the same address and r/w values.  When I dont get the error, most games on the 161 in 1 cart I have work just fine.

Address     Write      Read
00D00000       5555         5545

Now I have done a bit of learning and yes I have reviewed Channelmaniac's great repair logs but I have some questions.  On an MV-1C, which backup ram chip handles the "45"?  I cant seem to find hard info on which chip handles the first two digits and which handles the last two.   Also, I dont have any "obvious" visible trace issues, but I also havent been able to find a good pinout map / schematic to know where to check from the legs of each pin to their destinations. 


I do have access to a logic probe, oscilloscope, etc.  I have ordered some quad OR gates and RAM chips and have SMD rework and soldering tools to be able to change them, but Im still at the basic "Which RAM chip is which" problem at this point.  Can anyone provide any insight?

Thanks!!

[Josh128]

Just a followup, fixed the issue.   When trying to pinpoint the proper backup RAM chip,  I didnt understand at first that the CPU D lines are all tied to the I/O pins on the RAM ICs and I  so I checked the address pins on the CPU first.  When I didnt get any continuity on anything I checked the address lines on the CPU and realized they matched up, but each line goes to both chips. 

Then I realized the CPU D lines are tied to the I/O lines, and I was able to pinpoint the correct chip.  Good thing about my error is that at least I verified that continuity was good on pretty much everything.
Anyhow, I swapped the culprit RAM and it worked.  Thank you so much and I have some info to add to your repair logs if you would like-- the backup RAM location for the MV1C board.
D7-D0 = RAM 2, which is the upper RAM IC in the backup area
D15-D8= RAM4, which is the lower RAM IC in the backup area