600XL Memory Test Issue

[shoestring]

If you install the 4464s as is, the machine will still see the 16kb.

 

Check the 138 demux as well, I don’t have a schem infront of me right now but I believe that handles the output enables to the roms.

[asommer]

I switched out the memory and the same ROM test failure occurred, I've ordered some 74LS138 replacements, so I'll report back when they get here.

 

Thanks again

[+Nezgar]

I have the 64k upgrade kit, would you recommend doing that?

You can swap in the 2 new RAM chips from your 64K kit to test, they will just function as the same 16KB RAM until you complete the rest of the upgrade. Good for a test.

[asommer]

I've replaced the 138 demux with a new 74LS138 and have the same issue with the ROM test. When the machine works (usually when it's cold and only for a few minutes) the CE pin on the ROM has 0.9 volts (low) and when the ROM test happens the voltage is 1.5v. The same can be seen on the OS pin of the MMU.

 

Not sure what to try next?

 

Thinking to get a new MMU from Best Electronics cause they're pretty inexpensive and either way it may be nice to have a spare. Also, the traces between the MMU and ROM look good and test fine for continuity.