Cool, thanks. Yeah, my plan is to get the machine working "vanilla", and from there I can decide what to mod/refurbish. This is also fully hobby project, so I may get this done in fits and startsrubber_jonnie wrote: ↑Sun Sep 23, 2018 3:43 pm My experience with the Marpet style RAM upgrades is that the MMU header can often displace the connectors in the MMU socket, and in fact I had exactly this issue on exactly this model of mainboard not long ago.
One part of the puzzle with this particular board is that the MMU is actually fitted the OPPOSITE way around to other boards I have, in other words, it's a 180 degree flip, so when installing the MMU adapter for the RAM upgrade, you also need to flip that 180 degrees too. That had me foxed for a few moments unti I noticed it was different to other motherboards in my collection.
I also ended up having to do a complete reflow of every last solder joint on the MMU and Shifter adaptors, and also the SIMM board.
Once that was done, everything sprang into life with 4MB of RAM .
My advice though, is to get the board working with just it's 512kb of ram to begin with. Put everything back to factory normal before even trying to get it going with the RAM upgrade.
So sort out the resistors, clean the MMU pins, check all the MMU socket pins are good, clean them, clean the MMU chip pins, and reinsert, rembering that the orientation is 180 degrees out on this mainboard, so line the dot on the MMU up with the dot on the socket to be 100%.
If the mainboard won't work in standard guise, trying to troubleshoot with extras attached is going to be a lot harder.
You can see more here: https://www.exxosforum.co.uk/forum/viewt ... &hilit=ram
The Marpet upgrade is entirely out, apart from the soldered socket over the shifter chip that you can see in the photos from earlier. Today I resoldered the resistors, and since my last post I popped out the Glue and the MMU chips and I didn't see anything obviously out of alignment in the 60 seconds I spent there. But maybe there's something more nuanced that my eye isn't trained to see on these chips. Just how sensitive are they?
The Marpet instructions seem to point to the MMU for "black screen" boot failures. But if I were to doubt myself, I'd think that one of the resistor connections I resoldered isn't quite right.