Trouble pushing 4MB MMU board into socket

[sandord]

I checked for shorts again and used solder whick to remove as much solder as possible from the holes.

Now I get this:

PXL_20250928_112332976.jpg (98.51 KiB) Viewed 1675 times

[sandord]

The camera and the lighting makes it look a bit messy but I think there are no bridges.

PXL_20250928_121340109.jpg (486.78 KiB) Viewed 1665 times

PXL_20250928_121636160.jpg (502.02 KiB) Viewed 1665 times

[rubber_jonnie]

OK so black screen with a white border usually means a RAM problem.

So my money would be on the Shifter and MMU adapters having moved during the removal operation, or the cable has shifted.

[sandord]

OK so black screen with a white border usually means a RAM problem.

So my money would be on the Shifter and MMU adapters having moved during the removal operation, or the cable has shifted.

I've refitted the MMU board (this time I pushed the MMU down all the way first), beeped the cable at the connectors and refitted the shifter. Sadly, it didn't help. The screen is still black with white border. I have a scope, maybe there are some signals I can test? I have only very little experience with that.

[stephen_usher]

Check for shorts between address lines and ras/cas lines. It could just be a solder spatter bridging something.

[rubber_jonnie]

After looking at your pic from underside of the board, I'm in agreement with @stephen_usher in that if everything looks good from the point of the SHifter & MMU adapters (Also check for bent pins on both) you may have shorts.

The holes are not very well cleaned at all I'm afraid and you need to get up close and personal with a magnifying glass or preferably a microscope in order to see if there is anything going on, it could be something as simple as a solder whisker. If you're struggling to clean the holes and pads, use some flux to help and if necessary add some leaded solder to what's there to help it melt easier.

Perhaps even use a multimeter to check for shorts. It'll take a fair amount of time, but it will find any shorts.

I know I said jokingly not to remove the RAM, but this is one of the hazards in doing so when you have a working machine.

I have a 520 STM (No built in floppy) and I got it working after lots of RAM faults, somewhat related to the piggyback RAM in it. I cut the piggyback RAM off and restored it to just 512KB, running great, and then decided to desolder all the piggyback RAM legs that had been left soldered on after I cut off the IC's. That killed it and I ended up pulling all the RAM and giving it a Shifter/MMU style upgrade.

My point? Sometimes it pays to stop when something is working.

[sandord]

Thank you both for your guidance. I have managed to get the machine working again!

The culprit was a very small piece of solder that I found using the microscope, after hours of removing visible solder using whick and flux.

I'm happy to have improved my desoldering skills as well

[rubber_jonnie]

Thank you both for your guidance. I have managed to get the machine working again!

The culprit was a very small piece of solder that I found using the microscope, after hours of removing visible solder using whick and flux.

I'm happy to have improved my desoldering skills as well

[stephen_usher]