STM white screen but suspect RAM issues

[StepehnM]

Hi all,
I was given an STM some time ago and only just fired it up after finding the corresponding power brick. It was showing a white screen on power up and didn't drop to the GEM desktop. My first thought was RAM so opened it up to find piggy backed RAM from hell.
Any suggestions?
I would like to upgrade this to 4MB but where would I start?

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[exxos]

First see my video on similar problems..

[stephen_usher]

First of all, never try to upgrade a non-working machine as you never know if it's the system or the upgrade at fault.

My first diagnostic, in lieu of a diag cartridge, is to do the basics, i.e. power, clock, reset and halt. However, the last one could be due to other parts of the system playing up. Does the reset line stay low? Does the system behave differently if you hit the reset switch?

Personally, the first thing to try is to reseat the custom chips, using some good contact cleaner before putting the chips back in. A bad contact on the MMU will produce a white screen. This is the least potentially destructive task.

I would advise that you get a the diag cartridge before going further than this and connect to the serial port for early messages.

Beyond this you'll need a good oscilloscope and some knowledge about what the normal signals look like.

[rubber_jonnie]

IMHO when the screen is white you're more likely looking at a ROM issue than RAM.

I'd start by looking at the MMU. GLUE and Shifter especially the ICs in the PLCC sockets.

A tweak and clean of the PLCC sockets and clean the Shifter socket would probably be where I'd start. A diag cart may also help.

Bad RAM usually shows up as a black screen with a white border or on screen corruption

[StepehnM]

Thanks for the tips and where to start. Ordered a Diagnostic ROM to help with repair, will keep you updated as to how I get on.

[StepehnM]

While waiting for the Diagnostic Cartridge, the PLCC puller arrived. Armed with the puller and contact spray I went ahead and removed all the socketed chips and cleaned the legs and IC sockets. Replaced all the chips and turned on.

Woop Woop!

IMG_20240413_145752.jpg

Looks like I have a running Atari ST like the one I originally owned.

Thanks for the tips and guides everyone.

[rubber_jonnie]

:girldance:

[stephen_usher]

Yay! As I suspected! :-)