After replacing every faulty chip I could find, I'm still having some issues.
The 68K board now has only half the RAM since a good 1/3 of the DRAM is faulty. The board does still support that amount which is fortunate. But the 'HALT' signal appears as soon as the Cifer is switched on - easily seen thanks to a handy LED on the board. There is another LED that seems to be used to indicate errors but I'm not really sure what it does - it seems to be labelled as 'BITE'. 'Bit Error' maybe? In any case, I discovered that if I removed one of the 74LS166s then the board would start. It would even seem to run the IEEE488 bus too as I could search for a boot disk and it would actually check for winchester disks on the bus (obviously finding nothing on the 68k board). Unfortunately, it now seems to have died. With the 74LS166 removed the 'HALT' error LED is off but the other one remains lit and the IEEE488 bus does not now start.
The Gfx board partially works - or at least it did. When it starts up, pressing F6 and then 'Graphics' puts the thing into Tecktronix 4010 emulation mode which should, in theory, allow me to type on the screen but with the characters being graphics rather than text. Up until a few days ago, the mode indicator would change on the status bar at the bottom but letters/numbers were really slow to appear and the mode would switch back to text without me pressing the graphics key. Now, there is garbage that appears on the screen, sometimes letters or numbers can be typed on screen but they fade out to nothing. When garbage appears on screen, it flashes in time with an audible high pitched noise from the board.
The SASI board just appears to be completely dead. It is connected to the XEBEC 1410 but nothing seems to be pulsing or strobing on either board to any great degree.
I've attached some (not great) scans/photos of the schematics if anyone is interested.
Any advice on troubleshooting is gratefully received...