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Finally some progress! Lifted, cleaned up and re-soldered QFP 208 fine pitch Sonora chip. Not funny and impossible without a microscope as shorts are tiny even when soldering with soldering paste. Chime of death is gone and I was able to boot MacOS, keyboard and mouse are fully functional...

This is a common ground board, I usualy take "-" of PRAM battery connector, PSU GND pins or edges of connectors whichever is closer to the part I am measuring.

C21 is a cap on 8V power rail which is powering audio related stuff. Check PIN14 of UB10/DFAC chip. Reverse polarised caps explode as they become shorts for reverse voltage. UB10 could have been shorted, if PIN14 stays low = 0V after the cap has been correctly installed you are ouf of luck...

Well then, your decission opens new paths ;) Wash it. I do wash my boards (with all power sources removed) with regular dishwash(er). Then rinse in isoprop alcohol and bake in the oven at 50c for abt. 2hrs. I do wash and rinse both sides. Then do the caps, all electrolyitc ones. Have you...

Interesting, with SCSI chip removed (U8) good board chimes good as well. So neither RAMDAC nor SCSI are really queried for chime of death. Removal of on board VRAM or RAM results in chime being distorted. Soldered both RAMDAC and SCSI from bad board into a good one and both are okay. In the...

Washing in isoprop. with battery removed will not do it any harm but caveat here, these are old boards. You may knock-off something by accident even with a tooth brush. First question as I said is, do you want to play with it or return it as the item is not according to the description. You...

Provided you don't want to return LCIII (I would not accept return after someone messed with the board) definetly do the caps first. While this is not 100% bullet proof, I use vinegar to deactivate leaked acid and then flush it with isoprop. alc. Do this with caps removed and let the area soak...

Slow progress due parts and covid shipping delays... got myself two boards in the mean time so have now access to some parts. This project is now driving few other projects and got out of proportions ;) > Still waiting for more LA probes to be able to handle 64+ channels and do...

Measured everything 100% and no issues found. Looks like VSYNC/VBLANK is generated by Sonora chip while RGB comes from RAMDAC. Since the machine chimes (reads rom/executes code) should be no reason for no video and all traces/discrete parts are good there. It should display error hex code and...

Quite a pickle... these retro boards usually take me max few days but this one is stubborn. If I had more parts progress would be much faster but I am reluctant in exposing the only other working board to damage so not touching proprietary chips I cannot source. These 20yrs old chips probably...

Reading this other ROM boot process reaches P_mBootBeep Plays boot sound, pokes VBLANK interrupt as CRT seems to be wanting to turn on and then it should initialize ram contents? Without ram chips on board it plays boot sound but no video and no chime of death. With ram soldered in, it...

Progress has been made! It now chimes, unfortunately just to a second later play chime of death but there is no video. 

btw. just replaced the MCU hoping it would be semi-dead - no change. Working out QFP without stencil is refreshing :) 25% of LA wiring. bad thing without socketed option is that I am unable to flip the board.

in fact just a generic Motorola MCU but with specific programming so true, only another board semi-dead or dead for another reason. Good to know usually they suffer from failing quartz crystal which is easy to fix.

I equipped myself with 102 channel LA from 00s so... now just to learn the hw, sw, and wire it up all together. This one is a funny ride! All caps were replaced at start of this fun, clock chip measured etc. Visual inspection under microscope did not show any damage. Perhaps I have to do this...

This one worked out well. Replaced 99% of caps and re-soldered wires going to the base and the image is on. It will not power on without source attached though. On the board the main IC is a proprietary one but also other parts/drivers are long obsolete. Best path is to get a similar/same board...

Indeed timing could be an issue indeed. Full of hope I switched LS245 with F245D to discover nothing improved :( Managed to get my hands on a working board and do some measurements. BERR behaviour is different, working board does not have continuous BERR assertion. So there is some kind of an...

With ROM removed there's no life on the bus at all. De-soldered ram chips again. With or without ram chips behavior does not change. Soldered in another set of 8 chips with 70ns better than 80ns originals. I also found disassembly of mac plus rom. Not the same machine as LCIII but I would expect...

n-th day of battle. still loosing though. Replaced all F245 with new LS245 bus transceivers, replaced all ram chips. Switched ROM chips to good known ones, measured activity on CE and OE pins, still no progress, no dong, no image. There is activity on data & address lines but a pain without...

Found it here: http://rtellason.com/manuals/Apple M1212 Monitor Schematic.pdf Not greatest scan but with good magnification somehow works. Most non obvious parts like ICs or EHT appear unobtainable from regular sources, same as for apple ASICs.