hi, u'r right, here image of the PCB i have.
ram is totally populated.
on BLACK is highlighted what was reworked from previous owners
on GREEN what i 've worked for now
- the 4700uf CAP was not original, and was moving around not sitting solid on the PCB.
i changed it with a new one, don't care about the hot glue, i used a radial cap so i had to solder for longer legs, and secured on PCB with glue)
- i socked chips 7405 and 566 for test reset circuit that seem ok (i should check other chips for reset circuit? or is only the 7405?)
Voltages from PSU, tested on drive solder points and VCC of discrete logics seem ok (stable 5v and 12v)
About the "BLACK MARKED" of the previous owner is clear to me that:
CPU got desoldered , dunno if there was already a socket here , but 68k pins have some solder on legs. Anyway this not seem to be the problem (tested with an A500 68k, nothing change)
GLUE and MMU got for sure reworked, dunno if changed or socketed on the past. Solder points was full of flux. I've carefully reflowed pins and cleaned now seem fine
the 2 ROMS got reworked on past ... are on sockets, dunno if got changed or if are originals. Dunno if was originally on sockets or not.
the 3 resistors are clearly reworked from components side. I bet he disabled 512k ram cutting resistors legs from the top, then he soldered back from top side again.
i dunno if this was a fail on repair, or there was a success, or was an upgrade from 512 to 1Meg RAM ... i received the machine like this , not working.
Anyway on my opinion is born with full populated ram, smd solder on ram seem perfect.
on general, i've reflowed and clean from flux what was clear got reworked on past, i've clean some corrosion and tested continuity on some random pins that was reworked (for exemple i've tested continuity on all CPU pins solder side, and some address lines component side CPU. All is ok, so i am pretty confident the previous owner have not broke any trace ... at least i hope ...).
Of course, i have checked not everything, just where i've clean some corrosion and where i am pretty sure there was a rework.
SOCKETED CHIPS ARE:
the 2 roms
GLUE and MMU
U49 (the one on the center, video, or shifter i think from what i've read)
7405 and 566 (from me, that i am sure no rails lifeted, i test every pin/rail when i desolder before put the socket)
Machine is on HALT state, i have clock on Pin15 CPU but low on Pin17.
zero pulse on Addresses and Data BUS. Mostly all stuck high.
can't see nothing on video because i'am still waiting for a scart cable i've ordered, dunno if will be a blank screen or something different (i'am pretty sure will be a Blank Screen ^_^)
testing with OSC, dunno how and what i've done, happend 1 or 2 time to see on Pin17 a HIGH HALT, but after a reset back to LOW.
on 100 tests, maybe, is happend 2 times ...
pin22 CPU BERR and is always high
Pin48 (CS) is HIGH (no pulse)
Pin46 (DTACK) is HIGH (no pulse)
DMA looking for pinout from the ST schematics, i've found that:
Pin39 CLK is ok, clock
Pin38 RDY is low (no pulse)
Pin3 FCS is High (no pulse)
this is the situation ... and what i've check reading around and on service manual
if need, i can post an image on solder side too
hope for some help