2 Board Stack ... Seller claimed this as not booting/black screen ... gave it the usual quick clean, check for shorts, straightening of pins etc and booted up ...
After some weird initial on screen activity as seen above left (which I later realise is part of the boot sequence) the screen settled to what you see above right ... but it did seem to be changing (as in not static), and there were signs of activity before that screen kicked in where some characters displayed ok at the top of the screen, so I did think the game was possibly running ... this was backed up by the fact there was lots of normal looking activity on the CPU, program RAM and ROM etc.. The (all socketed) RAMs tested good and ROMs verified ok too so I switched attention to the video board and set about checking the contents of the ROM's (all verified ok) and the 3 socketed 6116 RAM (one each for sprites, FG tiles and BG tiles) and ALL 3 failed !!! (2 really badly and 1 with just one bad pin - at some point I'll try clean to this leg, it might just be a bit of corrosion and poor contact) The 3 RAM chips are highlighted below ....
So I replaced those 3 RAM chips and booted to what at first I thought was a fully working game ...
Being completely unfamiliar with this game, it wasn't until I played it I realised something wasn't quite right as some enemy sprites looked to just be hovering in random places
So I fired it up in MAME and realised I was missing FG tile layer which made complete sense !!
I worked out those 3 RAM chips I replaced each one handled a different layer so I checked each of those and all signals looked good on all 3 so I moved onto finding the FG tile ROMs ... there are 4 of these and their data outputs go to a LS273 flip flop. All signals looked good going from the ROM into and out of the LS273 .... from here the 8 data lines go to a LS157 multiplexer and again everything in and out looked good ... I continued to follow now 4 signals from the LS157 ... off the video board, through the ribbon connector and onto the main cpu board to a LS25 ( quad input NOR gate ) ... the 4 inputs all present and correct on the scope ... but the output stuck HIGH all the time, confirmed by Slice (below left) ... also confirmed as shorted to the 5v rail with my multimeter !! Classic failed IC .... maybe ..... before I replaced it I just thought I'd check where that signal goes in case it was the IC downstream pulling that line high, or if there was a short on a trace somewhere ... and found that that signal goes to a LS148 .... which someone before me had previously (changed?) socketed !!!!!
So I pulled out the socketed LS148 and the short disappeared, the output on the LS25 was healthy, confirming the LS148 to be bad, further confirmed by an out of circuit test (below right) ...
The path the ROM data takes is shown below, and where the bad LS148 (which was killing our FG tile data) is ...
So a working LS148 went in the socket and restored the FG tiles !!! 👍
NOW its working 100%
Cleaned up properly and another unloved bootleg saved 👍
After some weird initial on screen activity as seen above left (which I later realise is part of the boot sequence) the screen settled to what you see above right ... but it did seem to be changing (as in not static), and there were signs of activity before that screen kicked in where some characters displayed ok at the top of the screen, so I did think the game was possibly running ... this was backed up by the fact there was lots of normal looking activity on the CPU, program RAM and ROM etc.. The (all socketed) RAMs tested good and ROMs verified ok too so I switched attention to the video board and set about checking the contents of the ROM's (all verified ok) and the 3 socketed 6116 RAM (one each for sprites, FG tiles and BG tiles) and ALL 3 failed !!! (2 really badly and 1 with just one bad pin - at some point I'll try clean to this leg, it might just be a bit of corrosion and poor contact) The 3 RAM chips are highlighted below ....
So I replaced those 3 RAM chips and booted to what at first I thought was a fully working game ...
Being completely unfamiliar with this game, it wasn't until I played it I realised something wasn't quite right as some enemy sprites looked to just be hovering in random places
So I fired it up in MAME and realised I was missing FG tile layer which made complete sense !!I worked out those 3 RAM chips I replaced each one handled a different layer so I checked each of those and all signals looked good on all 3 so I moved onto finding the FG tile ROMs ... there are 4 of these and their data outputs go to a LS273 flip flop. All signals looked good going from the ROM into and out of the LS273 .... from here the 8 data lines go to a LS157 multiplexer and again everything in and out looked good ... I continued to follow now 4 signals from the LS157 ... off the video board, through the ribbon connector and onto the main cpu board to a LS25 ( quad input NOR gate ) ... the 4 inputs all present and correct on the scope ... but the output stuck HIGH all the time, confirmed by Slice (below left) ... also confirmed as shorted to the 5v rail with my multimeter !! Classic failed IC .... maybe ..... before I replaced it I just thought I'd check where that signal goes in case it was the IC downstream pulling that line high, or if there was a short on a trace somewhere ... and found that that signal goes to a LS148 .... which someone before me had previously (changed?) socketed !!!!!
So I pulled out the socketed LS148 and the short disappeared, the output on the LS25 was healthy, confirming the LS148 to be bad, further confirmed by an out of circuit test (below right) ...
The path the ROM data takes is shown below, and where the bad LS148 (which was killing our FG tile data) is ...
So a working LS148 went in the socket and restored the FG tiles !!! 👍
NOW its working 100%
Cleaned up properly and another unloved bootleg saved 👍
