I desuicided this original PCB but after booting up I got this
After checking rams and buffer they were all good.
Just to avoid any doubts I dumped without convinction all the graphics eproms which normally are very reliable.
To my surprise when I lifted eprom PWE02 with the pcb in operation, the parts of the background corrupted turned to black revealing that at least the problem was connected to the circuit around the eprom.
Placing the eprom on my programmer I got this:
Turned out that the VCC pin of the eprom was making bad contact internally.
Got this Sunset Riders PCB from my friend Joachim:
He told me PCB suddenly stopped working while he was playing at it.When I powered it up I got this :
Board was constantly resetting sign that the watchdog circuit was active.The main 68000 CPU was halted and its data/address bus in high impedance state.For first I disabled the watchdog by closing the jumper ‘JP’ near the JAMMA connector:
The /RESET signal was correctly generated by pin 10 of the custom SIL ‘051550’ but its clock input on pin1 was stuck high :
This signal labeled ‘AFR’ is generated by pin11 of a 74LS138 @13G:
Obviously, since the CPU was halted, I got no activity on inputs and outputs of this 74LS138 but I found that its pin 15 (an output) was almost shorted to ground (only 6.3 Ohm of resistance):
This was clearly abnormal, two were the possible causes : either this output was shorted internally to ground, either something was tiying it externally.I removed the 74LS138 but it was good and pad of pin 15 was still almost shorted to ground.Part of schematics posted above shows pin 15 connected to pin 1 and 15 of some 74LS253 multiplexers.Using again my multimeter in continuity check I could find the point of less resistance in the area of the two 74LS253 @14B and 13B:
I couldn’t know which was exactly the bad one so I desoldered both and tested out-of-circuit.The one @14B failed:
Actually it was the japanese revision called ‘Akumajou Dracula’.The board was in clean condition:
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But this was the scenario after I powered it up:
Vertical lines across the screen and missing sprites were the most noticeable issue.As usual I started my troubleshooting with a visual inspection focusing in the tiles and sprites generation circuit and I promptly noticed this:
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Someone (for sure not a great expert…) previously attempted a repair and replaced the two ZIP M5M4464L DRAMs @H3-H4 but mounted them backwards.After correctly installed a pair of new chips I got this improvement:
There were still jailbairs across some sprites.From my experience in past repairs I knew this kind of issue are caused by the other couple of ZIP DRAMs @L20-L21 (where the ‘007121’ custom @J16 accesses to)
I replaced them and sprites were fully restored.I was declaring the board as fixed but I noticed that some layers (more than the colors) were missing on some backgrounds like title screen :
With the help of schematics I went to check all the parts of circuit that could be involved until I notice that the input pin 14 of a 74LS399 @D16 was stuck low while all other were correctly toggling:
If I shorted this pin with the adiacent one (13), all the layers came back although some colors were wrong:
The signal labeled on schematics’GA1C02′ is generated by the custom tilemap generator ‘007121’ @J16:
So this lead me to play the card of replacing this ‘beast’ in PGA package of 208 pin:
Armed with a heat gun, I removed it :
and transplanted a good know working one taken from a Combat School donor PCB:
And I was right since since this fixed completely the board:
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