Today I could test the two PAL dumps we had on database from a Megablast PCB.They were submitted from an unknow source.I added proper chip labels and PCB location too.Actually the PCB has six PLDs but four of them are shared with Liquid Kids which have been already dumped previously by me.
I dumped the two PALs from a Taito Super Space Invaders ’91 PCB (single layer version).Both dumps are tested and working in a GAL16V8 targeting device.
I had on the bench this Aliens PCB:
Board had sever sprites issue:
I performed a MASK ROM check which reported two bad 4Mbit devices @K2 and @K8:
I was able to exactly reproduce the issue by using two dummy ROM files in MAME:
So, without further delay, I went to remove these devices and install two 40 pin sockets for the 27C400 EPROM replacements:
With these replacements fitted sprites were perfect but I noticed that sound samples (like voices of main character or enemies) were missing:
Sound samples are stored in a 2Mbit MASK ROM @D5 (which is not checked by the self-test) and its data are processed by a custom marked ‘007232’ which is a PCM controller:
Probing the 2Mbit MASK ROM revealed no activity on data and address bus, all pins were stuck high so I removed it:
and replaced it with a 27C400 (with doubled content since no 2Mbit MASK ROM replacement exists).This restored all sound samples.End of job.
Yes, yet another Rainbow Islands on the bench!
Once powered up the board, I was greeted by this static screen:
The watchdog was active sign that there were troubles in main code execution.For first I dumped the program ROMs and they were good (although they were from the Extra version so board was actually an hybird since C-CHIP was the one from the normal version).Then I passed to analyze the WORK RAMs (two Toshiba TMM2063) and I found weak signals on data lines of both:
This lead me to remove them, they both failed when tested in my programmer:
With good WORK RAMs the board successfully booted but sound was missing at all:
Fitted a missing 1000uF 16V capacitor @C84 in the sound section was not enough to restore sound:
I noticed that the RAM of the Z80 audio CPU was another TMM2063 which seems to be very unreliable part (like Fujitsu TTLs, I’d say..) so I went to piggyback the one @IC44 and sound was restored.The chip failed the out-of-circuit testing:
Yes, yet another Rainbow Islands PCB saved from scrap pile!
I got this CAPCOM CPS1 A-BOARD (the old hardware revision) from my friend Joachim for a repair:
He said me that board had some input problems.This was confirmed once I powered it up since most of player 1 and 2 controls were irresponsive but, after few times I cycled power, suddenly the board lost the red color:
I started to troubleshoot first the input issue.As usual I inspected visuallly the board and found some corrosion near the JAMMA edge, some of the traces from the involved input pins to custom resistor/capacitor arrays and 74LS245 were eaten:
This was confimed also using my multimeter in continuity test.With some patching work and jumper wires I restored the connections and after this all controls were correctly working:
As for the red color issue, schematics for this hardware are available so I went to look at the involved circuit:
I found weak data signals on the two CXK5814 SRAMs (compatible with the TMM2018 suggested by schematics and PCB silkscreening) @4D and 5D (good on left, bad on right) :
Since the data bus is shared between these two chips I could not know which was exactly the bad chip affecting the other so I piggybacked both them and when I did it on the one @5D, the red color was restored.So I desoldered the chip and it failed the test in my programmer:
Fitted a new SRAM chip fixed the board completely.