Got this bootleg board for a repair.
The game could be played with music and sound but had a problem on the text layer:
I started probing around until by shorting carefully pins of some TTLs I found the circuit of the text layer on the upper board.
A 74LS86@5J had all its output in the grey area (2.2V):
Changing it with a new one solved completely the issue:
Got a very clean Megazone pcb for a repair. The board was in near mint conditions but it didn’t boot and also the Sync was missing.
Looking at the schematics available online, I noticed that the quartz responsibile for the CPU clock and sync was the one placed on the bottom board.
Tested with my frequency meter and it was dead.
It is a very rare 18.432mhz OSC which to my knowledge is used only by early 80s Konami boards.
Luckily I had a Double Dribble which I used for spare parts which had the very same OSC.
After changing it, the game booted perfectly.
For you information, if you try to install the more common 20Mhz OSC, the colours are off and the scrolling is jumping often, so unfortunately it’s not a good replacement.
The board had no sound but I immediately noticed that there was no noise coming out of the speaker which is the evidence of a faulty amplifier.
The amplifier is infact the very unrielable LA4460N, so after confirming the the music could be hear on input pin 2 , I exchanging it and fixed completely the game.
Bought on Ebay this cheap untested Double Dragon original PCB:
Board came in not very good shape, CPU board was really dirty as you can see from picture above.I powered it up and got a solid black screen so I decided to washed it.After dried and removed the oxide from JAMMA connector :
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I powered the board again and this time it successfully booted but after started a game I noticed some sound FXs were wrong, replaced only by some noise :
Sound FXs are generated by two MSM5205 ADPCM chips which read data samples from two 27C512:
The two MSM5205 were tested good on all pins and piggybacking them didn’t change nothing but when I went to test the two 27C512 I found that pin 7 (address line 4) of the one @IC95 was silent so it wasn’t being addressed.Schematics showed it connected to pin 3 of a 74LS393@IC61 :
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Jumpering the two pins fixed the issue and all sound FXs were restored but it was an ugly solution so I followed the trace until the point of its break.This lead me under the socket of the 27C512 @IC95 where a pad was corroded and lost contact to pin 7 of this EPROM.I restored the connection with a piece of AWG30 wire after breaking the plastic of the socket in order to reveal the pad:
Another arcade PCB preserved!
Some days ago I had on the bench this Bad Dudes PCB for a repair:
Board played fine but clearly had color palette troubles which I narrowed in CPU board by swapped a good one:
Putting my fingers on board some colors were changing so I could locate the palette RAMs in three TMM2018 chips:
Probing the chips I found that pin 6 (address line A2 in common) of all of them was completely silent, this was confirmed also by my scope:
According schematics this signal should come from pin 9 of a near 74LS157 @15H :
but I couldn’t measure any continuity with my multimeter.A closer inspection revealed some corrosion on pin 6 of the TMM2018 @IC78 :
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I removed the chip and found that the pad was completely corroded and lost contact with its traces:
I promptly patched them and this fixed the game completely:
The board played completely blind.
With a logic probe I tested all the 74Ls07 which had fixed input and outputs as expected, therefore no activity at all.
Tested the clock (pin 11) on the 74LS273@9C and 10C and it was fixed high.
Unfortunately the schematics of the game found in internet are not complete, so I had to trace back manually and found out that the output pin 14
of the 74LS04@17E was stuck, while the input was floating.
Replacing the chip with a good known one didn’t fix the problem!
So I looked on the back of the pcb and found that a pin of a chip was touching the line of the output it shorted it !
After lifting the pin, the pcb showed the game in colours but all the screen was blocky.
While searching for the source of the problems I shorted two pins of a custom chip by mistake and the pcb rebooted without any issues.
It was clear it was some kind of reset problem for the graphic chips.
The 68000 had it’s own circuit but the customs had a separate reset.
Looking back on the schematics, I found out that the system reset was given by the custom 051550 which I couldn’t find on the board….
Infact it was missing and I didn’t notice!
Soldered a new one from a scrap pcb and the game was fully restored!
