Aug 102017
 

Got this faulty pcb for my collection

 

Game was resetting over and over right after the first stating screen without even the possibility to see the diagnostic screen “RAM ROM OK”

The code was running and the watchdog was triggered by some event which I eventually found in a faulty 2114 SRAM @E15 on the video board

 

Marked in red on the actual pcb:

After changing it, the boot process went on until it displayed RAM and ROM OK but it reset again just before screen with the white grid.

At this time I was complete blind , missing any programming skills I couldn’t check what the code missed to go on.

I had a suspect on the sound part and after checking the clocks on the 8039 MCU I noticed it was completely missing.

The oscillator was working correctly and from the schematics I could see that the clock for the 8039 and the AY is generated using some 4bit binary counters 74ls293, very difficult to find on other boards.

 

Probing pin 9 of 74ls293@A12 it was stuck high while on pin 10 I had the clock.

I proceeded to test it out of circuit but was tested good.

Not sure about the reliability of the tester, I found a Starforce boot which use one 74ls293 and I installed it in a socket on megazone.

Still reset and no clock from pin 9

The output of the IC@A12 goes to pin 8 of a 74ls240, therefore I decided to test pin 8 of IC @D14 against +5V and it gave me a short.

After changing it the game finally booted without any other faults

 

Devil World repair log

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Jul 212016
 

Got this Devil World PCB as non working. This game is running on a Konami Twin 16 hardware. It is a two layer board with the first one devoted to CPU/sound and the second one to GFX/video. The two faulty/replaced chips were on the video board and are highlighted in red.

devilworld1

Luckily the board was booting and got stuck on the RAM/ROM test screen with ROM #18 reported as bad (chip #1 on the board picture above).

devilworld2

That ROM is part of a 4 ROMs row which contains the GFXs. They are 40 pins Konami chips and looking in MAME I could see that they were 512kb MASK ROMs. I tried dumping it with my programmer but was unable as it found one dead pin. Compatible EPROMs for these are 27C400 so I burned one and replaced it.

The game was now booting but with a color problem on some tiles.

devilworld3 devilworld5

After a deep look at the video board I could find a Fujitsu 74LS153 located at 10H (chip #2 on the board picture above) with floating outputs (while inputs had healthy pulsing signals). I piggybacked it and got everything corrected. I desoldered the faulty one, put a socket and replaced it. Board is now working perfectly.

devilworld4 devilworld6

Bucky O’ Hare repair log #2

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Jun 182016
 

Got another Konami game with the usual jail bars defect that is so common on these games

bucky3

 

I ran again the maskrom test and it reported bad maskrom 8B.

After desoldering it and testing as 27c160, the programmer complained about missing contacts on several pins.

bucky

After replacing the markrom with a 27c160, the game was 100% fixed

bucky2

 

bucky4

Sunset Riders repair log #4

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Jun 182016
 

I got this pcb for a repair.

Gameplay was fine but graphics had black lines and music had a lot of white noise.

ssrider

 

I ran the maskrom check and it reported two roms as bad, one of which was infact the rom with the sound data.

 

ssrider2

 

It doesn’t mean necessarily that the maskroms are bad, can be also that the custom chips have some pins not soldered correctly or the chips themselves being faulty.

The only way is to desolder the maskroms and check on a programmer if it reports pins not connected which means that the chip internally is broken.

After a check with the romset, I decided to dump rom 16k as 27c400 (4mbit eprom with maskrom pinout) and rom 1d as a 27c800 (8mbit 16bit eprom).

Both were reported as having pins non connected properly.

ssrider3

So I replaced maskrom 16k with a 27c400 and the markrom at 1d with a 27c160 eprom (16mbit 16bit eprom) after having doubled the original file in order to prevent the game to access the empty space in case the additional addrr line is not connected to GND or 5V.

This fixed 100% the game

ssrider4

ssrider5

Mar 292016
 

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.

megazone

 

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.

megazone3

megazone4

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.

megazone2