Arcade PCB Repair Logs

Arcade PCB Repair Logs #3: Legendary Wings (Boot)

Hello true believers,

I originally had this Legendary Wings marked "working". Thought I'd give it a few rounds of play time, and listen to some of that beautiful Capcom arcade soundtrack. Check it out:

 
Courtesy World of Longplays

So I fire it up, and... I'm not impressed. The music sounds a little thinner than I remember. I loaded the ROM up in MAME to compare and yes indeed, it actually sounds like the board is missing about half the instruments. My eyes turned to the sound section of the PCB and behold, the reason it only had half of the sounds:

 

That's right, the bootleggers failed to install one of the YM2203C synthesizer chips and the matching Y3014 DAC. Searching through my scrap boards, I quickly came across the needed parts. The 2203C was a very common FM synth chip with some analog sounds, generally used for sound effects. In fact from what I can read, the YM2203 essentially includes an entire AY-3-8910, which has been used on numerous boards.

Anyway, back to the board at hand. With all sounds present, I began to play, noticing a few little graphics glitches, almost like little sparkles on single pixels. these pixels began to stretch into lines, and eventually I had a mess of horizontal lines all over the screen. 

In the picture the manifestation is the dark vertical lines you can see in the water. It's upside-down because the problem was exacerbated in this orientation (and my monitor doesn't rotate that way). There's a "FLIP" dip switch, and for whatever reason the glitches are reduced when it's engaged. I thought it would be better to troubleshoot when it was at its worst.

I...spent...HOURS going over this board. I spent a lot of time looking around the controls section, because it appeared that the coin input was broken. Pressing the button wouldn't reliably insert a coin, but if I pressed it a bunch of times really fast it would count each one. After replacing a few '367 ICs that all tested good, I eventually realized that the coin would only register if it was pressed and released relatively quickly. This must be programming to help prevent stealing credits, somehow.

I reasoned that the sparkly lines issue was on the sprite layer, because occasionally I would knock out the sprite layer while poking around and the lines disappeared with it. Another thing was that the sparkly lines were showing up only as the CPU continued to run. For example, If I happened to poke something that froze the CPU, the sparkles didn't just freeze, but they would disappear. I also noticed if I reduced the 5V voltage down to about 4.85V, the sparkles would disappear but then eventually the CPU would crash. Finally, the sound Z80 would conk out sometimes after poking around near the main CPU.

Notice how I said CPU a whole bunch of times? Well in the end I measured a few of the CPU lines and saw they were quite low, as in the "high" signals were only up at about 3V. This was apparently enough to psyche out a few ICs, because I replaced it with another Z80B and the sparkles disappeared.

Welcome back, Legendary Wings!

Lessons learned: 

  1. Some games are strict about coin inputs.
  2. A CPU can "work" but not work well enough to run everything.
  3. Bootleggers may use cunning cost savings techniques.