Had another Wonderboy in for some action recently, Gt-ray offered some faulty boards for this repair and the faulty ones happened to be exactly what I was after, so this fella arrived for some TLC.
The board was in good nick, a couple of chips had been replaced by Gt-ray himself in an attempt to fix the board. The chips replaced were ones I had found to be faulty in previous repairs. This approach is not likely to lead to success as there are many chips involved in any single thing you see on the screen, so even if the fault is similar the chances the exact same chip is the cause is pretty low.
Aside from those chips there was this at IC55 and IC69...
...little links of burnt wire badly soldered across a couple of chips - very odd.
As I had foolishly sold my Sega System 2 adaptor with my last Wonderboy board I had to make up another one to even power this up, a very tedious job s, this one isn't going anywhere as I hate making up these things. It's such a dull job that I only wired up the power lines, the video and the sound at this stage, enough to power up the board and see what it does.
As expected, it was not well...
[video=youtube;L5zgzdLsalE]http://www.youtube.com/watch?v=L5zgzdLsalE[/video]
... aside from the messed up colours which seemed to change at random, the screen was very glitchy and covered with noise from time to time.
First thing to do was to undo the crazyness on the LS85.
I had assumed that this was to connect up an unused gate on the chip to take over from a bad gate that the PCB connected to. Often TTL chips contain the same gate two, four or six times and in many cases the board doesn't use them all. However this wiring makes no sense whatsoever as the chip is a 74LS85, a single
logic unit and tying these lines together is going to bugger up the function of the chip entirely by joining four input lines to ground, actually only 3 as one of the wires was just not actually soldered to anything on one end. After trying to see any sense in what had been done I simply removed the chip and the bits of wire, fully expecting to find the chip was bad. Except it wasn't, my VP280 EPROM reader gave the LS85 a clean bill of health, and despite it being a Fujitsu TTL chip it went back on the board. 74LS85s are not common chips on arcade boards and I couldn't find a single one in my scrap PCB bin. The game was such a mess and so unstable when running that I couldn't actually tell if this had made any difference at all.
I had a poke around the video mixer section of the board with the scope and didn't find anything that looked bad, but as there were a collection of Fujitsu 74LS153 multiplexer chips in that area I fired up the HP 10529A logic comparator to compare a known good ls153 with the ones on the board. Due to the complexity of the 153 (12 input data streams producing 2 outputs data streams) it is impossible to see if it is doing the right thing with a scope, or a logic probe. As long as the output signal looks healthy you have no way to tell if the actual data it represents is bad, without feeding the same inputs to a known good chip and comparing the outputs to the on-board chip - which is exactly what this device does.
The LS153 at IC8 was bad, giving a constant mismatch between it and a good chip. So this was de-soldered and a good one soldered in its place.
Firing up the board again gave this...
... and no glitching or flashing either.
As the board was back in good health I removed the weird wire link on the the 74HS109 chip to see what affect it would have - absolutely nothing, so it stayed off.
I have seen some shotgun repair attempts before but usually that just involves replacing chips semi randomly, this is the first time I have seen evidence of what I can only assume is someone going over the board shorting things together and seeing if it makes things better or worse, if it seems to improve things then they wire in a link in and carry on.
If anyone can suggest any more logical suggestion I would be interested to hear it as my explanation is hard to believe.
Last thing to do was to wire in the 8 player 1 control wires (Coin, Start, Up, Down, Left, Right, Fire, Jump) into the harness to confirm the board was actually playable and I ran through the first four stages without any issues at all.
Another Wonderboy fixed!
