Chopper I (Bootleg) Repair Logs
...it was wrapped in bubble wrap with a label saying "sound fault". The board has also suffered in the scrap box under the weight of other boards as the boards were well bowed in on each other.
Even tho they were held at each corner this meant track damage where they had come in contact with each other was quite likely. Thankfully the boards had enough spring in them to stay about 1cm apart even when lying down.
When I connected it up to my cabinet there was complete and total silence, not even the pop you get through the speakers when a game is powered up. It also had another fault, all the bullets and explosions were missing. For some reason this board really doesn't like my test bench monitor, or visa versa, so I had to take photos on the cab which isn't as easy.
Anyway the board set is a two pcb stack, with 3 50 way ribbon cables connecting them, and each board has a daughterboard. The first thing anyone faced with a board set like this that is misbehaving is to reseat the ribbon cables. Physically disconnect them totally and then reconnect them. Oxide builds up on the contact surface over time and increases the resistance of the joint, until they lose connection totally. A reseat scrapes this oxide away restoring the contact, however in this case nothing changed. However the daughterboards are equally affected by oxidisation and on this board it was just a dual bank of pins that plugged into a set of standard DIP sockets on the main board, a pretty cheap and nasty mode of connection. Reseating the daughterboard on the CPU board didn't change anything, but a reseat on the lower board's daughterboard restored all in game objects.
If it hadn't fixed it I would have gone looking for track damage where the boards had rubbed together, luckily there is only a small scuff mark in the middle of blank space.
That left the sound, the fault that had consigned the buggered board box in the first place. Someone had clearly been buggering about with the amp chip and had left it in a bit of a mess.
What is odd is that the board was designed and silk-screened for a 13001 amp chip, yet it has an NEC 1241H on board, odder still is that this amp chip is the original one on the board. The soldering is clearly original and the other holes were never used. This might explain why there are so many missing components in the amp section, support components for the other amp chip type. However, unused holes would have been filled by solder when the board was flow soldered, so the gaping holes at c84 and c87 were where parts used to be parts that had been torn off.
There was also a 4.7uF 16V cap at C91 that had been wrenched to one side and then bent back, one leg was torn out but had been pushed back into place so it was not obvious unless you were looking for cap damage. The unusual point here was that I actually had the original missing capacitors, one was loose in the bubble wrap the board came in and the other was loose at the bottom of the box. Quinny had basically shipped me the box he found the boards in, plus a lot of newspaper packing so bizarrely I was able to refit the original capacitors. They matched the brand of all the other caps and thankfully they were both 220uF 16V caps so I didn't even have to try and find out which one went where. With those soldered back on the board and the 4.7uF capacitor replaced I powered it up again, and got music, sampled speech and sound effects, as these photos clearly show
Just had a play test of the board and it is 100% again, have temporarily sandwiched some foam between the boards to push them apart as they are now re bolted together. Bent boards will often go back almost flat again if they are allowed to over a long time, or over a shorter time if they have some encouragement. So in a couple of weeks it will be back as good as new.
Repair Logs converted to wiki format by Brad from Aussie Arcade.