Arc-aid: part 2!
- Thread starter Nes4life
- Start date
Hmm, Centipede's not playing quite so nice!
It basically doesn't do anything, just a black screen. Looking at the timing section the initial clocks out of the immediate crystal section are good 12Mz, 6Mhz and the clocks out of the first counter are good (6, 3, 1.5, 0.75) but not much coming out of the next counter. De-soldered the second counter chip and it's fine which means something that is connected to those signals is shorting them to ground.
Now it's a real pain in the arse as it's pure time consuming grunt work to go over the schematics and look for anything that those signals go into, pull it and see if the timing comes back up. Rinse and repeat until you've found it. Hopefully I can find 1 or more chips where ALL of the signals go into as opposed to individual ones.
Also forgot I have the board from the Asteroids here as well
It basically doesn't do anything, just a black screen. Looking at the timing section the initial clocks out of the immediate crystal section are good 12Mz, 6Mhz and the clocks out of the first counter are good (6, 3, 1.5, 0.75) but not much coming out of the next counter. De-soldered the second counter chip and it's fine which means something that is connected to those signals is shorting them to ground.
Now it's a real pain in the arse as it's pure time consuming grunt work to go over the schematics and look for anything that those signals go into, pull it and see if the timing comes back up. Rinse and repeat until you've found it. Hopefully I can find 1 or more chips where ALL of the signals go into as opposed to individual ones.
Also forgot I have the board from the Asteroids here as well
And then there was centipede...
I hate "fixes" like this though!
The counter section in centipede consists of a 12Mhz crystal, a simple divider, an inverter and then a number of counters first for horizontal, then for vertical. Basic checks told me that the circuit was good and yet I had a completely black screen and no life. Odd. So I tested this stuff with a couple of bits of kit and the second counter in the chain appeared to be bad. Inputs going in, outputs stuck low. So I pulled it only to find it tested fine out of circuit. Hmmm. So this is where I assumed it would be something connected to the outputs dragging them down to ground.
SO, I scanned over the schematics and I could only really see a 74LS157 and a 74LS74 and I pulled both. Both tested good. Umm, what? OK, so queue a couple hours going over and over and I just couldn't see anything up. So I socketed all three counters that are involved and the inverter. Put new chips in all of the sockets and the game worked. Bingo me thinks!
Not happy with not knowing what's going on, one by one I put the original chips back in the sockets and blow me, but the game still works. So what was wrong?? Search me! Dry joint maybe? No idea.
Hence, I hate things like this.
I still need to rig up a sound amp and check that the sounds are working OK, especially given that the corrosion on the PCB is under an op-amp in the sound circuit. But I'll do that another night. Oh, need to check the trackball though. As crap as it is, I played a couple games with joystick and it plays fine. High scores don't save but they can't because my rig doesn't supply the voltage required to produce the +/-22v required for the EAROM.
I hate "fixes" like this though!
The counter section in centipede consists of a 12Mhz crystal, a simple divider, an inverter and then a number of counters first for horizontal, then for vertical. Basic checks told me that the circuit was good and yet I had a completely black screen and no life. Odd. So I tested this stuff with a couple of bits of kit and the second counter in the chain appeared to be bad. Inputs going in, outputs stuck low. So I pulled it only to find it tested fine out of circuit. Hmmm. So this is where I assumed it would be something connected to the outputs dragging them down to ground.
SO, I scanned over the schematics and I could only really see a 74LS157 and a 74LS74 and I pulled both. Both tested good. Umm, what? OK, so queue a couple hours going over and over and I just couldn't see anything up. So I socketed all three counters that are involved and the inverter. Put new chips in all of the sockets and the game worked. Bingo me thinks!
Not happy with not knowing what's going on, one by one I put the original chips back in the sockets and blow me, but the game still works. So what was wrong?? Search me! Dry joint maybe? No idea.
Hence, I hate things like this.
I still need to rig up a sound amp and check that the sounds are working OK, especially given that the corrosion on the PCB is under an op-amp in the sound circuit. But I'll do that another night. Oh, need to check the trackball though. As crap as it is, I played a couple games with joystick and it plays fine. High scores don't save but they can't because my rig doesn't supply the voltage required to produce the +/-22v required for the EAROM.
Amazing work Martin, it goes to show you with time and patience (and perhaps the odd bit of test equipment or two) these things can be brought back to life again.
Really enjoying the detailed posts, I'm learning lots of good stuff reading them.
Hopefully I will have a brick for this cab soon so Xevious can have its one back.
