Hey guys I thought I'd share with you info about my 1st ever PCB repair which was a Nintendo Vs PCB. Hopefully it *might* help someone else?
I bought an untested Nintendo Vs PCB a few months back that turned out to be faulty, so I thought I would give it a go and try and fix the PCB as I don't think many people repair these either in the US or UK.
The PCB started out by displaying a blank screen:
Or by displaying an out-of-sync type screen like this:
A fellow arcade collector came across a fantastic paper by the University of Illinois that gave me loads of information about the pinouts of both the CPU and PPU of this board and I used this info to check that the reset (3) and clock pin (29) on the 2A03, and PPU (18) were working properly. It also enabled me to identify the address and data lines on both to ensure they were looking healthy.
Everything checked out
Here is the article I used:
http://www.cs.uiuc.edu/homes/luddy/PROCESSORS/Nintendo.pdfNext up I removed the RAM at 1E and 6E and replaced with sockets with 6116P-2 ram. A fellow collector had the idea that we could replace the RAM with equivalent 2Kb NVRAM modules. That way we could power up the board, let it run for a few seconds, power it down, remove the NVRAM module and read out the contents of those rams in an eprom reader. It would then be possible to compare the contents to the MAME equivalent of that game in the debugger and that would let us know whether the CPU was doing what it should do as regards writing the correct data to those particular rams. It would also confirm if the CPU was executing code correctly and would eliminate it from the problem.
Before that I checked the RAM @ 6E and found all pins pulsing apart from pin 21 which was dead. A fellow collector told me that pin was the /WE (write enable) pin and should be pulsing to indicate the CPU is reading from, and writing to the ram. Following it back to its source on the schematics it comes from pin 8 of the NAND gate @ 5B. The 2 inputs into that NAND gate are from pin 31 of the CPU (via 3 inverter gates @ 5E) into pin 10 and from pin 34 (R/W) of the CPU into pin 9.
I probed the NAND gate and also pins 31 and 34 of the CPU for activity. If the CPU isn't executing code (or has crashed) then you'd expect to see no activity on pin 34. Pin 31 is a clock output signal from the CPU and you'd expect it still to be pulsing even if the CPU isn't executing code or has crashed. Pin 31 and 34 on the CPU were pulsing, but the Nand gate at 5B looked broken. Pin 8 was dead. I piggy backed it with another LS32, and pin 8 started to pulse and look correct, and so did all the pins now on the RAM.
Next up I wanted to find out what was causing the out-of-sync image on screen I was getting. I found the possible out-of-sync problem area on the schematics which centered around the LS04 @1L. Pin 5 was not acting as expected, and piggy backing with a known good LS04 made no difference. I removed the transistor @ Q8 to determine if that was causing the fault and that looked ok, so swapping the LS04 out with a new one rectified the problem.
The next part was a bit of luck really. I could see that there were issues with the RAM @ 8L. Looking at the schematics I traced this back to the LS157's @ 5K and 6K. I piggy backed both and the game sprung into life. I removed the IC @ 5K and the game continued to run. Removing the IC @ 6K killed the game entirely.
So I ended up with the game working. The graphics issues that are still visible are caused by a faulty PPU:
I then checked the PCB in Red Tent. Unfortunately the other side had issues. So I planned to take a look at that once I've got my test setup sorted for looking at the other side of the PCB on my test monitor
I made a start looking at the 2nd side of the board. It is very poorly as predicted :-(
It took me a while to figure out the best way to see the second screen. The option I choose was fairly easy.
https://www.youtube.com/watch?feature=player_embedded&v=Jm4DUXRVswYThis was a weird one......Mario Bros does not work on either side of the PCB and even on it's own at the bottom. I get exactly the same issues
I have also swapped out the roms, CPU and PPU for known good working ones and it does exactly the same!
So I burnt another set of Golf roms, and put that in the top side and bottom side and they both boot up fine, apart from the top side has no green. Moving the green pot makes no difference.
I *think* this is potentially an issue with the LM324 @3P, as I can see that pin 3 looks stuck
As to why Mario Bros doesn't work....I have no idea
So this is where I was upto.
I thought the issue might be to do with RAM speed but it doesn't really make much difference.
https://www.youtube.com/watch?feature=player_embedded&v=TD0eg2MgwyMGolf seems to work ok on both sides, but has colour issues on the 2nd side
I fixed the colour issues on the PCB.
3P, pin 3 stuck low
1P, pin 8 dead
https://www.youtube.com/watch?feature=player_embedded&v=rEeghdfeU3cI've finally managed to fix this damn Nintendo Vs PCB!
The Mario issue was caused by a broken LS157 @ 5K
I think I can finally take this off my test bench at last, and start playing it
https://www.youtube.com/watch?feature=player_embedded&v=J7IwLeWFmhM
Show Posts
