Ghosts n Goblins - player sprites missing (so close!)

[srarcade]

I've had this ghost n goblins board for ages. I was almost about to throw in the towel and send it off for repair but I decided to give it one more stab this week and I'm glad I did because I've made some progress.

This board as you know has no visible traces on it and you must rely on the photocopied schematic on the internet.

Issue 1: garbled graphics on the screen, random bits of sprites everywhere
Fix 1: Piggybacked a spare 2114 static ram over 11E, 11H, 11J, 12E, 12H, 12J. I found when on 12E, 95% of the garbled graphics were cured. I pulled 12E, installed a socket, put in my spare 2114 and now the screen looks great. I have 1 tiny bit in top left that still has an issue but I am out of spare 2114s, I will socket and replace the next bad chip when they arrive and it should cure the rest.

Issue 2: No player/enemy sprites of any kind on screen. Game coins up and plays fine otherwise.
Check 1: I've been basing my investigation around the K and L columns on the B (video) board. I noticed a strange issue, most of the clock inputs were pulsing very very faintly. As I moved down the schematic the pulse got poorer and poorer. The originating clock signal comes in on connector pin C3 and goes directly to pin 2 of 6K (although the schematic says pin 2 is ground and pin 1 is in). 6K is a 74LS32 and/or gate. Pin 1 is ground which is always low so really the output is always going to be whatever pin 2 (the clock) is sending. Can someone explain this? I don't see the purpose of this. Anyway, I thought why not jump pin 2 to 3 (out) and this cured my fading clock signal. It did not seem to make any change on the game.

Check 2: I have further been checking out multiplexers and flip-flops in K column and L column, following all signals on the schematic and haven't found any stuck outputs yet. Another fix log with same problem said it was a bad 9K (74L139) but mine seems to check out.

If anyone has any ideas or suggestions, i'm all ears. I think I'm really close to this- it doesn't seem like its going to be something major at this point.

Thanks

Ryan

[channelmaniac]

If the original device outputting the signal doesn't have enough drive capabilities then they will put a gate like that in to add more drive capabilities. Think of it as a digital amplifier of sorts.

Digital logic gates have different drive capabilities based on the family (74, 74L, 74LS, 74C, 74HC, 74HCT, and many more) and the technology (open collector outputs, tri-state outputs, etc...) You'll hear two different terms with regards to that: fanout (# of gates the output can drive) and capacitance which refers to the capacitance of the gates and the signal paths. Stay within the parameters and things work well. Go outside of those and chips start to glitch or fail.

Replace the bad chip. Don't jumper the input to the output or you'll drive the failure further back up the digital logic chain.