Hope this one isn't to long. I don't own the cabinet so I had to do it all on my bench. I hooked up a couple 25k pots and switches and had a go. I ran into an issue with the the MAME ROM set. ROM E would not pass the self test. I was able to pulll a good image off the originals and send it off to the MAME devs. So if any of you are working on this board the good ROM will be in MAME release .189
Game PCB: Midway Road Runner 8080
Symptoms: nothing on the screen, ROM/RAM issues, missing sound and gun positioning
Steps:
To start with I removed Road Runner daughter board and used my known good Space Invader daughter board and test rom. which allows me just to focus on the mainboard
Issue 1: blank screen
When I first powered it on I saw nothing on the screen, no sounds, it seemed dead. The first thing I usually check in this situation is the timing. Using my oscilloscope I made sure the CPU was getting a clock on pin 22, it was. Then I went through the counters 9316/74161, they looked fine. I had good signals going into A5 and it seemed a valid sync signal coming out so I looked at the video side of the signal out of C4. C4's output was not moving and while checking the inputs I noticed pin 15 (load) was not changing so the RAM data was loading into the chip. I followed it back to A7 and D6 those chips seem to be acting correctly for the signals they were receiving. I moved on to B4. The inputs were good but no output. I replaced and got a screen with white vertical lines.
Issue 2: White vertical lines
You see these lines when you remove all ROMs. I had a Space Invaders test ROM in there so it they should not have been there. Seems like I should start looking at the ROMs. The address lines going in to the ROMs looked good and so did the data lines coming out. Followed the instruction bus from the ROMs to the 74153's (A2, B2, C2 and D2) There was no output at A2 so I replaced it and my test rom started working and displayed a RAM error at G14
Issue 3: RAM error
Replaced G14 with a new RAM and the motherboard booted to the test screen. On to the Road runner daughterboard.
I removed the Space Invaders parts and installed Road Runner ROMs and daughterboard.
Daughterboard
Issue: moving Graphics were missing
When I had it all set up for Road Runner I noticed all moving graphics were white squares. Not uncommon to have white lines running through the graphics but when everything is missing it usually is A4 (74175) I tested using my logic comparator and it did come up as bad. I replaced A4 and I had the moving animals.
Issue 4: no sound
There was no sound at all so I started working backwards from the speakers starting at P1 (LM377) the final amp. Using my signal tracer I was able to hear all the game sounds at pin 7 (input) of P1 but nothing was coming out of pin 2 (output). Voltage was good so it looks like I need to replace the chip. Hate replacing this chip there is such a big ground plane and heat sink it's hard to melt the solder unless you cut off the heat sink. Bought new amp and heat sink and replaced it. Sound came back.
Issue5: gun tracking
Everything is working now except when I go into test mode the marker for the gun location is travailing diagonally across the screen/ the control pots make some change to the location but it is always moving. Using my oscilloscope, I trace the signals from the pots back. Ever thing looks good until I get to B1 (LM324) pin 6. The signal is not reading any higher than about 2 volts. Looks like it is either B1 (LM324) or C1 (ZN425E) messing up. I had a LM324 but not the ZN425E so I replaced the B1 and there was no difference so I ordered the ZN425E and replaced it and the aiming worked. everything seems to be working
Repair: replaced on motherboard 7420 @ B4, 74153 @ A2, 2107 RAM @ G14
on daughterboard 74175 @ A4, LM377 @ P1, ZN425E @ C1