Omega Race Board Repair

musicman282

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I've pulled the trigger and purchased myself a non-working omega race machine. I was pretty excited when I picked the went to pick the machine up and there was absolutely no acid damage on the cpu board. I figured this would be an easy candidate for repair. The cabinet was in decent condition and all the electronics looked complete. When I brought the game home, I fired it up for the first time and all the lights came on but the monitor had some random lines on it. I opened up the back and saw that the LED was lit on the gameboard so I assumed I had 5+ going to the game board. I decided to clean and reseat all the chips on the board since that is usually what I do when I receive any game. The chips had a lot of corrosion on them and it took me quite a while to clean them all. I put the boards back together and popped them in the machine fired it up and it was doing exactly the same thing. At this point I measured all the voltages coming out of the power supply and they were all within specification. I also measured the 5V rails at each of the rows of chips to makes sure the 5V was consistent on the board. Next, I put the game into test mode and it started flashing ram error codes. I would assume the 2114 ram errors are very common since these are CMOS chips that tend to be highly static sensitive. I ordered some more 2114's from ebay and replaced all of the ones on the board that the error codes pointed to and fired up the game. When in test mode the game now does nothing, no flashes no sounds. The spot killer LED is on the board. I attempted to cycle the test mode into the screen test to try and verify the XYZ voltages coming off the board but it appears that there is no signal coming from the board. I do not have a oscilloscope so I'm kind of limited in tracing analog signals on the board. The game does have some weird behavior though. I tried playing it blind and the game beeps when coined up and the coin meter clicks. The start buttons seem to light up according to how many coins are put in, but the game doesn't appear to actually start when you press the buttons. They just continue to flash and there are no sounds. I tried probing the boards with my meter to check the voltage at some test points to further investigate and accidentally shorted out the -15v regulator at U101. DOH!! Well I'm going to have to wait for that regulator to come in order to continue my troubleshooting. Does anyone have any suggestions on what could be causing my problems. I've been studying the schematics and have a pretty good idea on how the circuit works, I just am wondering if there are common problems outside of battery corrosion that happen with this game.
 
Well the ones I've worked on, had problems in the up/down counters, video section, Ram, Roms, random TTL causing watchdog problems. I've never really found anything common unless it's induced.

Just start at the beginning, and work your way to the end.
 
Well the ones I've worked on, had problems in the up/down counters, video section, Ram, Roms, random TTL causing watchdog problems. I've never really found anything common unless it's induced.

Just start at the beginning, and work your way to the end.

That's not the answer I wanted to hear :) Oh well, this will give me a nice challenge to test out my shiny new oscilloscope.
 
Once I get the transistors swapped on my linear PS, my bottom end will be stable and I can start working with you dudes on the OR board troubleshooting. I've fixed every other game I have (at least the ones I have all of the parts for) so, either OR or Monaco GP is next on deck...
 
I replaced the 7915 voltage regulator and some rectifier diodes that blew when I shorted out the regulator. Plugged the machine in and there was no smoke, but the game still does not work. I scope the vector output area of the board and see that there's no data going into it. I assume there's something going on in the program area of the board that is preventing it from even attempting to output any video information. I probed the pins on the processor and there's data traveling on all the address lines, data lines and the reset line remains high. I'm kind of confused as to what to look for now. I ordered a rom burner so I can read the roms to determine if they are good or not. Any suggestions as to where to look next? All the voltages on the board are good. The board will randomly give me ram error beeps that show all of the video ram being bad, I was thinking that it meant there was a bad chip in the buffer between the program bus and vector bus, but it is an intermittent problem. This is so frustrating because it seems like the board is doing something, but I don't have an in-circuit emulator to run specific tests on the board. Any help would be appreciated.
 
Well if it's indeed trying to run the ram test then the program is running, at least in test mode. That being the case then there should be signals trying to get to the video out section.

#1 power is sorted out
#2 Check the data line buffers, and the rom buffers for activity
#3 Get the Go, Stop and the like signals for activity.

You can still verify the roms out of circuit, and not be able to read them in circuit due to other faults.
 
Still stumped

Well I'm still baffled by this OR board. It looks like such a simple circuit on the schematic and I didn't think it would be this difficult to repair. I purchased a rom burner and verified the checksum on all the roms and they all checked out ok. I ordered a new DVG Prom from hobbyroms.com which won't arrive until next week. I tried probing through the board again with my logic probe and have now decided that I need a logic analyzer. Here's a question though. I hooked my oscilloscope up to the board and it isn't reading a nice square wave on any of the pins on the ICs. I've never had a board that I put a scope on do this. I've moved the ground to a couple of different positions and that doesn't seem to make any difference at all. I would expect the game not to work if the data is that dirty on the busses. Has anyone experienced this before. I am thinking that a capacitor in the power supply section could be bad. Any suggestions?
 
If you suspect a cap in the power supply, measure the ac voltages on the dc outputs, or use your scope to check for ac ripple on the dc lines.

Have you checked your clock circuit for proper operation?
 
If you suspect a cap in the power supply, measure the ac voltages on the dc outputs, or use your scope to check for ac ripple on the dc lines.

Have you checked your clock circuit for proper operation?
I'm not seeing any ripple coming from the power supply on the DC lines or the rectification circuits on the main CPU board. I'm not sure what I did exactly, but now the game coins up and plays blind, but there's no video on the screen. When I run the ram test it tells me that the #2 video ram is bad. The game should still output something if the #2 video ram is bad correct? I scoped the video section of the board and I'm getting no data on the vector address bus, vector data bus or vector output bus. I suspect that this could be caused by a bad DVG Prom. Am I reading the schematics correctly? I appreciate all your help Talon, I feel like I'm so close. I'm just hoping that once I get the video working the monitor fires right up. Knowing my luck so far, I'm not getting my hopes up.
 
You might have some corrupt graphics with a bad ram, but it all depends on where it's at. So I would at least replace it. I've not run across a bad prom yet on Omega race, but it's definitely possible. I would wait till you have the prom and replace both and see what happens. Then start working the counter section if you are still not getting video out.
 
You might have some corrupt graphics with a bad ram, but it all depends on where it's at. So I would at least replace it. I've not run across a bad prom yet on Omega race, but it's definitely possible. I would wait till you have the prom and replace both and see what happens. Then start working the counter section if you are still not getting video out.

Well I received the PROM today and installed it and now I have data flowing through the vector data bus. I feel like I'm one step closer. I also replaced the bad video ram and now have no errors. I still am not seeing any video and it appears that I have a problem with the DAC. My question is, what kind of voltage levels are supposed to be coming out of the DAC. I read the data sheet and it said it could output 10V but I'm just getting a DC signal out with a little bit of ripple. I would assume this isn't what I'm supposed to be seeing. The thing that confuses me is that I'm getting the same sort of signal out of both DACs but I'm getting plenty of data at the digital input pins. What are the chances that both DACs are bad? I'm thinking there's probably another problem, but I can see the data coming into the DAC input pins so I can't figure out why it wouldn't output anything.
 
Here's a question though. I hooked my oscilloscope up to the board and it isn't reading a nice square wave on any of the pins on the ICs. I've never had a board that I put a scope on do this. I've moved the ground to a couple of different positions and that doesn't seem to make any difference at all. I would expect the game not to work if the data is that dirty on the busses. Has anyone experienced this before. I am thinking that a capacitor in the power supply section could be bad. Any suggestions?

Most buses tri-state at times, whch ends up looking pretty ugly.
 
My question is, what kind of voltage levels are supposed to be coming out of the DAC. I read the data sheet and it said it could output 10V but I'm just getting a DC signal out with a little bit of ripple.

If you think there's "voltage" coming out of the DAC, perhaps you should read the data sheet again, as you have absolutely no clue what the circuit's doing.
 
I see, I was confused by the paragraph on differential nonlinearity where it states the DAC outputs a voltage proportional to the input voltage divided by the total possible number of states. My thinking was that the internal trimming resistor being jumpered to the output meant that you were taking a voltage reading across that internal resistance in relation to ground, but now realize that is silly since the current will just flow directly to the due to the lack of resistance in that direction.

so basically I need to measure the output voltage across the resistor to see what my signal is.
 
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