Midway Space Invaders MBoard Issue

Nethawk86

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Donor 2016, 2020
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Hello all, picked up a Midway Space Invaders that had some issues. First was someone pried on the flyback copper retaining rod and broke it clean off (not sure what the thinking on that was). But thats another problem for another day as I have another chassis/flyback retainer I am going to use. My main issue here was the game board. Upon plugging into a deluxe space invaders cab, I get a immediate constant loud humming with space ship sound but no image/coin up or anything else. So we replaced all caps (using radial caps instead), reflowed all connector solder, reflowed board interconnect socket, cleaned and reseated all roms/processor, even tried installing the braze kit (with eproms still seated and using the known working deluxe's processor)....all to still the exact same thing with no image and a loud humming sound mixed with background saucer sound.
Swapped boards out and confirmed its the motherboard and not the daughter/sound board. Yes we are getting neck glow as well as this is in a working deluxe cab and not in its original standard cab with flyback issue. Also, this being in a working deluxe cab....the power supply is known working as well. This board is insanely clean...I mean INSANELY!! Like fresh off the conveyor clean. All solder joints and traces are pristine. I see maybe 1 chip that seems to have been replaced, not sure on factory or not. What makes me scratch my head is that even with or without the braze kit installed....which has a ram/rom/fault checker...I should atleast get a test screen or beep letting me know if a fault is found, which leads me to believe that there is a problem before this so its not getting the right signal out of the gate.
Anybody have any ideas or have had a similar situation?
 
Take a gander at the four 74161s at location 5 thru 8. Are they pulsing? Or, you can always shotgun all 4. ;)
 
well, sounds like it is not fully booting to me, so i would remove all four roms and verify the 5, 12 & -5 voltages on the 8080, see if all address and data lines are working
i have seen a bad rom cause the multi kit not to boot
 
Start with making sure you have the 20Mhz clock on A7 pin 4. If not then A7 or the crystal is bad. If you do have clock check 5H pin 2 to see if you have a pixel clock. If not then C7 or B5 is bad. If you do have a clock then verify the video counters are running as Highscore suggests D5,E5,E6,E7. If those are all good verify you have composite blanking on pin 8 of A6. If that's all good check pin 13 of C4 that's the last stage of the video generation.

I agree it seems like the processor is not booting but even with the processor not installed you should get video (garbage screen but video). Sooo- either you have multiple problems OR the master clock is not running which feeds both the CPU AND the video generation.

* edited to correct chip numbers for Midway board *
 
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correct me if i'm wrong about this, but i was thinking that the clock is probably working since he is/was getting various missile saucer sounds - or is sound generation completely independent of the clocking on these
 
correct me if i'm wrong about this, but i was thinking that the clock is probably working since he is/was getting various missile saucer sounds - or is sound generation completely independent of the clocking on these

Just double checked this, some sounds use the master clock but the saucer sound and a few others are generated without the system clock. There's a few oscillator devices in the sound section so you can definitely get sound with no master clock.
 
ah, thanks for clarifying that, btw, which set of schematics are you getting your locations from? they don't seem to match the midway sets that i have
 
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Just to update this thread. Problem was found and it was one 22uf 25v Tantalum cap that went bad and was soaking up all the voltage. I went ahead and replaced both just to be safe. These were not included in the normal cap kit of course and I have been told they hardly ever go bad. Now onto fixing my monitor problem still.
 
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