I've got a Bally MPU I'd like to get working as a spare. Boots fine but all displays and all digits flicker, can try to get a video if my phone camera will cooperate. Driver board is not the problem, swapping MPUs fixes it. Where should I start probing around at? I figure the problem would be with the display blanking signal but this level of PCB repair is not my forte so looking for some advice
Cracked solder joints at the header pins maybe? Reflow the headers. Sounds like loose/bad connection.
Quoted from slochar:You can check the display frequency 555 timer but first resolder j1.
Quoted from JethroP:Cracked solder joints at the header pins maybe? Reflow the headers. Sounds like loose/bad connection.
I wanna say I did resolder the headers but not sure. Will try that first
Flickering displays in this system are 'almost always' dry jointed connector pins o the displays (mainly) and you need to reflow them ALL not just one display because dry joints on one display can cause ALL of them to flicker - they are all basically interconnected (simplifying how they work).
no pullup resistors on the displays. open circuit on inputs might cause them to oscillate with other nearby noise. connectors connectors connectors.
slochar has it. All displays flickering is typical of the 555 display interrupt generator circuit malfunctioning. Takes a scope to properly test it. However you can try hit and miss by replacing the resistors and capacitors in the circuit.
Checked out the display interruptor circuit and found that R21 (36k ohm resistor) was way out of spec. Changed that, tried to boot the MPU but now it doesnt boot, LED stuck on. Im trying the barebones boot method of just having U6, U9 and U11 in. I was swapping chips around with other MPUs, and that circuit isnt related to the reset circuit so gotta figure out that first 
Quoted from Knxwledge:Checked out the display interruptor circuit and found that R21 (36k ohm resistor) was way out of spec
How far out was R21? Games with 7 digit displays actually shipped with a 24k resistor at R21 for faster (420Hz) display interrupt generator.
Quoted from Quench:How far out was R21? Games with 7 digit displays actually shipped with a 24k resistor at R21 for faster (420Hz) display interrupt generator.
Lol I think it was 24k. Would having that smaller resistor in a 6 digit display game cause that flickering?
No. Having the smaller resistor value will increase the display interrupt rate. This means the displays will refresh more frequently which should actually reduce the flicker. But the faster interrupt rate then means the CPU time is reduced for doing other 'functions'.
I would check your connections such as plugs with bad contacts and/or cracked headers as mentioned above. Since this happens with more than one board then I would look at plugs first.
Quoted from Knxwledge:Boots fine but all displays and all digits flicker, can try to get a video if my phone camera will cooperate.
I know your camera might find it hard to capture but we probably need to see a video.
I've seen people use "flicker" to describe other faults like strobing, ghosting and other artifacts.
Quoted from Quench:I know your camera might find it hard to capture but we probably need to see a video.
I've seen people use "flicker" to describe other faults like strobing, ghosting and other artifacts.
Yeah "strobing" might be a better word. It's very rhythmic and consistent, not an occasional flicker. Will take a video of it once I can get it to boot again
Quoted from G-P-E:No. Having the smaller resistor value will increase the display interrupt rate. This means the displays will refresh more frequently which should actually reduce the flicker. But the faster interrupt rate then means the CPU time is reduced for doing other 'functions'.
I would check your connections such as plugs with bad contacts and/or cracked headers as mentioned above. Since this happens with more than one board then I would look at plugs first.
It actually only happens with this MPU board, I've been testing it in our shop game we keep around for testing purposes. Swapped the MPU back to the working one to work on some displays that came in and it worked great
Quoted from pins4u:Shotgunning and second guessing - replacing random parts on a whim, almost never fixes a problem. It almost always adds additional faults!!!
I tested for and thought I found an out of spec resistor in the circuit I was advised to look at, how is that shotgunning and second guessing? Thanks for contributing nothing
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