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Author Topic: LED pinball display issues.  (Read 1479 times)

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lilshawn

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LED pinball display issues.
« on: October 10, 2019, 12:59:02 pm »
I can confirm this issue is confined to the display board itself...as other LED displays operate fine on the machine.

From what I can ascertain there is a some kind of row clocking error happening.  I have 3 LED boards exhibiting this behavior. 2 boards are REV C while another is REV A none have any modifications or factory mods. all board behave identically in every machine they have been tested in. it's been so long that I don't recall if these boards worked for a period of time then ceased... but i do recall one display acting this way out of the box and we just went ahead and used it until the machine came back to our shop for service.

in any case, a graphic pixel meant to be drawn in (for example) the 3rd row is instead being drawn in the 2nd row...with a partially illuminated "ghost pixel" in the proper location directly below it in the 3rd row as it should be... as if there is some kind of problem in the clock signal making the row clock earlier than expected but also when it should.





as you can see in the attached pictures, the top row of pixels of the letter T in "TEST" should be 6 pixels from the bottom of the display, but it begins drawing them 7 with the partially illuminated "Ghost pixels" showing up at the proper row. as well as a display test with a horizontal line and it's accompanying "ghost" pixels below in what i'm to assume the proper space. (cannot verify since I can't just illuminate one specific line but other evidence suggests this)

the "ghost pixels" illuminate at about 60% intensity.

the image is perfectly static. it does not fluctuate or jitter... at least visibly to me anyway. I'm unsure if it has perfect frames and imperfect frames regularly interlaced or if it's a constant row by row issue.

jennifer

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Re: LED pinball display issues.
« Reply #1 on: October 10, 2019, 01:38:50 pm »
My first thought being a leaky zener  (<?) Somewhere in that circuit feeding back a false trigger (all-be-it a low voltage) causing the consistent behavior...Although beings how it is SMD you would play hell to fix such a problem...You should put a scope on that when you get some time (lots of fun test points I see) and let's check this out. 8)

lilshawn

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Re: LED pinball display issues.
« Reply #2 on: October 10, 2019, 03:17:58 pm »
best i can tell the row clock, row data, and latch signals run into the connector from the driver board through a 220 ohm resistor (also tied to ground through a small value capacitor) into a 74HC04 inverter IC and then run to some serial data driver IC's (74ACT299) where they drive the LED rows (through transistors).

swapping the invertor doesn't seem to help. I have no source currently for the serial driver IC's but the board has 4 of them... i cant see all 4 behaving the same on 3 separate boards. i have a feeling it's one single part common to all 4 drivers.

there is some very small value decoupling IC's on the power rails driving them and i've seen if adding additional capacitance helps (it does not.) i've seen some factory mods on vishay displays  that have the clock signal tied to ground through a 22pF ceramic on the driver IC's (different 74HC164 still doing the same business though) apparently to eliminate row clocking errors. a similar bodge was applied to the boards I have, with no change.

the boards are actually quite simple in theory, horizontal drivers, vertical drivers, and a way to time it all.

I just can't source why the timing is screwed up.

Ken Layton

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Re: LED pinball display issues.
« Reply #3 on: October 10, 2019, 10:37:16 pm »
Have you talked to the display board manufacturer's tech support? Or at least to the distributor you bought the display from.

lilshawn

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Re: LED pinball display issues.
« Reply #4 on: October 10, 2019, 11:41:01 pm »
yeah, i'm on email with stern. he's offering me up a schematic so i think he's got no idea what might be the issue.

something to stare at...maybe it'll jostle something in my brain once i've looked at it long enough.

if only i had a mountain of money to buy one of those expensive ass keysight scopes with the multi-channel logic analyzer on it.

https://www.keysight.com/en/pd-2730652-pn-16861A/34-channel-portable-logic-analyzer?nid=-32136.1186281&cc=CA&lc=eng

jennifer

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Re: LED pinball display issues.
« Reply #5 on: October 10, 2019, 11:43:46 pm »
I like the way you keep going back to that clock theory, as there may wind up being some truth in that, But why?...those drivers would have a pretty large window of tolerance one would think...It almost seems like a feedback into the ground rail drawing it down, get it on the scope man! That will show you in real time what's going on.
« Last Edit: October 10, 2019, 11:47:38 pm by jennifer »