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Taito Space Invaders. Rolling screen.


aaablettt

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As i understand it rolling can be a v-sync problem. Does the output to the monitor have seperate h-sync and v-sync or a composite sync? Generally you can tell by the number of wires, 5 wires for composite sync (RGB, GND, Sync), 6 wires for seperate sync (RGB, GND, v-sync, h-sync).

 

Also do you know which board type you have? And do you have access to an oscilloscope?

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As i understand it rolling can be a v-sync problem. Does the output to the monitor have seperate h-sync and v-sync or a composite sync? Generally you can tell by the number of wires, 5 wires for composite sync (RGB, GND, Sync), 6 wires for seperate sync (RGB, GND, v-sync, h-sync).

 

Also do you know which board type you have? And do you have access to an oscilloscope?

 

Thanks for chipping in.

This board set outputs a composite video so no sync wire at all. Just a ground and a video composite. I have swapped monitors and got the same issue then I swapped motherboards and issue ceased.

No oscilloscope, just a logic probe.

 

 

Sent from my iPhone using Aussie Arcade

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Thanks for chipping in.

This board set outputs a composite video so no sync wire at all. Just a ground and a video composite. I have swapped monitors and got the same issue then I swapped motherboards and issue ceased.

No oscilloscope, just a logic probe.

Aussie Arcade

 

Ah, right. Not sure exactly what board you've got, but if its the L shaped one I've had a look at the schematics and the first place I'd start looking is an 74LS55 IC named 6B on the schematic. Try the logic probe on pins 1-4 and 10-13 and see if any of them are stuck high or low. The output of that IC (pin 8) is marked as "Comp Blanking" so a reasonable place to start.

 

I'm assuming aside from the vertical rolling the image is otherwise fine?

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Ah, right. Not sure exactly what board you've got, but if its the L shaped one I've had a look at the schematics and the first place I'd start looking is an 74LS55 IC named 6B on the schematic. Try the logic probe on pins 1-4 and 10-13 and see if any of them are stuck high or low. The output of that IC (pin 8) is marked as "Comp Blanking" so a reasonable place to start.

 

I'm assuming aside from the vertical rolling the image is otherwise fine?

 

Yes, the image is fine otherwise.

I have probed around 4F and 6B. Nothing obvious but I'll go again.

Cheers.

 

 

Sent from my iPhone using Aussie Arcade

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Changed out the 10mf cap.

 

Did that fix it?

 

If not... it looks like there are some lines that go high low at different frequencies. Some of the inputs to the blanking logic IC (6B) are these signals. Looks like they use the fact that they will all be high together at a certain interval (effectively dividing the lowest frequency by a 2^x number) to do some sort of timing. Checking those lines are pulsing at the right frequency would be helpful but i'm not sure how you can do that without a scope.

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Nice work. Hadn't thought about the output as I assumed the h-sync was being done by the same IC and would have had more picture issues if the h-sync wasn't working. Guess the h-sync is done another way.

 

Might be worth a quick write up in the repair logs incase someone else ends up with the same issue.

 

Next I need to figure out how to fix the missing [emoji102]

 

With regards to the eyes, that is quite strange. If it was a data problem you'd expect only certain chars with no eyes. The other through I had was that potentially the signals were having trouble going low after being high for a while, but that would mean the antenna would have the same problem.

Edited by crackedMagnet
didn't notice the missing eye remark.
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Thanks for your replies Matt.

Its awesome when you guys take the time to write your views down for consideration. Another perspective is always welcome.

The eyes....

I'll swap out a daughter board to eliminate that part of the equation and come back.

 

 

Sent from my iPhone using Aussie Arcade

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Re the missing [emoji102]:

Thought through the problem like this...

The Braze kit does a self test and says bit fail.

I look at the data bus on the daughter board and see 8 ICs that run the bus. They are unmarked on the schematics I have and added to that the three data select pins on theses 8 chips (74LS151) were not connected on the board as depicted on the schematics. So, I probed the data select pins 9,10 & 11. Two of these pins; 9 & 10 both wiggled at some stage during the attract or game play but pin 11 was held low.

That prompted me to look at the controlling IC (74175) at locn. [5] and to find the pin that is tied to pin 11 on the 8 x (74151). It turned out to be pin 2 on [5].

I cut the pin 2 [5] with an eye to resolder if I was wrong but the signs were good. c6bbe1a9ac53970ffd42073940a7c204.jpg

I am waiting for a 74175 to replace [5].

 

 

Sent from my iPhone using Aussie Arcade

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