Robotron Missing Colors

Logical Highs

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Hello All,
I have a Robotron CPU board that is only outputting red. I swap in a different CPU board and all the colors are fine, so I know it is on that CPU board. Any help where to start looking? Technique's on how to narrow it down? The best way to test the video RAM?

Chuck
 
RAM is tested at power up. If the game doesnt identify any as bad then its all most likely good.

the IC at 1A could be bad (74LS374) or the diode resistor matrix on the output of that chip may be damaged, or Q1 and Q4 are bad or missing.
 
Check Q1, Q2, and Q3.

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I pulled the drawing set and probed the chip at 1A and it didn't have any inputs, so I probed the two video ram chips and one has good outputs, but the other does not. The inputs look good so I am now trying to find a 74LS189N. I had someone send me a link to unicorn electronics that seems to have them for $3.49, but they require a $25 minimum purchase.
 
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Looks like ebay, Amazon, and Aliexpress have sellers that have the 189 (I assume you don't actually need the N suffix).

DogP
 
I finally found the time to put that chip in and and this Robotron board is now happy with all colors! Thanks for the help.
 
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