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Colecovision Video RAM Issue Help?


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#1 KylJoy OFFLINE  

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Posted Thu Oct 11, 2018 10:07 PM

I am not 100% positive but I believe I have a video RAM problem in a Colecovision.  Here are two images:

 

During a blank screen interval:

20181010_194039.png

 

Following the VRAM test of the diagnostics cart:

20181010_193828.png

 
I have performed a 5v Video RAM replacement in other ColecoVisions and don't want to do that right now.  I just want to get this unit fully working for now.
 
Can anyone tell me if there is a way to identify which RAM chip(s) is(are) bad from the images or a good way to test each chip?
 
Thanks!


#2 Ruggers Customs OFFLINE  

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Posted Thu Oct 11, 2018 11:52 PM

Logic probe or an oscilloscope.

 

 

Here's the datasheet for the 4116.

 

https://console5.com...8/85/MK4116.pdf



#3 Ikrananka OFFLINE  

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Posted Fri Oct 12, 2018 10:14 AM

Wow - that's interesting graphic corruption - never seen one quite like that before.  Wish I could help.



#4 -^CrožBow^- OFFLINE  

-^CrožBow^-

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Posted Fri Oct 12, 2018 10:17 AM

Could this actually indicate an issue with the video processor chip itself and not necessarily the vram chips? I've not seen this kind of corruption either.



#5 KylJoy OFFLINE  

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Posted Fri Oct 12, 2018 4:46 PM

Could this actually indicate an issue with the video processor chip itself and not necessarily the vram chips? I've not seen this kind of corruption either.


It's a lot easier to socket and replace that one IC so I might try that and see if the issue clears up. But I'd still appreciate it if someone could point me in a very specific direction.

#6 KylJoy OFFLINE  

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Posted Sun Nov 11, 2018 11:28 PM

Well, I totally lucked out and replaced the only bad RAM chip with the first socket I installed.  It was U17.  :)

 

Here is a pic of the socketed RAM:

20181111_214028.jpg

 

I used a spare RAM chip from a console that I performed the 5v RAM upgrade on in the past.  It is interesting to me that two different brands of RAM were used on the motherboard previously.  It looks like original work so I am guessing it was original like that.  Anyone know differently?

 

Here is how the screen looks now:

20181111_214357.jpg

 



#7 Ikrananka OFFLINE  

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Posted Mon Nov 12, 2018 8:31 AM

Congrats on the fix.  Interesting to know that a faulty VRAM can cause that kind of graphic corruption.






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