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Colecovision corrupt graphics after 5V vram mod

5v vram mod graphic corrupt

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#1 zCRP OFFLINE  

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Posted Wed Aug 9, 2017 3:33 PM

Hi, I just modded my colecovision by replacing the vram chips and making the 5v only modification. This CV is also AV modded.

 

When I turn it on everything works perfectly, but after a few seconds the whole screen becomes garbled. See video below.

 

https://youtu.be/9oTzC-_4fhA

 

I assume that the vram chips are fine, and the kind of corruption does not look like what is usually experienced with failing ram (I refer to the screenshots in the technical manual). 

 

I have cleaned the power switch repeatedly, so for now I would exclude that as well.

I have also tried to use a second power supply, and I get the exact same symptoms.

 

Thus my focus is now on the VDP failing. Given what you see in the video, would you replace it? I have two spare VDP chips, just wanted to hear second opinions before starting (another) surgical intervention.

 

Thanks

Chris


Edited by zCRP, Wed Aug 9, 2017 3:49 PM.


#2 Keatah ONLINE  

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Posted Wed Aug 9, 2017 3:58 PM

If you let it run for like 10 minutes, turn it off, then back on, does it work? Or does it get corrupt right away.

 

I'm thinking something is overheating or temp sensitive. Might be a good place to start noninvasively.



#3 zCRP OFFLINE  

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Posted Wed Aug 9, 2017 5:04 PM

I have just had the CV off for approx.45 minutes. Then I turned it on with Q*Bert inserted, but have not touched the controller. The blue screen with invitation to press 1-6 shows up correctly. I have waited a minute or so and nothing seems wrong. The I press 1 to start the game, and after 4-5 seconds the screen starts scrambling. Now if I turn off the console and turn it back on after 2-3 seconds the screen becomes garbled right away (i.e. the blue screen is already wrong). If I wait a minute or so before turning back on, I get the same behaviour as at the beginning: the blue screen "works", as soon as I start a game graphics corrupt after a few seconds.

In all this, audio and input continue to work correctly, so this is a video only issue.

Is the VDP overheating? If yes, why? This whole mess has started only after I replaced the VRAM.



#4 5-11under ONLINE  

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Posted Wed Aug 9, 2017 6:43 PM

You could look for a bad solder connection, but otherwise I'd guess VDP.



#5 zCRP OFFLINE  

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Posted Thu Aug 10, 2017 11:58 AM

I took an infrared snap of the CV (see attachment). As can be seen, the places getting hot are as expected (CPU, vram etc.), the only unexpected place getting hot is where the RF chip is, more or less at the center of the board. not sure if that is the capacitor on the left getting hot or the chip itself. Incidentally, this is close to the region where I removed the coils in order to change voltage for the ram chips.

 

The CPU tops at around 50°C, the ram chips around 40-42, the place I mentioned above reaches about 45. Those temperatures seem all pretty "safe" to me, given that chip specs usually mention 0-70°C operating ranges.

 

I will now try re-checking all solder spots for the vram chips I replaced. Any idea on why a bad solder spot could cause a delayed corruption such as I am experiencing?

 

Next on would be replacement of VDP. How do I get the heatsink off, and what do I need to do to attach it to the replacement chip?

 

Thanks

Attached Thumbnails

  • File 10-08-2017, 19 41 44.jpeg


#6 Tursi OFFLINE  

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Posted Thu Aug 10, 2017 12:34 PM

Strange.... but I wonder about the heatsink on the VDP. Since you were in there, perhaps you damaged the thermal connection between the VDP and the heatsink. If the VDP was overheating and the heatsink was not making good contact, you wouldn't see that from the top (maybe from the side?)

#7 zCRP OFFLINE  

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Posted Thu Aug 10, 2017 2:31 PM

I do not see any sign of the VDP heating abnormally.

 

I just resolder all vram chips, but that did not make a difference. I also checked all contacts between the vram and the VDP and they appear fine.

 

While poking around with the multimeter I noticed that my power brick supplies +5 and +12V as expected, but the negative voltage is -2 instead of -5. Could this be the issue?



#8 zCRP OFFLINE  

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Posted Thu Aug 10, 2017 5:32 PM

Never mind my last post, I read in other posts that -5v is not relevant here.

 

So I went ahead and desoldered the VDP. I carefully removed the old chip, installed a socket, then tried the old chip and two spare chips. Unfortunately, the situation only got worse, in that previously the console would work for a few seconds, while now I get the gibberish right away.

Underneath I can hear music and reaction to joypad inputs, so I would say that CPU, ram, audio and inputs work correctly, only the video portion is not working.

 

I have no spare vram chips to test, but I will look for some online. What else can I try at this point? The console has socketed vram and vdp. I doubled check all pins for continuity and found no lifted traces. Please help! ;)



#9 Osgeld ONLINE  

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Posted Thu Aug 10, 2017 5:53 PM

Not totally helpful but artic silver makes two wonderful heat sink 2 part epoxies

Since it was asked and they perform great and are fairly cheap

Edited by Osgeld, Thu Aug 10, 2017 5:53 PM.


#10 Keatah ONLINE  

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Posted Thu Aug 10, 2017 6:27 PM

If I'm reading that IR image right, it looks like the areas immediately to the right of the ram is getting warm. Warmer than the chips themselves. Just questioning the uneven heat distribution. Are the bypass caps for the chips fighting noisy power? Just a thought..



#11 Osgeld ONLINE  

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Posted Thu Aug 10, 2017 9:04 PM

If I'm reading that IR image right

 

A flir image without a scale on the side is almost pointless, it could be a 100c difference or a 3c difference tween orange and yellow depending on the min,max, &average as the color patterns are non linear 

 

(ie dark blue to light blue could be 20c delta, but from orange to yellow could be whatever, I can take a very similar image showing a product where the dark area's are ambient and the glow area's are 145c giving a 120ish delta, or I can produce the exact same image pointing the stupid thing at my leg) 

 

FLIR actually has a nice post processing tool that takes the image metadata and make it into data one can process for free on their website 


Edited by Osgeld, Thu Aug 10, 2017 9:07 PM.


#12 zCRP OFFLINE  

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Posted Fri Aug 11, 2017 2:48 AM

I have noticed that the flir App has misaligned the infrared image with the optical one, in fact the "hot spots" correspond to the chips, not to the caps on their right. Regarding the temperature range, the dark areas are in the 29-30°C range, while the brighter ones are in the 40s with some getting as high as 45. The RF out chip (which should not be used since the console is AV modded ;) is one of the hottest getting to 45.

 

But sadly, as I mentioned above, after tampering with the VDP I get the garbled screen right away when I turn on the console, even after it was off for  a long time, so not sure it is an overheating issue.

 

I guess while desoldering I fried some other component or broke some trace, but I could find no evidence of either. Does anyone have other suggestions for further diagnose?

 

Thanks






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