Colecovision Video Output Repair

[Ham62]

I have this Colecovision a friend gave me to repair a few months ago. Basically, when it's running I get a TV signal, it displays the background color but it does not render any tiles or sprites on top of the background.

 

I originally thought this was faulty VRAM and the CV just wasn't able to write to the memory addresses where the charactermaps are stored, but after doing the 5v RAM conversion I still get nothing but the background color on the output.

 

https://console5.com/wiki/Colecovision_/_ADAM_5V_RAM_Modification

 

I've checked every connection listed on the console5 wiki for the RAM mod probably 5 times and cannot find anything that's disconnected.

 

Where do I go next with this? I know the games are running because after passing the blue setup menu I can hear the games playing in the background, but nothing displays except the background on the TV. I don't think this would be the video chip would it? It still gives a stable color signal when running a game, but I don't have a donor system to pull a replacement video chip from to test it.

 

I've included a YouTube video of the console loading up Donkey Kong to demonstrate the weird behavior. 

 

 

[Ikrananka]

First step would be, with the system turned on, to measure voltages from your power supply and around the board at the various VCC pins on the chips.  In particular check you're getting correct voltages at the VRAM and the video chip.

 

Do you have an oscilloscope or a logic probe?

[Ham62]

Unfortunately I have neither an oscilloscope nor a logic probe.

 

Checking the voltages around the board they seem to be within spec. At the power supply I have +11.6v, -5.15v, 4.98v respectively. On the board I'm reading 4.85-4.86v on the RAM chips and I have 4.84v at the video chip.

 

Do you have anywhere else I should be checking for voltages on the board, and what I should be expecting at those points?

 

Likewise, is it possible there's a broken trace somewhere between the Z80 and VDC that might be causing it to not load the character maps into memory?

[ChildOfCv]

Wow, never seen a problem like that before.

 

Check that pin 13 of the video controller chip has continuity with pin 5 of either SRAM chip (U3 or U4).  It also needs to reach other chips of course, but the fact that the game runs seems to say that the other A0 connections are being made properly.  This is the difference between sending mode commands and sending data.  I guess you could also make sure the data lines make it too.  For that:

 

U3   ->    VDC

14            17

13            18

12            19

11            20

 

U4   ->      VDC

14             21

13             22

12             23

11             24

 

If all of that is making it into the VDC but it won't show any graphics, then the VDC itself is the likely cause.  It looks like the heat sink is installed, but is that something you had to fix?

[Ham62]

Yeah, all of those connections you listed seem to have continuity fine, and the heatsink was still installed when the console got to me so it shouldn't have been overheating from the VDC.

 

I'm really kind of hoping that chip isn't bad, as I imagine finding a new one is not cheap...

 

I just remembered I have an old Arduino board laying around somewhere, would it be worth trying to set that up as a makeshift oscilloscope to further debug this? I'm not sure how accurate it would be but it might be good enough to give an idea of what's happening on the VDC?

Edited December 29, 2020 by Ham62

[ChildOfCv]

I suppose the Arduino could be programmed as a logic probe.  For that matter, maybe there's already a logic probe program out there somewhere.  You'd want to check pins 1-11 and 25-32 of the VDC for digital activity.  All should be constantly high/low.  I guess it could be worthwhile to check pins 40 (clock) and 34 (reset), but because you're getting the correct background colors, I have confidence that those are working as designed.

[Ianr757]

On 12/28/2020 at 6:37 PM, Ham62 said:

Yeah, all of those connections you listed seem to have continuity fine, and the heatsink was still installed when the console got to me so it shouldn't have been overheating from the VDC.

 

I'm really kind of hoping that chip isn't bad, as I imagine finding a new one is not cheap...

 

I just remembered I have an old Arduino board laying around somewhere, would it be worth trying to set that up as a makeshift oscilloscope to further debug this? I'm not sure how accurate it would be but it might be good enough to give an idea of what's happening on the VDC?

Not sure if you have already solved this issue or not, but I had this exact problem with my CV. I replaced both the VDP and VRAM to get it working. And don't worry, the VDP (TMS-9928) is only about $10 US. You can easily find replacements on ebay.

[Ham62]

I know I'm resurrecting an old thread, but I hate searching the web for a problem and finding a dead thread with no confirmed solution posted at the end.

 

I finally finished procrastinating and got around to desoldering the old 40 pin VDC and installing a socket + new VDC I got off ebay. Lo and behold, I've got sprites and graphics on the TV now!

 

I've just gotta get some thermal adhesive to properly secure the heatsink on the new chip and it'll soon be good to go!

 

Thanks for the help guys!