Test cart (especially RAM) ? Bad graphics/colors

[david.winter]

Hi,

 

I just got a european CBS Colecovision which I'm trying to fix. For those who don't know, this outputs RGB video which should be crystal clear unlike RF. The problem is, I get buggy colors and graphics. When this happens, alterations are stable, they never change randomly.

I tested two carts: Mr Do! & World Games.

Mr Do! seems to work almost without trouble, but somtimes during the "Player 1 Get Ready" screen, I get two or three stable sprites on the left. I couldn't make this happen again but testing World Games shows the real thing... See screenshots below.

I suspect the video RAM to have something bad but I'm not sure. It could also be the TI VDP chip or maybe the cartridge connector.

 

To see what I'm talking about, I made a quick html page with several screenshots.

It's at http://www.pong-story.com/cv/cv.htm

 

Any help appreciated. Thanks !!!

 

 

David.

[newcoleco]

Hello David,

 

I've never fixed a video glitch problem before, so I can't help you really. What I did experience in the past was changing the power supply to notice a difference, and I don't know why, but sometimes it works.

 

I hope someone will be able to help you.

 

As a side note, your dedicated Pong web site is still incredible, and I still have the ColecoVision game you did trade with me long time ago.

 

Good luck and take care!

 

Daniel

[R.Cade]

I've got one that does this and it changes when I wiggle the RF output socket. I think mine has bad solder joints on the board, for sure.

[PkK]

Hi,

 

I just got a european CBS Colecovision which I'm trying to fix. For those who don't know, this outputs RGB video which should be crystal clear unlike RF. The problem is, I get buggy colors and graphics. When this happens, alterations are stable, they never change randomly.

I tested two carts: Mr Do! & World Games.

Mr Do! seems to work almost without trouble, but somtimes during the "Player 1 Get Ready" screen, I get two or three stable sprites on the left. I couldn't make this happen again but testing World Games shows the real thing... See screenshots below.

I suspect the video RAM to have something bad but I'm not sure. It could also be the TI VDP chip or maybe the cartridge connector.

 

To see what I'm talking about, I made a quick html page with several screenshots.

It's at http://www.pong-story.com/cv/cv.htm

 

Any help appreciated. Thanks !!!

 

 

David.

 

Looks like the typical problem to me: Graphics errors caused by the supply voltag ebeing to low for the graphics chips. Typical solutions for this problem are a better power supply and cleaning the power switch (see e.g. the "ColecoVision Problem" thread in the hardware forum for details).

 

Philipp

[youki]

I had often this kind of graphic glitch on some arcade PCB i own.

 

In must of case, just a good bath solve the problem.

 

Remove all component that are on socket. and plunge the PCB in a bath of water and soap. Then rinse well and let dry 4-5 days. Usually with arcade PCB it solve lot of issue.

 

But you can try also another power supply.

[Bruce Tomlin]

Arcade PCBs are exposed to a lot of dust, some of which may be a bit conductive, and that is why cleaning them works for you. But the Colecovision is not a coin-op.

 

 

In the case of the Colecovision, the problem is typically video DRAM chips that have gone bad, and bad power, usually due to oxidation inside the power switch. No amount of soaking in water is going to fix a bad DRAM chip.

 

Often a DRAM chip goes completely bad, and you can tell which one by how the "insert cartridge" screen looks. Actually testing for bad bits sounds like an interesting idea, though. I might see what I can do when I get a couple of my current projects finished.

 

Also, if there is a dead DRAM chip, it might be good to figure out a way to show it even with the chip dead. Maybe something like "U8 BAD" in big block letters. Or even choose name table entries to work around it. (Hmm, I've seen bits show up bad in the name table, but the patterns are still OK? And I only just now realized this after how many years?)

 

It would also be nice in addition to the controller test I've already written, to have one specifically for testing if either joystick input chip has gone bad. Something like you turn the stick in all directions, and it marks inputs as being good.

 

Of course there will always be problems that can't be tested simply by software diagnostics, like the one time I got a "sepia-tone" CV. One of the inputs to the video modulator had a bad solder joint and it was causing all the colors to come out brownish.

[david.winter]

Thanks for your hints. I replaced all the electrolytics in the AC adaptor.

After a first try I noticed a slightly better picture but still with glitches.

I then turned the power switch on/off many times very rapidly and this apparently solved the problem.

I guess the contacts got enough oxidation to become more or less resistive and generate random voltages, which the video chips didn't like I'm sure.

 

Thanks again