Need help with Colecovision video output scrambled

[zCRP]

Hi all,

I have a PAL Colecovision, which was last used some 8 years ago and was then working perfectly. Currently I do not have any device (TV or else) accepting RF input, so I got myself an AV mod kit and installed it without any problems.

Upon connecting the console to a plasma screen and turning it on, I noticed that the screen was "hick-upping", every few seconds. I tried other games, and in some cases other video artifacts appeared. I am not speaking of "memory" artifacts, such as random characters, corrupt sprites etc., but video artifacts, like when an rf signal is not well tuned, or when one looks at an NTSC signal on a PAL tv, or horizontal distortions in portions of the game screen. I tried again switching games, and the artifacts kept getting worse and worse.

Now, upon turning on the system, the welcome screen (coleco logo + game name) is basically just black with some barely visible gibberish, the difficulty selection screens is a black screen with some blue flickering, and if I start a game by pressing "1", sometimes I can see bits and pieces of the game screen. Some games, like Activision's HERO, work a bit better in that I am able to make out the game screen, with background and sprites, but it is still unplayable. Audio works perfectly.

 

I read about screen corruption problems due to the power switch, so I tried taking it apart and cleaning + lubricating it, but to no avail. I also tried soldering both the +12V and +5V connections to always on, which should get rid of any switch problems, but nothing changed. Obviously I am unable to revert back to RF output easily so I have no way to tell if the AV mod was wrongly done or broke something, or whether there is something else broken in this colecovision.

 

Does anyone have a suggestion of what else I may try to debug this issue?

 

Thanks

CP

[adamcon]

Hi all,

I have a PAL Colecovision, which was last used some 8 years ago and was then working perfectly. Currently I do not have any device (TV or else) accepting RF input, so I got myself an AV mod kit and installed it without any problems.

Upon connecting the console to a plasma screen and turning it on, I noticed that the screen was "hick-upping", every few seconds. I tried other games, and in some cases other video artifacts appeared. I am not speaking of "memory" artifacts, such as random characters, corrupt sprites etc., but video artifacts, like when an rf signal is not well tuned, or when one looks at an NTSC signal on a PAL tv, or horizontal distortions in portions of the game screen. I tried again switching games, and the artifacts kept getting worse and worse.

Now, upon turning on the system, the welcome screen (coleco logo + game name) is basically just black with some barely visible gibberish, the difficulty selection screens is a black screen with some blue flickering, and if I start a game by pressing "1", sometimes I can see bits and pieces of the game screen. Some games, like Activision's HERO, work a bit better in that I am able to make out the game screen, with background and sprites, but it is still unplayable. Audio works perfectly.

 

I read about screen corruption problems due to the power switch, so I tried taking it apart and cleaning + lubricating it, but to no avail. I also tried soldering both the +12V and +5V connections to always on, which should get rid of any switch problems, but nothing changed. Obviously I am unable to revert back to RF output easily so I have no way to tell if the AV mod was wrongly done or broke something, or whether there is something else broken in this colecovision.

 

Does anyone have a suggestion of what else I may try to debug this issue?

 

Thanks

CP

I am thinking that the refresh rate of your new tv is WAAAAAY faster than the ColecoVision can even try to match

[SiLic0ne t0aD]

There is also a possibility that your Colecovision may have failing DRAM chips..

Since you already did some modifications, I'd consider replacing/upgrading the stock 4116 DRAM with new 4164 DRAM chips which require +5 volts only and eliminate the need for -5V, +5V and +12V used with the originals. Just a thought..

 

I bought mine from Here. Good luck!

[zCRP]

Wouldn't failing ram chips have "artefacts" such as random characters or corruption as symptoms? What I am experiencing looks much more like a video signal issue.

 

Regarding the refresh rate, this may be an issue and I will try the console on a CRT screen as soon as I can get hold of one. But wouldn't I then have the issue always with the same degree? As mentioned above, the console kinda worked when I first turned it on, but video degradation has worsened over time.

 

Thanks

[zCRP]

Hi, sorry if I resurrect this topix, but summer is usually when I can dedicate some more time to retrogaming things Further, I was finally able to get hold of a Sony PVM monitor, which should be more than capable to display the signal provided by the Coleco console.

 

So basically I have this PAL Colecovision, to which I have applied an AV mod:

http://www.coolretroprojects.com/ColecoVision-A-V-Mod-Kit-PAL.html

 

As mentioned in my previous post, the output is "wavy", but while I do not think there are any RAM related artifacts, it does look to me like a problem with the composite signal. Here is a video I made which shows the issue:

https://www.youtube.com/watch?v=qLh3vL5Lv_w

I have also tried attaching an expansion module 1, and the output has exactly the same problem: I can hear audio perfectly fine but the video output is "wavy". So again, based on my limited understanding of the hardware, this should not be a RAM or VRAM problem, as there are no display artifacts. Since the same problem happens with the Atari module, I guess that the problem is somewhere in the final stage of signal generation.

Does anyone have a suggestions for further investigation? Could the AV mod be the culprit? Unfortunately restoring RF would be extremely hard, and I have no display accepting an RF signal.

FWIW, I have a logic sniffer, a multimeter and know how to use a soldiering iron, but I definitely need pointers as to where to look.

Many thanks!

[Bmack36]

You should look into getting a AV mod with the LM318 chip instead. The "Ben Heck" transistor mod is prone to overheating and has some brightness issues. Both myself and Yurkie sell these mods. Feel free to contact if you have any questions.

 

The circuit is not too hard if you are interested in doing one yourself.

 

Thanks

[zCRP]

Thanks, I will definitely look into this alternative mod. In the meantime do you have any suggestions on any diagnostic steps I could try to find out where the issue is?

[+nanochess]

The VDP is bad. When it loses synchro almost always is the VDP.

[Bmack36]

Does the RF output do the exact same thing? If the RF looks good (or as good as RF can) it might point the the AV mod failing. If not probably the VDP like nanochess said.

[zCRP]

As mentioned, I am unable to verify if RF works. It used to work a few years ago

 

I will probably try to get a replacement VDP chip. Just to make sure I tried everything before desoldering, is there anything else I can try?

[ColecoDan]

Since you said the Atari module was not working either I would do the RF conversion first because the Atari module is not going to use the coleco VDP chip.

 

Of course both could be bad but I would do RF first since it is easier than changing a chip.

[zCRP]

Hi, yesterday I opened the CV and played around a bit. I checked all solderings of the AV mod but everything seemed fine, then turned it on, and suddenly the picture was a tiny bit better. Now occasionally the picture is steady and I was even able to play a game of Donkey Kong. Strangely, the third screen would appear shifted upwards by a fifth of the screen height. Anyway even this did not last long. Now I usually get the rolling picture effect.

 

So my next steps would be (in order):

1) maybe I could try amplifying the video signal a bit further? For example, what happens if I just route the AV mod's video out into a circuit like this:

 

2) change AV mod to LM318; unfortunately all instructions and schematics I found are for the NTSC version. Does anyone have any idea how this mod would apply to the PAL version?

 

3) switch out the VDP processor, but I will leave this as a last resort.

 

Thanks for any help

[zCRP]

Further update: yesterday I tried solution 1, i.e. add an additional amplifier in series to the composite output that I get from my AV mod. The picture now appears to be a tiny bit better, the screen sometimes stops rolling and stays put, albeit often with the bottom half of the screen on top and viceversa. Bottom line, not a big enough difference, so I do not consider it a step forward and I rule out the hypothesis that the picture signal is too weak. Now either the first AV mod somehow distorts the signal, or the issue comes directly from the motherboard.

So today I will try to replace the AV mod I bought online outright, putting my own 2N4401 based version in place of the kit, and play a bit with resistor values. Unfortunately I do not have any theoretical knowledge of circuits and transistors, thus this will be mostly a try and error exercise. Also, the transistor's collector is connected to +5V. What would happen if I connect it to +12V?

 

I currently do not have an LM318 chip to play with, but I wonder whether the difference between the two could be so big as to identify the transistor based solutions as cause of the rolling picture.

 

BTW, while messing around I noticed that wiggling the wires that go into the (male) power plug had some impact on the picture, in that I was able to influence the picture distortion by pushing the cable (mostly the white +5V one). So I added some solder on all contacts, and now this wiggling effect is gone, but the picture still has all the rolling and shifting etc. I have already cleaned, bridged etc. the power switch, and I also have two different PSU units to try, so I believe that I can rule out any PSU or voltage related issues.

 

Again, any suggestions are welcome. In any case I will document my findings here, maybe they will be useful for other users.

[zCRP]

Quick update: after messing around with RAM tests etc. for some time I went back to the basics and rebuilt the transistor based AV mod from scratch. The instructions posted on the site are a bit nonsensical, for instance it gets ground from two separate points. So I redid everything joining all ground connections, and voilà, suddenly it works perfectly!

[Bmack36]

Glad to hear you got it fixed.

[Jinks]

Great to hear! If you would like to share your solution I might try it on my ntsc coleco if that makes any sense.

[zCRP]

Apparently my cheering was premature. For a few days everything worked fine, but now suddenly video jerking issues have started surfacing again.

 

Here is a youtube video showing the output I am currently getting:

https://youtu.be/wIXFZ529EaA

 

One weird thing is that video distortion is apparently dependent on what shows up on the screen. In this video, one can see that the startup title screen works fine (although this is not consistent, usually requesting a few resets in order for it to settle down), then the skill selection screen is distorted, as well as the game startup screen (in venture that is the screen depicting the collected treasures). But once the actual game starts, somehow everything appears to work fine again. So I am a bit at a loss as to what is happening here.

 

If I understand correctly, the video signal follows this path (please forgive my ignorance in electronics matters):

1) since my CV is PAL, the VDP chip is a 9928, thus outputting a YUV signal

2) this signal is somehow converted into composite, which I then pickup from a transistor (Q24, which according to this schematic should be a 2N3904, but in my CV is a BC something equivalent)

3) this in turn is fed into a transistor based amplifier which finally feeds into the cinch video out connect

 

So previously I thought the problem was at step 3, and in fact I believed that by resoldering eveything and getting better ground I fixed it, and it looked like this for a few days. Now, I already ordered a replacement 9928 chip, but before desoldering that one is there any way I can check if anything is wrong in step 2? I would rather replace Q24 than the whole VDP.

 

Thanks for any input!

[Bmack36]

If the atari module does the same thing, it should not be the vdp as the atari module video does not go through the vdp.

[zCRP]

The issue definitely affects the atari module as well. Is it thus safe to conclude that the VDP is fine?

[Bmack36]

I would think so. The fact that you worked on the AV mod and it got better for a little bit makes me think it is most likely an issue with the AV mod.

Here is a link with info on the LM318 mod, not sure if it is applicable to PAL systems.

 

https://www.a1k0n.net/projects/coleco/sch.html