Black Screen issue with Colecovision

[vgcollector]

Hi, I am new to this forum but I have been collecting and repairing video game systems for a while now. My favorite system is the Colecovision. I have learned a lot about how to fix broken ones. However, I have not been able to fix the black screen issue. I always clean disassemble the power switch and clean it and resolder it. This fixes a lot of issues. The one thing that stumps me is black screen issue where when powered on your Colecovision displays a blank black screen sometimes with a hum. Has anyone come up with the steps to try and fix this? I am not the best at checking voltages, but I do have a voltage tester, but I don't know what pins to check. If anyone can help it would be much appreciated. I have read a lot of forums on this subject, but very few end with the fix. Thanks in advance.

[alortegac]

Assuming you have checked the obvious. Check the Y1. I had a similar issue once and I leg of Y1 was detached.

Try the atari module to see if you get picture to rule out rf modulator.

Just my 2 cents.

[vgcollector]

Hi, thanks for the tip. Where is the Y1 located on the system board. I looked for it but couldn't find it.

 

Thanks for your comment. I just saw it.

[vgcollector]

I found the Y1 connector on the 74LS138 chip. So did you have to resolder the chip? I just want to make sure I know that your terminology when you use the word "leg: are you referring to one of the pins of the chip?

 

Thanks again.

[nick3092]

Not sure if you know what Y means in terms of the notation. Y is the notation for a crystal or oscillator. They usually look like a little metal pill or can. It should be fairly easy to find once you know what it looks like.

 

Edit: looking at some images of some Coleco pcbs, it may be just to the right of the cart slot.

Edited February 5, 2019 by nick3092

[ChildOfCv]

Is it a 60Hz hum like you sometimes hear from cheap devices or transformers?

 

Did you try a different power supply to rule out dirty/bad power? Also, check the pins from the power brick. It diagrams them on the brick label so you know what each one should read.

[vgcollector]

Thanks for the information. Here is what I have done so far.

 

Tested power supply for voltages and they all check out.

De-soldered power switch cover and cleaned the power switch.

I do get video and audio using the Atari Expansion Module so the RF should be ok.

Cleaned cartridge slot and checked for anything in the slot.

When turned on, the screen is black and sometimes has a hum which varies from time to time. This happens regardless if a cartridge is in the slot or not.

 

I have 3 Colecovisions doing this same thing, Most of these boards seem to be in good condition and one Colecovision was super clean so I was surprised when it didn't power up correctly. I appreciate the help with this one. I know this issue has been brought up in these forums before, but I have never seen a clear cut solution. I am a novice on the electrical stuff and some of the terminologies, so I appreciate the clarifications.

 

Thanks,

Vgcollector

[Pixelboy]

I do get video and audio using the Atari Expansion Module so the RF should be ok.

Not necessarily. The Atari Expansion Module circumvents the ColecoVision's video processor in order to output to the TV directly.

[ChildOfCv]

Not necessarily. The Atari Expansion Module circumvents the ColecoVision's video processor in order to output to the TV directly.

The RF is just the final stage that modulates the audio and video signals to a TV channel. So as far as that goes, it sounds good.

 

But yeah, the Colecovision's display controller could be an issue. Put Frogger in and see if you at least get audio when you turn it on. It's one of the few games that doesn't start with the Coleco Pause, and it starts playing music as soon as it starts up. So if you hear audio but see no video, you can be somewhat sure that the VDC is dead.

[vgcollector]

I will try the Frogger that is a good idea too.

 

Thanks I appreciate all the suggestions.

 

Vgcollector

[vgcollector]

Not sure if you know what Y means in terms of the notation. Y is the notation for a crystal or oscillator. They usually look like a little metal pill or can. It should be fairly easy to find once you know what it looks like.

 

Edit: looking at some images of some Coleco pcbs, it may be just to the right of the cart slot.

I will look for the oscillator or crystal. What is the function of the oscillator?

 

Thanks,

Vgcollector

[ChildOfCv]

Without the oscillator, nothing runs. It gives the CPU something to synchronize its internal electronics to. It also gives the sound chip something to work on. It provides the RF circuit with the clock for encoding color. And it provides the video controller with its necessary synchronization.

 

It's tempting to think "Oh, so the RF works, with the Atari module, so it must have a good oscillator." Well, not quite. The RF module also has its own internal oscillators, one for sound modulation and one for TV channel output. So the color modulation was already done in the Atari module and it skips that stage of the RF encoder, going straight into the channel modulator. So with external video, you can still get good RF output with a dead CV oscillator.

[vgcollector]

Do you know what chip number it is? This would explain a lot. Is there a way to check voltages on that chip to make sure it is working properly?

 

This is excellent information. I appreciate you sharing this!!

 

I am going to try and put together a lot of the info out there specifically related to the CV motherboard like what is the function of each chip number. That will help me and others identify problems with the chips for troubleshooting.

 

Thanks,

Vgcollector

[ChildOfCv]

That can be tough between revisions. But on Rev J at least, the crystal works with U22 and some surrounding capacitors and resistors to build the oscillator.

 

I created a schematic of the entire system along with a PCB diagram, using my rev J board. https://drive.google.com/open?id=1lVH7_HhNAEBQ5yUY1j8FsJktdYjzmgUc

 

Console-PCB.pdf is a diagram of the PCB and the locations of all of the components. Console-schematic.pdf is the schematic.

[vgcollector]

You really have put a lot of time into this. Thanks for all the great documents.I can pretty much fix the controllers, the Super Action Controllers, the roller controller, and most issues with the Atari Expansion module. I am still working on my skills with the driving controller. This one issue is one of the few that I have not been able to isolate, but it would be nice to be able to come up with a check or checks that would identify which chip or chips are failing. I am an electronics novice when it comes to things like voltages and knowing what a chip does, but I am good at troubleshooting. De-soldering the chips on the pcb are tough. I have done the controller chips, and the memory chips, but that is about it so far.

 

Is there anything out there that will allow you to test each chip for voltages to make sure they are getting or putting out the correct readings?

 

Thanks,

Vgcollector

[ChildOfCv]

The best beginning tool for testing is a logic probe. They can be fairly inexpensive, especially for slower ones that only work with vintage electronics. They're useful because they light up green for high, red for low, or blink both for changing signals. So it gives you a great way to at least see if something is "alive".

 

I've considered building a diagnostic device out of an Arduino or similar device, and a 30x2 card edge connector. You can test a huge amount of the Colecovision system directly from the expansion port without even opening the case. But that is still just a pipe dream at the moment.

[vgcollector]

I went and bought a logic probe. Now I will have to look at some youtube videos to learn how to use it. Are you able to test most of the Colecovision chips with this? This could be a real game changer if I am able to find a bad chip with it. You have been most helpful.

 

Thanks,

vgcollector

[vgcollector]

Do you know if I can attach the black lead to the +5 on the power plug in on the Colecovision and the red lead to the -5? Then do you know what pins to check on the chips?

 

Thanks,

Vgcollector

[ChildOfCv]

Connect the black lead to ground. If you connect it to -5 and +5, that would be a 10V difference and would probably kill the probe. Don't try to probe negative or +12V lines with it.

 

As long as you know what to expect, it's useful. It can certainly tell you if the clock inputs are working (flashing lights), and if the +5 and gnd pins are good. You can poke at data and address lines to see if any communication is going through the system. For instance, the RFSH, M1, and MREQ lines from the processor should always be blinking. Any clock line should be blinking. If any data or address line is making it to the chip, chances are it will be blinking too. The higher address lines (A15 through A10 or so) may not be too active, especially without a cartridge in.

 

Chip selects are trickier though, as they will only go low when that particular chip is singled out for communication. The ROM chip's CS pin will probably be blinking though, possibly even both SRAM chips. Audio and video output won't be digital, so who knows what the probe would say looking at any of them.

 

As a start, look at U22 pins 2, 4, 6, 8 and 10. They should be blinking. Same for U8 pins 6, 5, 8 and 9. Just for grins, U8 pin 13 should be high and pin 7 should be low, though it's because these are hard-wired to power and ground respectively.

 

U7 pin 4 should be blinking. Pins 11 and 12 should be low and U7 pin 10 should be high.

Edited February 7, 2019 by ChildOfCv

[vgcollector]

Where should I plug in the red clip? I will try and do this today or tomorrow.

 

Thanks for the specific information.

 

Vgcollector

[ChildOfCv]

One possible location might be on the +5V coil near the main switch. But you would need to make sure the clip doesn't come into contact with the 12V coil right next to it. Probably with a good amount of tape on both sides of the +5V coil to prevent it from touching anything nearby. The black one can easily clip to the metal on the switch.

[vgcollector]

One other question from this newbie about using the logic probe. Do you test this with the power switch on? If turned on, do I need a cartridge in the slot.?

 

I plan to try it out tonight. FYI I did test with the Frogger cartridge and I got no sound. That oscillator may be the culprit since you said it starts everything else.

 

Thanks,

Vgcollector

[ChildOfCv]

The device under test does need to be powered on. The system can work without a cartridge, but you may see more interesting signals while the cartridge is in.

[vgcollector]

ChildofCV, thanks for your help. I am working on some other things, and then I will get back to the Logic probe. Have you heard much about the Atarimax for the Colecovision? I am thinking about getting one because it has the Colecovision diagnostics. That would be helpful in testing the memory.

 

Thanks, I just wanted to let you know what I was working on.

 

Vgcollector

Edited February 10, 2019 by vgcollector

[alortegac]

I am sorry I did not received any notifications after my last input.

I see you have been making significant progress and happy you have been assisted by a true expert (childofcv).

 

Did you say you have 3 colecovisions doing the same thing?

 

Before you do anything else, I would try a different tv set.

Just my 2 cents

 

Regards!

 

Al

Edited February 11, 2019 by alortegac