Blackscreen CBS Colecovision

[relo999]

I've got one of those nice "blackscreens of can be anything" on my recently acquired colecovision... But I can't seem to fix it. It is a revision D PAL system. I suspect it having a short somewhere on the 5V line. Powered off it gives 5V but turned on 1.38V. And my guess is low voltage to chips = black screen.

 

-Cleaned it at least twice with alcohol.

-Jumped the powerswitch, but that was a no go.

-I've checked the PSU (its one of those plastic bricks unable to work on if I wanted) but that seems to be fine, as I could power a gameboy over the 5V line of the PSU.

-I've replaced the RAM chips with CD4516BE using the 5V mod, thinking one of those chips might have caused the short.

-Recapped it but with similar result. (there was some lower resistance between ground and 5V line, but that was gone after but apart from that nothing changed)

-Checked if it could be on the RF board, but that seems to get it's 5V from the 12V line.

 

So what else might cause this problem, and if its a short how can I find it?

Edited June 25, 2014 by relo999

[relo999]

Detached the RF board, no change. So not a problem on the RF board...

[BigO]

My general thoughts/reactions:

 

It gives 1.38V measured where? Is that a regulated power supply? If only 5V is showing up at the unloaded output of an unregulated 5V power supply, that could be a problem.

 

Do you have any way to more extensively load test the power supply exclusive of the console? Not sure it it's even a possible failure mode, but maybe too much of a load is causing the power supply voltage to drop. The Gameboy load may not be enough to induce the failure. Just a wild guess with no specific knowledge of the stated problem, but it does potentially narrow the focus rather quickly.

Edited June 26, 2014 by BigO

[relo999]

I measured it at the plug and power connector (off and on) and after the inductor it becomes more ~1.28 with not much variety after.

I think it's a regulated power supply but I'm not sure.

 

I don't have much for a more extensive load test I believe but I'm going to test it on my for parts SNES and check if it could power it and if voltages change.

Edited June 26, 2014 by relo999

[relo999]

*Can't edit post*

 

The SNES did get voltage trough, but not enough to power it (it needs minimum 9V). And still no drop in voltage.

[Osgeld]

drop in voltage indicates a shorted out component

[relo999]

There wouldn't be an easy way to check for that?

[relo999]

I've found what MIGHT be the problem, I've removed the RAM chips and now it does get 5V and a quick tone+scrambled colored screen. I can add up to 2 Chips, at any spot, after which that voltage drops (with 3 or more) again also those 2 become incredibly hot...

Edited June 27, 2014 by relo999

[Osgeld]

but to say where does it get stupid hot, that's probably where to start looking =)

 

I do that on occasion at work, cant find a short, well let it ride for a few moments and pop out the infrared camera heh

[relo999]

Yea that was my thinking also, but I can't find a flaw... Maybe I overlooked it some times. Might be the wires I placed to recover the traces I broke with removing the chips. Maybe you can spot an obvious flaw? (I've removed the CD4516BE's which had the notch the closest edge of the board)

 

[relo999]

if I read this right: http://console5.com/wiki/4516 and http://www.datasheetarchive.com/dlma...DSA-228419.pdf it seems I've placed the IC's backwards. Now I do get 5V but it also seems that they are incompatible for the most part, could anyone confirm it?

 

At any rate still no picture, if they are compatible I might have broken them by putting them wrong way around or it's a different problem. Also may I not that now the actual Videochip get warm instead of the RAM.

Edited June 27, 2014 by relo999

[Uzumaki]

Some of the sockets are installed backward. It could cause confusion to whoever gets your system in the future. The notches on the socket (and the chip) are supposed to match the silk screening. When you put the RAM chips, did you make sure all the notched side are facing the right way? In the picture you used, the notches should be pointing toward the top of the image.

 

Sockets itself don't have polarity and it will be fine on its own, just leave a sticky note inside to remind yourself or inform other people in the future to ignore the notches on the socket as they are wrong.

[relo999]

Yea I was thinking the same myself, when I put them in I didn't check that.

[relo999]

I just received new ICs as I noticed they weren't compatible and changed them. But i still get a black screen, what else could be the problem?

[relo999]

The IC's that I replaced was the TMS9928A with the CD4516BE. But I've just tested/ordered MK4516N-15 (becuase CD4516BE I saw that it had the wrong pinout and probably different type of IC) as those are mentioned on the mod page, just to be completely sure to use the correct IC.

But I still get only a black screen, but now do get a tone quite consistently.
What I think is weird, and I have mentioned it before I think, is that I get the correct channel on multiple channels. And the weirdest is that the best (only one with a tone I think and no noise on the black screen) channel is on 57 MHz, seeing how it is a german colecovision that frequency should be radio (and unpickable by most, if not all, mid/early 80's European TV's).