Colecovision won't power on - help?

[reverendshoebox]

Hey all.

 

I have a Colecovision that won't power on. No black screen, no BIOS screen, nada.

 

I've verified via multimeter that the power supply seems to have 12, 5, and -5 volts where it should, including where it connects to the board, but when I switch the power on, the 5 volt reading jumps down to 1.6.

 

Is this an expected drop, or an indication that I've got a bad power supply?

 

If the former, where should I look next?

 

Thanks much,

-=ShoEboX=-

 

[SignGuy81]

That isn't an expected drop and more than likely indicates a bad component pulling the power supply down.

[reverendshoebox]

Thanks for the response. Do you mean a bad component in the power supply or in the Colecovision itself? And if it's in the Colecovision, how do I go about diagnosing the issue further?

 

-=ShoEboX=-

[SignGuy81]

I believe you have a shorted component inside the Colecovision itself. Is any part inside the Colecovision feel hot? See if any of the chips feel hot.

[+grips03]

Measure +5v to ground on top of all the ICs, ohm leads right on top of the IC legs. Write down the resistance going as many places as the ohm meter supports.

 

Example 50.001 and 50.009 ohms.

 

Does the hot to the touch IC also show the least amount of the resistance?

 

If yes, perhaps that is the bad IC. Clip the ground leg of that IC, does the resistance change between ground and +5v?

 

If resistance does not change, then resolder that ground you just cut and keep trying.

[reverendshoebox]

Hey all. I ended up trying another AC adapter and I'm up and running. Thanks for the advice, greatly appreciate it!!

[paulderr]

Wait! You said the power supply was putting out the correct voltages. How did just changing to a new power supply fix the issue if the one you were using to begin with was working? I am having the same issue but only have one power supply. I have two Colecovisions and neither power up at all. I test the power supply and the voltages are correct but get nothing from the Colecos.

[ChildOfCv]

If the power supply has a bad regulator, it can still read correct voltage with no load, but fail under load.

 

As someone mentioned though, the board will have a characteristic DC resistance to ground on the +5V rail. On mine (rev J) it will be about 72 ohms. Any less than that, and you might have a component drawing too much power, which can also lead to power supply failure.

[alortegac]

Hi, would please explain the process of doing this measurement? I just want to be completely sure I will do it correctly when needed. I have come accross a couple of CV consoles that appear to draw too much power. I thought it was a component is short or failing but I did not know how to best identify it. Thanks a lot! Al

[ChildOfCv]

The easy way is to unplug the CV, turn it on, open the expansion port, and check the resistance between the two large contacts on the left side (one on top, one on bottom). On my board that reads about 75 ohms.

[alortegac]

Hi, I must be doing something wrong readings are 321 ohms 341 in 2 different consoles. Any ideas?

Thanks!

Al

Edited February 9, 2019 by alortegac

[ChildOfCv]

Looks like you took the readings in the correct way. It could be a difference in meters (mine's a Fluke 117). But the important point of checking here is to make sure there are no shorts. Yours is definitely not shorted on the +5 rail, at least. Once you've checked the +12V rail and the -5V rail in a similar fashion, you know it's at least not going to kill the power supply. If you still see a voltage drop when you turn it on, the power supply is then the most likely culprit.