Black Screen issue with Colecovision

[+KylJoy]

On U5 of "black screen" unit:

 

1 - sometimes pulses, sometimes high, sometimes low

2 - sometimes pulses, sometimes high, sometimes low

3 - sometimes pulses, sometimes high

4 - usually pulses, sometimes low

5 - usually pulses, sometimes low

6 - high

7 - usually pulses, sometimes high, sometimes low

9 - high

10 - high

11 - high

12 - high

13 - high

14 - high

15 - high

 

On U5 of known good unit:

 

1 - pulses, then goes low

2 - pulses, then goes low

3 - pulses

4 - pulses

5 - pulses

6 - high

7 - high

9 - high

10 - high

11 - high

12 - high

13 - high

14 - high

15 - pulses

 

Thanks for the assistance!

[ChildOfCv]

So the processor is hanging out in the E000 range, according to that.

 

Did you make sure to press reset to see if you get at least an initial pulse on pin 15?

Edited March 18, 2021 by ChildOfCv

[+KylJoy]

4 hours ago, ChildOfCv said:

Did you make sure to press reset to see if you get at least an initial pulse on pin 15?

I can't see any initial pulse on 15 of the bad unit.  FYI, this very IC seems to work fine in a known "good" unit.

[ChildOfCv]

12 hours ago, KylJoy said:

I can't see any initial pulse on 15 of the bad unit.  FYI, this very IC seems to work fine in a known "good" unit.

You mean you pulled the IC and tested it in the other unit?

So the problem is the input to U5 then, not the output.  Its inputs are driven entirely by the CPU.  If you never see a pulse on the BIOS line at all, even after hitting reset, that means the CPU isn't doing what it's supposed to.