464 Plus Black screen (My 3rd bad board)

[Overkill]

Hello all again, this is the 3rd board I'm trying to repair.
 
>Black screen, but I can see it's outputting some video, get normal colour flash before black screen with normal horizon interference.
 >My scope sees data on address lines and data lines.
 >I've replaced the RAM with good ones.
 >I've checked all connections from the RAM to the ASIC.
 >I've checked all the connections from the CPU to the ROM cart.
 >I've tested the DM74LS27 Triple 3-Input NOR Gate IC16.


I'm not sure how the CPU's Wait, Reset and other pins should look like.
Could a black screen be caused by the AY chip?
Any other things I can check?

[GUNHED]

3 bad boards?

- Use new PSU, with 5 Volts
- Use new monitor to show pocture
- Press keys to hear sound (indicates some functions)

[Bryce]

Could a black screen be caused by the AY chip?
Any other things I can check?

Only if the AY had a bad short and was pulling the 5V Rail down below 4.75V.

Check if the ASIC is outputting data on the 12 colour output pins.

Bryce.

[Overkill]

3 bad boards?

- Use new PSU, with 5 Volts
- Use new monitor to show pocture
- Press keys to hear sound (indicates some functions)

sorry for confussion, I don't have 3 boards with the same fault, the other two boards are in other threads.

[Overkill]

Could a black screen be caused by the AY chip?
Any other things I can check?

Only if the AY had a bad short and was pulling the 5V Rail down below 4.75V.
Check if the ASIC is outputting data on the 12 colour output pins.
Bryce.

Thanks for the advice.
 
It's interesting what you say about the 4.75v, because the board that works apart from some crashing and messy sprites only does that on my cheap (very cheap) bench power supply which drops to 4.6v.   If I try to run that board on a good 5v power supply I only get a black screen. 

[Bryce]

Could a black screen be caused by the AY chip?
Any other things I can check?

Only if the AY had a bad short and was pulling the 5V Rail down below 4.75V.
Check if the ASIC is outputting data on the 12 colour output pins.
Bryce.

Thanks for the advice.
 
It's interesting what you say about the 4.75v, because the board that works apart from some crashing and messy sprites only does that on my cheap (very cheap) bench power supply which drops to 4.6v.  If I try to run that board on a good 5v power supply I only get a black screen.

If you have a board that only (and messily) runs at 4.6V and doesn't at 5V, then you have a chip that's starting to fail (probably damaged or contaminated silicon). If it's a Plus, then the first candidate would be the ASIC, but it could be the CPU, AY or RAM too. But, if the PSU is dropping to 4.6V, then the computer is pulling a lot of current, so the damaged part is most likely getting a lot hotter than it should. So it should be quick to find the culprit.

Bryce.

[Overkill]

The voltage drop is strange because my bench supply says it's only pulling the normal 0.4A

Sorry, I'm confussing matters, because the strange voltage drop is another board.

Back to this board, I've tested the 12 colour outputs, I tested them at the inputs to IC15 for ease.  They don't seem right, some have a good square wave, some have a much smaller square wave and some are held high and some have nothing, they don't look the same as a board that puts out a picture.

I have found that the INT on the CPU is held high at switch on, and just stays high.