TI-99/4A QI repair - Startup process question

[Hans23]

Hi,

 

I have a QI motherboard on my table that I'm trying to fix - It does not start and the screen stays black.  I have verified that the ROMs are good, and I see that the machine executes code after reset.  It does so for a couple of seconds, then halts.  What I don't see, however, is selection of the scratch RAM.  I would assume that the scratch RAM is accessed very early in the startup process, so not seeing selection pulses seems odd.  My impulse is to declare the gate array to be faulty, but as I am seeing several other output signals from the gate array (ROM chips selects etc.), I wonder how likely it is that one of its outputs or logic functions fails.  Maybe this is a common failure mode with the QI board, which would of course suck, as getting a replacement for the GA would be difficult. 

Any hints would be appreciated.  Thank you!

-Hans

[GDMike]

This guy troubleshooting this 4a was neat.. don't know if there's anything there to help

https://youtu.be/tPDP4izVqrk

[+Ksarul]

The logic diagram (down to the gate level) for the Gate Array is on WHT, in the 99/8 folder. The chip name is HOMBRE. I scanned it years ago and put it there, not realizing at the time that it wasn't a 99/8 chip (I found that out a few years later as part of a conversation with the designer, Mike Bunyard).

[Hans23]

1 minute ago, Ksarul said:

The logic diagram (down to the gate level) for the Gate Array is on WHT, in the 99/8 folder.

I have been looking at that diagram, it is very helpful and helped me figure out that the scratch RAM is never selected - It is a great piece of documentation, thank you!  I'm contemplating whether it'd be worth trying to add a separate decoder that generates the chip select to figure out whether only that signal is not generated correctly by the HOMBRE chip.  I think what I'm basically asking is whether such partial failures in the gate array are a thing, or it'd be a waste of time to try that.  Another option would be to try coming up with a CPLD based replacement.  Of course, this all does not make a whole lot of sense, but it would be fun