OK - device fitted. Main difference is that the screen is full of characters, and does not clear.
UI10/11 pin 1 - go high and stay there. Checked at 4.30V with a meter.
RAM chips - pin 4 has no reading (ie it stays at the same level with and without the scope connected. I checked the voltage at 0.40V on all RAM chips.
Pin 15 goes high and stays there - again checked with a meter at 4.30V constant.
Scope readings to follow.
Colin.
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Originally Posted by SiriusHardware
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...and in a straightforward manner that would be ideal. Unfortunately I don't have sufficient knowledge to comment on the viability of this as an option
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Me neither,  at least not without using a diagnostic ROM, which remains an option.
The only other sure way to do it is to remove them all intact and use something else, like an Arduino, to independently test them.
Or remove them all intact, fit sockets and if possible fit combinations of them to try to find a combination of 8 out of 16 of them which will work as 8K, if that is a valid combination in these machines. Then cycle through the remaining ones to find out which ones stop the machine from working, much as you did with yours.
My suggestions before that:-
Fit the NOP tester in place of the CPU and check again to see if there is activity on UI10 / UI11 pin 1. If so, then it just shows that the CPU is currently getting into a loop or a mode where it is just not attempting to read from system RAM, and we can disregard the lack of activity there when the CPU is fitted.
Similarly, the RAS and CAS lines of the system RAM, pins 4 and 15 on each 4108 chip. Is there activity there during a NOP test?
Lastly, still while running the NOP test, scope the BD0 to BD7 lines again to see what they look like.
UI10 pins 11-12
UI10 pins 13-14
UI10 pins 15-16
UI10 pins 17-18
UI11 pins 11-12
UI11 pins 13-14
UI11 pins 15-16
UI11 pins 17-18 |