OK - the voltages all check out OK when powered on. As do the resistances on the RAM chips.
However, the resistance measurements on UI10/11 do not.
For example (meter set to 20K at all times)
UI10 6/7 (0V) - 1.54
UI11 6/7 (0V) - 2.03
UI10 2/3 (0V) - 2.07
UI11 2/3 (0V) - 1.56
UI10 4/5 (5V) - 1.61
UI11 4/5 (5V) - 2.29
UI10 2/3 (5V) - 2.16
UI11 2/3 (5V) - 1.67
Am I making the right comparisons here?
I have not scoped anything yet - would that still be useful?
Colin.
Quote:
Originally Posted by SiriusHardware
No problem aj, at least yours has synchromesh.
I've just remembered that the system RAM is separated from the CPU by not one but two lots of the dreaded 74LS244 buffers, one set being our old friends UE10/U11 which we never changed, and UI10 / UI11 which separate the RAM data pins from the buffered databus provided by UE10 / UE11.
A while back in #832-#833 you (Colin) did some while-you-wait resistance measurements around the system RAM area and you did find one possible anomaly where the circuit node of UI11 pins 2/3 - J2 pins 2/14 - I2 pins 2/14 had a noticeably different resistance to +5V and 0V than the other seven similar nodes. This could indicate a problem with UI11, UJ2 or UI2.
Could you maybe do those measurements again, power off of course, but this time use a different resistance range for all the tests to see if that amplifies or exaggerates the difference between that node and the others?
Then after that, make comparative scope observations of these pairs of connected pins (as they are connected toegether, you need only scope the first one in each pair)
UI10 pins 8-9
UI10 pins 6-7
UI10 pins 4-5
UI10 pins 2-3
UI11 pins 8-9
UI11 pins 6-7
UI11 pins 4-5
UI11 pins 2-3
The signals on those pins may look quite chaotic, but what you are really looking for is a significant difference in the height or shape of the signal on one or two compared to the others.
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