White - Yellow - Row 4 - Short

[thejefu]

I have awoken from a long migraine induced sleep, but have made headway solving what is 'hopefully' my last problem with this project!

Got the display fixed to unveil the error 'White Yellow Row 4 Short'

I have been following the System 11B Repair Manual and have completed the following steps:

1. Disconnected 1J8 and 1J10 during a switch edge test, error persists (isolated to MPU).
2. Jumper tested all columns, fine.
3. Jumper tested all rows, pin 4 of 1J10 fails to close.
4. Skipped logic probe step (don't have one, can buy one if needed).
5. Checked 560ohm resistors at SR10, all return 560ish.

I then got stuck at this step:

Then check SR10/SR11 between the 1J10 connector pin and the *second* SR10/SR11 pin listed below - 1k ohms should be seen. Then check continuity from the second SR10/SR11 pin to the U30/U39 chips.

● 1J10 pin 1, to SR11 pin 2, to SR10 pin 1/2, to U39 pin 12 (switch row 8). U39 NAND gates pins 12,13 (high) and outputs pin 11 (low) to PIA.

● 1J10 pin 2, to SR11 pin 3, to SR10 pin 3/4, to U39 pin 8 (switch row 8). U39 NAND gates pins 8,9 (high) and outputs pin 10 (low) to PIA.

● 1J10 pin 3, to SR11 pin 5, to SR10 pin 5/6, to U39 pin 2 (switch row 6). U39 NAND gates pins 2,1 (high) and outputs pin 3 (low).

● 1J10 pin 4: KEY

● 1J10 pin 5, to SR11 pin 6, to SR10 pin 7/8, to U39 pin 5 (switch row 5). U39 NAND gates pins 6,5 (high) and outputs pin 4 (low) to PIA.

● 1J10 pin 6, to SR11 pin 7, to SR9 pin 2/1, to U30 pin 13 (switch row 4). U40 NAND gates pins 12,13 (high) and outputs pin 11 (low) to PIA.

● 1J10 pin 7, to SR11 pin 8, to SR9 pin 4/3, to U30 pin 9 (switch row 3). U40 NAND gates pins 8,9 (high) and outputs pin 10 (low) to PIA.

● 1J10 pin 8, to SR11 pin 9, to SR9 pin 6/5, to U30 pin 2 (switch row 2). U40 NAND gates pins 2,1 (high) and outputs pin 3 (low) to PIA.

● 1J10 pin 9, to SR11 pin 10, to SR9 pin 8/7, to U30 pin 6 (switch row 1). U40 NAND gates pins 6,5 (high) and outputs pin 4 (low) to PIA.

My inexperienced brain can't make sense of these steps or I'm getting dyslexic about how '/' works. Anyone explain the following steps to me like I'm an idiot? I can follow straight forward steps but there comes a point where compound sentences and 1/2 leave so much ambiguity I start lighting matches. I can already tell I'm not going to manage the next step either. Can someone shed some light on the meaning of 'high' and 'low'?

 

[thejefu]

Had a response in PM. Will check continuity and have a logic probe on the way. Will update.


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[Pick Holder]

Do system 11 use a ULN2803 chip ?

 

[thejefu]

-delete-

 

[thejefu]

Traced the issue back to no continuity between pins 1 and 2 of SR9. Replaced and working perfectly!


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