S.Elliott's blog

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A collection of simple (simplistic) tests for diagnosing a non-booting Disk II or A2 disk controller:

• Monitor commands to test 6 of the 8 data latches in the 9334 chip
• Monitor commands to load just one sector, then beep and break
• Attach an LED to troubleshoot individual latches Q0 through Q3
• Use a flashlight to "candle" circuit traces
• Use wood or bamboo to scrape debris off the circuit board

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Some experiments and variously messy results from trying to map how often the video scanner reads each byte of screen memory while creating each video frame.

Green indicates bytes that were detected only once per frame.

Purple indicates bytes that were detected twice per frame.

Orange indicates bytes that were detected thrice per frame.

Blue indicates bytes that were detected four times or more per frame.

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Apple chose bare headers for the disk connectors on the Disk II controller, which make it notoriously easy to connect the drive cable incorrectly.

Jameco's "Shrouded Box Header 20 Pin 2316R-20G" makes an ideal replacement.