Unfortunately I had another issue with my 4P. It was working fine about a month previous when I last used it, but when I went to power it up to test some disks the screen was quite funky. It's was like a single line of random characters (although the random characters are the same on each power up). It also looked like the screen was scrolling super fast vertically, therefore it seems like the screen is full of the same line of characters repeating. That’s the best way I could describe it.

Pushing ‘BREAK’ produces a blank screen. After swapping boards around with my 2P I narrowed down the cause to a problem being in 540 video board.

Dave from osiweb.org forums pointed out that the column counters seemed to be fine, as did the row counters as far as the 8 rows of each character, and that I should look at the row counter chain farther down the line. He suggested U5H in particular which lies at the end of the the row divider chain.

I tested U5H and found that the outputs were not pulsing and were stuck high. I decided the evidence was fairly well weighted towards the 74LS163 being dead, so I went ahead and changed it out as I had some spare in my parts bin. First I dropped in a nice machined socket of course. The machine is now up and running 100%. Thanks Dave. I love the easy fixes.

Interestingly enough, when I first fixed this machine about a year ago there was another 74LS163 I had to replace on the video board to get it working. I guess these 74LS163's are prone to dying.

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