PDP 8/L repair log - hopefully the LAST installment!

The 8/L is finally fixed! The final interesting failures were: The Group 2 microinstructions (CLA, IAC, etc) would not work. Changed the relevant 7400 - still only weak wiggles. Tracked it down to a dead short on the AC CLEAR line. Turns out that one of the ground bus strips that run the length of the backplane had buckled just enough to touch the exposed wire at the bottom of the pin it was supposed to make a circle around! That was really fun to locate. Lastly, the TTY receiver card M706 would only work when on an extender card. When in the backplane it would receive only one character and "hang" until power cycled. I spent a lot of time in a fruitless search for a bad backplane contact. Finally I found on the stop bit flipflops that their PRESET pins were floating (a frequent bad habit of DEC in their early design was to leave uncommitted TTL inputs to float, which works fine as long as it's an electrically "quiet" environment. Which a card handling 20mA TTY signals is NOT). A quick 1K pullup to +5 and it hasn't dropped a bit since. I've just finished constructing my "DF32x4" disk simulator (PC boards in a 1U [1.75" high] rack chassis). I'm starting to debug now - so far I had a bad IDC connector and (of course) no spare. I will make Eagle .SCH and .BRD files available to anyone who wants them IF it ever works ;) -Charles [demime 1.01a removed an attachment of type image/jpeg which had a name of mvc-457e.jpg]