Comment by budman1
9 hours ago
I am a bit surprised, but I guess everything eventually wears out.
In the 1980's I worked as a field engineer that supported a lot of pdp-11's. They were very reliable for the time; tape drives and disks were the #1 maintenance items. To actually have to open up the processor and change a board was not a regular activity.
Other machines of that era, like those from Gould or Perkin/Elmer or DG gave regular practice in the art of repairing processors.
Guess I expect them to work forever. Like a Toyota.
I encouter two main failure modes. First, the bipolar PROMs degrade at the atomic level, the metal ions in the fuses tend to migrate or 'regrow' over decades, causing bit rot. Second, the backplanes suffer from mechanical fatigue. After forty years of thermal expansion and structural flexing, especially when inserting boards, the traces and solder joints develop stress cracks. Both are a pain to repair.
https://retrocmp.com/articles/trying-to-fix-a-dec-pdp-1134-b...
Excellent work.
The feeling of accomplishment when the machine boots after a major repair (almost) makes it all worth while.
(i think i would have found a used backplane...fixing it was crazy clever)