Check if the -5v charge pump is stable and supplying the correct voltage.
I recall a problem with a PDP 11/34 and a non-DEC memory board in which the on board pump went slowly bad. Depending on the data content and tolerance margins for the memory chips, we saw very erratic problems. Jerry > On Sep 7, 2015, at 5:09 PM, Noel Chiappa <j...@mercury.lcs.mit.edu> wrote: > > So I've mentioned how I've seen this wierd behaviour where QBUS memory boards > that hadn't been used in a long time didn't work when first plugged in, but > started working later. > > I just had something even weirder happen, and am curious if anyone has an > plausible explanations. > > So I had a dead M8044 (MSV11-D), symptom was that you could write -1 to any > location, it read back as 0. Quite repeatable, I can power cycle the machine, > take the card in and out, etc, etc. > > So I throw it on an extender, and start chasing. I have a two instruction > loop (write location 0, loop), and I'm watching the data going into the > memory chips on the card, and it all looks good. So I add a third instruction > (read location 0, after the write), and continue chasing. > > Data looks good coming out of the chips; then it goes to an octal latch. So I > look at the latch enable, and that doesn't look so hot - just a tiny little > ugly spike. So I look at the source of that, and it's a D flop. So I look at > the D flop's clock input, and it's also a nasty little spike. So that comes > from the output of a triple-AND, and so I start looking at the inputs of the > 3-AND. And when I put my 'scope lead on the second input... the memory > suddenly starts working! > > Well, I could see that - the added resistance or capacitance or whatever of > the probe might have had some effect on a circuit that was right on the edge. > But here's where the ghost enters the machine. > > I pull the 'scope probe ..... and the memory keeps working! > > I can power cycle the machine, leave it off for 15 minutes, power it back on > - and the memory still works fine! > > > Does anyone have _any_ idea WTF is going on here?!?! > > I feel like I'm in some sort of AI koan... > > Noel
