VAX 11/725 and 11/730
There's a story I want to share with you.
I was sent out to fix a VAX11/725, that would quite frequently halt (as in, couple of times per week) for no apparent reason. No crashes, and they would find the poor thing in the morning, powered off I went, checked, found no probable faulty component, replaced a sensor board, but the problem persisted. I replaced a power harness. No dice. We hooked up a logic probe and it, after yet another of those halts, pointed to the airflow sensor. Which was clean, but still, it _had_ triggered the power controller. The intake filters were OK, not squeaky-clean, but I have seen much worse, and those systems didn't trip.
We got a swap unit in (an 11/730) and I took the 725 away to the PRC. Took it apart, cleaned it, replaced every bit of harness that even just looked scuffed, replaced the temp and airflow sensors, and I think I replaced the sensor board yet again, for good measure. Then ran it, without a single hiccup, for two weeks.
So back to the customer it went, the very cleanest 11/725 in all of Europe, and lo, it was halted again the next bloody day. Something that the 11/730 had not suffered from, even though being in the same place, connected to the same power and everything.
Now the time of the halts as seen from the last opcom message would seem to implicate the cleaners, but the system was in a recessed space with its power cord plugged in somewhere behind it, and there was a free wallsocket in plain view for the cleaners to use. They would have to reach over and behind the system to unplug its cord, and plug it back in afterwards; one could think that the 730, being higher, made that more difficult. If that was the modus operandi in the first place, which was rather unlikely. Still, the correlation was unmistakeable.
Dirty power and other such causes had been ruled out in the meantime, so finally a colleague went and sat there, thermosflask of coffee with him, waiting for the cleaners to enter and reveal their nefarious ways. and sure enough, the cleaners came, one plugged the vacuum into the visible wallsocket and started vacuuming, the other emptied the wastebaskets, then took the waste bag from the paper shredder, tied it up and put it aside.
Right in front of the poor 11/725's air intake. Fwump. Clack.