For some light relief from rebuilding the more complicated UNIBUS machines, I decided to take a break and tackle something a bit simpler, a QBUS based uPDP-11. I have a few uPDP (BA-23) chassis lying around so I picked one of these as the basis for the machine.

As the machine had only left service a mere 5 years ago, I thought I would take a chance and test the power supply the evil way. With all the boards removed, I powered on. All started up well and I confirmed the correct voltages with a multimeter. So far so good

Before I started installing cards I took a break to download the required manuals from Bitsavers (http://www.bitsavers.org) and was deeply engrossed when my nose picked up the first warning signs that all was not as it should be. Sure enough, I looked up to find smoke streaming out of the back of the PSU.

I quickly removed the power and confirmed there wasn’t an actual fire inside the machine. After airing the machine for 10 minutes to rid it of the worsed of the smell, I removed the power supply and opened it up. I was met with an impressive scene of fluff, grease and a variety of insect remains but surprisingly no signs of damage or exploded capacitors.

After a thorough clean, I returned the power supply to the chassis and tentatively put the power cable back into the chassis. With fire extinguisher on hand I waited to see if the smoke returned. After 10 minutes and no smoke, I decided to risk a power on and eureka, all came up well and voltages were still as they should be.

So I was really rather lucky here, but I guess the moral of the story is never power on an old machine without first thoroughly inspecting it.