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The Restoration of the PDP-12 Minicomputer (PAGE 6)


M221 Fixed -- 2007 09 19


Turns out, the M221 board had a short between BS2 and BT2; there are four traces reasonably close together, and it looks like something scraped across them, connecting two of them. So now, I can execute my favourite little test program (ISZ/JMP/IAC/JMP)!

More than that, it passed my memory diagnostics for the first bank of core!

Second bank passed as well!

I reconnected the internal cables that I had disconnected, and now the AUTO key works as well. I've heard the speaker click, and some of the relays as well, so I think this machine is in very good condition so far.


Front Panel -- 2007 09 21


Front Panel of a PDP-12 S M L XL

This is what the front panel of the PDP-12 looks like.

In order to find the lights that didn't work, I took the entire front panel apart. I then removed all the LEDs that I hadn't seen light up, and tested them on a power supply -- they all worked. Then I tested all the transistors. Then I plugged all the LEDs back in. Then I tested the cabling harness from the front panel board to the N-row connectors.

After all this, and not actually doing any repair, more of the lights work :-) (I did have to replace one driver transistor, but that was my own stupidity, because I put in an LED with no series resistor and it seems to have blown the driver (and of course the LED). "DEC-2" can be replaced with a 2N2222.)

So, after 39 years, ZERO LEDs needed to be replaced. Most impressive.


Side Panel Restoration -- 2008 11 01


Side Panel of a PDP-12 S M L XL

Over the last six months, I completed a move from a small house where the PDP-12 was kept in the garage (!) to a huge McMansion with a walkout basement where the 12 lives happily indoors.

Next on my todo list is to fix (restore) this particular side panel. It's interesting that all the other panels are fine, except this one -- the paint has severely cracked and is coming off in places.


Side Panel of a PDP-12 S M L XL

So, I labelled each and every pot and switch, and carefully removed them from the cracking panel and replaced them onto a cardboard one (so that they retain their positions -- the power switch, of course, will not work, so I'll need to bypass it if I'm going to be doing any active operations).

Sanding the Side Panel of a PDP-12 S M L XL

As with any restoration work, elbow grease needs to be applied. I started off with 80 grit sandpaper to get the rough pieces of cracking paint off, and sanded in small circular orbits by hand for about 15 minutes. Then I moved on to 150 grit for about 5 minutes, and then finally 400 grit for another 5 minutes.

Completion of Sanding the Side Panel of a PDP-12 S M L XL

Smooth as a baby's bottom!

Botched Spraypainting of Side Panel of a PDP-12 S M L XL

I botched the spraypainting the first time round. I had it outside on newspapers, and did a wonderful job, and then the wind came up and blew the newspapers onto the panel. Damn! So then I botched it a second time. I sanded it off, applied a nice, wonderful coat, and then put it on the floor to dry. I had learned my lesson, and did this inside. Figuring I'd save on drying time, I turned on the fan and blew dust and wood chips onto the paint. Third time lucky? I waited for it to dry, sanded it down to the 150 grit level, and then spraypainted it again. This time I just left it on the floor with no fan. (The picture is after iteration 1, with the "scrapes" left by the newspaper visible on the bottom middle.)

After the third time, it looked pretty good -- there was just this one little area where the paint had bubbled up a tiny bit. I was prepared to live with it. But then, I had placed it on the mantle for my brother in law to pick up when he was here so that he could do the silk screening. Guess what? (This is starting to sound like a Monty Python skit) I dripped wax on it. @#@^#$!!! So, I sanded it down again, and spray painted it a FOURTH time.
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