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The Restoration of the PDP-8/I Minicomputer (PAGE 2)


First things first


I was assured by the seller that the machine had not been powered on in over twenty years. This immediately means one thing -- reforming the power supply capacitors.

Over the next few days, I moved the 8/I downstairs into the basement, disassembling it in the process. I took it apart into three pieces: the power supply, the main unit, and the rack.


Capacitor Reformation -- 2003 06 08


Next, in order to reform the capacitors, I had to borrow a friend's variable transformer in order to ramp up the AC voltage slowly. There are many websites describing the chemical process that occurs in a capacitor when it's inactive for a long period (years, if not decades), so we won't go into that here.


PDP-8/I Power Supply Removed from Cabinet S M L XL The PDP-8/I's power supply dismounted from the cabinet that it's in. The three large capacitors on the bottom left hand side are, left-to-right, C4, C3 and C2. C2 is rated at 57 milliFarads (although mF just confuses people, they like to think of it in terms of 57 thousand microfarads for some reason), and C3 and C4 are rated at 160 mF. The capacitor mid-way up on the rightmost side is C7, the "resonating cap" for the transformer. It's rated at 8 uF at 660 VAC. HIGH VOLTAGE DANGER! :-)

The first thing you need to do is disconnect the 5 VDC regulator from the capacitor terminals. This was done so that there wasn't any other load on the +8 VDC power supply (the +15 VDC and the -30 VDC power supplies didn't have any loads on them) -- all three power supplies had a high-wattage / low resistence resistor connected across them (probably to discharge the powersupply when it's powered off). I left these power resistors connected.

Once everything was unplugged (and the console "power switch" connector was shorted out to enable power), I started off at 5 VAC input and went up from there. Unfortunately, I hit a snag at around 35 VAC -- just as the fan was starting to spin up. The variac that I was using started to get hot. Here's a table indicating the voltages that I measured as I ramped up the power. I'd keep each 5 VAC increment active for one hour -- so for example, at 9 am I'd start at 5 VAC, and then at 10 am I'd ramp up to 10 VAC, and so on.
Input (VAC) C2 (VDC) C3 (VDC) C4 (VDC) C7 (VAC)
5 3 1 0.35 N/M
10 6.5 2.7 1.3 90
15 N/M N/M N/M N/M
20 14.5 6.5 3.5 188
25 17.3 7.9 4.3 224
30 21 9.5 5 274
35 24.5 11 6 312
40 27.5 12.5 7 350


N/M = Not Measured

Initially, this had me confused -- 40 VAC was about 1/3 of the 120 VAC target, and yet the capacitors C2, C3, and C4 were almost at their full values! At this point I made the executive decision that since the capacitors were almost at their full values, that the capacitor reformation had successfully occured, and I could go ahead and ramp it up to the full value. Ignoring the overheating variac, I quickly turned the voltage up. Imagine my amazement when the C2 value started to decrease! I knew that the AC voltage was increasing because the fan in the power supply was turning faster and faster, but the C2 voltage decreased and then slowly started to come back up. When I hit 120 VAC on the input, everything was where it should be, so I shut things down.


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