IBM 5150 Won't Power Up

After a very busy few months, I had some time this weekend for tinkering and decided to power up my IBM 5150.  Getting this computer running was a real scavenger hunt and it took months to track down a Model F keyboard (necessary because this was the only keyboard with requisite 5 pin DIN connector) and a video graphics adapter with a composite video output. But I finally got it working and was able to boot the dinosaur up on PC-DOS 3.3. So you can imagine my frustration when I flipped the monolithic red power switch to hear soft "click" rather than the airplane turbine whir that usually indicates a successful startup. I probably would have just tied the fifty-some pound computer to a rope and used it as a boat anchor if that click sound hadn't been so familiar. After spending most of my Sophomore year in an electronics lab, I will always know the sound of a resettable fuse detecting a short circuit.

The power supply directs its fare to the rest of the 5150's internals via four bunches of wires - two for the 5.25" disk drives (numbered p10 and p11), and two that connect to the system board (p8 and p9). I unplugged each of these in turn while testing the power and found that the fuse was triggered only when p8 was connected to the system board. The good news was that I knew roughly where the problem was, the bad news was that the problem was in the system board. Much more difficult to replace, if necessary, than a disk drive.

The power supply connections on the system board. +12V is third from the right.

Either way, I found a link to the 5150's technical manual and began flipping through it. The System Board section has ten pages of logic diagrams that detail the schematic layout of the board. P8 (labeled p1 on the board) provides 12 volts to the board while p9 (p2 on the board) supplies 5 volts. Sure enough, a quick check with a multimeter showed a short between the +12V pin and ground. To make sure it wasn't one of the cards in the extension slots that was causing the short, I removed the disk drive controller, memory expansion and video graphics adapter cards one at a time while powering up but none of these made a difference so the short was happening somewhere on the system board.

At something of a loss, I searched the Internet for talk of short circuits in old IBM's and didn't find much but  there was mention of tantalum capacitors and their tendency to fail after not being used for long periods of time. This seemed like as good a place to start as any so a quick look at the technical reference showed that the each of the power pins is decoupled by a 10uf capacitor. Decoupling capacitors connect the high and low lines to ground in order to reduce noise that might effect a sensitive integrated circuit. For an excellent explanation, watch this video.

After removing the system board, I tracked the shorted +12V pin until I found the decoupling capacitor. IBM used three-lead capacitors on their system boards. The middle lead is the anode and the outer leads form the cathode. The capacitors are configured this way so that they cannot be inserted backwards, leading to a thermal runaway (also known as an explosion).

You can see the power supply pins at the top of the board. The third pin from the right is the +12V pin. Follow it down and you can see where it connects to the center lead of the decoupling capacitor.

The faulty decoupling capacitor.

It took a little coaxing to get the capacitor off the board but after removing it a quick test with the multimeter revealed that the short had disappeared. Just to make sure I tested the capacitor as well and, sure enough, it was shorted. I made a quick trip to RadioShack and (for $1.79!) I got a 10uf tantalum capacitor to replace the faulty one. Making sure to place the anode (marked with a plus sign and a longer lead) in what was the center hole for the three-lead cap, I soldered the new one into place.

After removing the original capacitor...

... and adding the new one.

After quickly reassembling the rest of the components I flipped the big red switch and was greeted with the deafening sound of the power supply trying to suck the air out of my lungs! The 5150 runs like new and I can pleasantly wait for the next component to fail!