Lo ve Your Mac
Luis shares his recipe for a capacitor sandwich…
The Lisa power supply comes in a nicely removable unit, which suggests they were intended to be replaced by ordinary users. Actually getting inside the metal PSU case is a bit more work (see this month’s How To guide), but nothing to stymie the determined tinkerer with a decent screwdriver. Inside I was delighted to find the circuit board was exactly as I had predicted: almost entirely pristine except for two large and very obviously blown capacitors. Actually, they weren’t so much blown as exploded!
The sides of each one had shattered into resin fragments and to figure out the part codes for the capacitors I had to carefully gather up all the shards and re-assemble them, like the Triforce of Wisdom. They turned out to be 0.22 and 0.1 microfarad capacitors and, amazingly, the exact same part number is still sold at uk.rs-online.com.
Unfortunately, you can’t buy them in fewer than packs of five, so it cost me £15 for the two capacitors that I actually need, leaving me with another eight that I’ll probably never use. If anyone else has a Lisa that needs new capacitors, send a stamped addressed envelope to the MacFormat team and I’ll post you my spares.
Electric baguette
Capacitors are devices that store electric charge. They are used to smooth out voltage spikes, and as a sort of filter that blocks direct current but lets AC through.
Physically, each one is a sandwich made from conducting bread, and some sort of non-conducting filling. Intuitively, a vacuum might seem like it would be the perfect insulator but electrons are actually quite good at jumping an empty gap, so capacitors mostly have a dielectric filling.
Dielectrics are materials with polar molecules that can wiggle a bit so that their charged ends all line up with any nearby electric field. Electrons building up on the bread of the capacitor sandwich create an electric field that polarises the dielectric filling. This stops the electrons from flowing, and stores electrical energy in the polarisation of the dielectric. But like any sandwich, the filling can eventually go off. Applying a voltage to these stale sandwiches makes them give off hydrogen gas, and since they are tightly wrapped up in a resin or metal container, this makes them explode. It’s something like that, anyway. Anyone with any actual knowledge in the field of electronics (or sandwiches) is invited to write in and enumerate all the flaws in my analogy.
Risk of explosion
Desoldering the blown capacitors was surprisingly easy. Most modern day gadgets are assembled using high-temperature solder that you can’t melt with a cheap soldering iron like mine. Instead, you have to melt in some low-temperature solder, to create a solder alloy with a lower melting point. But the capacitors on the Lisa just came straight off. Maybe Apple didn’t start using high temperature solder until the 90s?
I swapped in the new ones and braced myself. You see, it was entirely possible that the blown capacitors were a symptom, not a cause, and as soon as I turned on the power, whatever other fault had made the old caps blow, would do the same to these ones.
On the one hand, I didn’t want to get too close to this, in case of flying shrapnel. On the other, I needed to make sure I got a photo of the carnage, just in case this was the only thing I had to show for my efforts. So I used a wooden peg to disable the interlock switch,
I cowered behind my iPad, ready to record any sparks or smoke
put on my safety glasses and cowered behind my iPad ready to record any sparks or smoke. When I flicked the power switch, there was no hint of drama, which was a bit of an anticlimax, but basically a good sign.
I put it all back together and shoved the PSU back into the Lisa. This was when I remembered that I had still not reassembled the motherboard power connector from the last time I tried to get the Lisa working. So I had to dismantle the top and front panels and plunge elbow deep into the dark interior, like a sort of robo-vet.
I wiggled the power connector back through the hole in the chassis to the PSU compartment and resoldered the wire for the case interlock switch. (Remember when I cut that, because I was too lazy to dismantle the switch?) After all that nether probing I couldn’t get the back panel for the Lisa back on, because I had pushed the motherboard power connector too far into the PSU compartment and there wasn’t room to slide the PSU all the way in. But it was connected at least, so I left the back panel off for the time being.
Vital signs
And so to the actual test. Heart in mouth, I flicked the power switch. Nothing. My heart sank back down out of my mouth and into my stomach. But then I remembered the interlock switch on the PSU. There is a hole in the PSU case that exposes the switch and a little prong on the back panel of the Lisa that is supposed to poke this to enable the power. And I still had the rear case panel off!
Excitedly I wedged the interlock switch closed with half a clothes peg, and suddenly there were terrible grinding noises. Success! Rushing round to the front, I saw snow on the screen.
It still wasn’t a working Lisa, but it confirmed that I have a working power supply, and that’s a step forward. The grinding noise was coming from the floppy drive, which was hardly surprising considering all the rust and spiders I’d seen in there. I hadn’t bothered to hoover them out yet because I honestly didn’t think I would get to the point where it became an issue. The snow on the screen is probably caused by something else anyway. But it’s digital noise, not just analogue static. So the CRT seems to be working as well. Maybe there is a fault in the firmware or the video RAM. Or maybe the motherboard isn’t fully initialising yet, but this definitely feels like progress. It may take another few transplant operations, but I will get this Lisa to boot yet!