winter's computers
found, loved, and nurtured for spring
my ugly geek secret

Monday, November 26, 2007

raising the dead

I've had a broken Mac Plus gathering dust in my collection for years. It's an all-in-one design with a high-voltage analog board (technical term: monitor guts) that controls the display and a one-piece motherboard (the computer itself). These Macs were fanless: wonderfully silent and cooled by convection, the case acting like a chimney. At least that was the plan. Excess heat and a couple of poor component choices on the analog board resulted in notorious problems like the analog board warping causing solder joints to crack.

This was the typical result.

I took my Plus apart recently to try to resolder some of the connections in the hope that they were just cracked. That didn't fix the problem, though, so I found a list of common problem components in the pickle's Low-End Mac FAQ and ordered some replacements from Digi-Key.

I enlisted Donnie to help me out and to give him a bit of experience with soldering and electronics in general. The tube circuit can store something like 30 thousand volts even after the power is turned off, so the first step was to discharge it. Donnie wasn't too happy with the idea of doing this, but I had him stand on a step stool and with one hand insert a screwdriver into the grounding lug while I wedged another screwdriver under the anode cap and touched the two together. Had it been fully charged we would have heard a loud snap and seen a big spark between the screwdrivers, but — much to Donnie's relief — the charge had dissipated since I had last powered it up and we didn't notice anything. I was a bit disappointed.

A paper shield covers the solder joints on the mask side of the analog board.

I found that PC expansion slot covers work really well for popping the four buttons that held the shield in place.

Looking more closely, there was a clear bulge in capacitor C1 (top right), the likely culprit. Donnie heated up the capacitor's solder joints and I pulled it out.

The replacement part didn't look anything like the original which made me wonder if this was the correct part. It's just the package type, though, and the capacitance and voltage rating looked correct so we went ahead.

I bent the leads and popped the replacement capacitor in. With a little instruction, Donnie did an awesome job of resoldering the connections. That soldering iron is older than he is; I got it from my folks when I was not much older than Donnie is now.

The replacement cap looked a bit odd but the fit was okay. After trimming the leads, we put the Plus back together and powered it up. The screen was disappointingly black for a moment, but then...

It worked! Wow. I was rather surprised how well that went. We didn't even have to adjust the video circuits.

It won't boot, though. The floppy drive is dirty and possibly broken. I may have to replace it. I also have to dig up a keyboard and mouse (I think I've got both but the mouse has no ball... insert your own joke here) and connect the external SCSI harddrive and see if the SCSI CD-ROM I've got works. I may have to do a bit of shopping on eBay to get this Plus fully functional... or I may try something a bit more radical...

No comments: