Macintosh Classic II Repair
A New Challenge
As one can figure out from my old nickname, I always liked old all-in-one macs, like the Macintosh 128k and Plus. Some time ago I found on eBay a Classic II near Milan for a reasonable price, of course broken and the seller didn’t offer the shipment. Without a clear purpose, I purchased it, even knowing that only the travel will take a lot of hours.
More than 120€ spent between the purchase and the travel, still it was worth it, as the machine was in perfect estetical conditions. It has been stored in a basement, so no UV light made it in need of retrobrighting. Obviously I couldn’t even hope to plug it in and start using it, as the previous owner described a smell coming out when powering it. I was curious like a cat, and I proceeded immediately to open it and study how to avoid being zapped by the CRT.
The Disassembly
The CRT contains high voltages, even after being powered off, as it acts like a capacitor. If the reader wants to handle a CRT, I strongly suggest to study how to properly handle these objects. In order to put my hands safely inside the computer, I needed to make sure that both the CRT and the input filter capacitors were discharged (the power supply discharges in like a minute by itself). There are plenty of videos of how people make arcs with old flybacks to properly discharge a CRT in safety.
Little dust, no dead mice and a broken fuse. The last time I changed a fuse without checking the rest of the hardware, I was presented with a big smoking flash and no power after, not going to repeat my mistakes, thanks.
I had a spare broken Macintosh Classic analog board handy, and that came useful one time to replace a flyback in a Mac Plus I repaired (as an alternative activity for a new year’s eve). Extracting it requires to unplug the board on the top of the CRT electron gun, which is freaking fragile, so be extra careful there. Remove the glue first.
After removing the analog board from the Classic II, I immediately noticed a short circuit on the high tension alternator circuit, right after the input filter. The power MOSFET that created the AC input in the main transformer was completely burnt (albeit in perfect estetical conditions), the gate pin was in conduction with the source. That made me thing that also the controller might be busted by the discharge, it’s a small chip nearby. To make things even more interesting worse, there was a black goo under the card. Bad, that was surely capacitor stuff that leaked out.
Ordering the Parts
I got the right parts from Mouser, which isn’t exactly cheap, but it had all I needed:
- Desoldering wick
- Flux
- Electrolytic capacitors (analog board)
- Electrolytic capacitors (SMD for the digital board)
- IRFBC40 compatible MOSFET
- Fuses
The controller seems to be a rare chip, I desoldered it from the other spare board I had, and the clock battery is taken from another computer.
While I was waiting for the parts, I started desoldering the bad components (noticing that they effectively did a mess underneath), and after some labour everything was properly removed.
The digital board was washed with distilled water, rinsed, cleaned with isopropyl alcohol and dried throughly with compressed air and an oven.
Fixing Stuff
Some chips had to be removed (including the floppy controller), and doing that definitely wasn’t a joke, but it was necessary, given the goo I found under them. Even the battery slot has been desoldered, cleaned and put back.
Once I had the components, I resoldered every capacitor and checked every solder point on the power supply in order to correct cracks (which are fairly common especially near the yoke connector).
Checked with the tester, remounted, anode cap, motherboard, hard drive, floppy disk, enclosure. I then proceeded to turn on the switch, and took some steps back before plugging it in a socket.
BOOM! Black smoke, fire, sparks, demons came out of the floppy drive, the cat died, jesus came back to visit earth, they elect Trump as president (EDIT: oh they did it in the end, nice).
Oh cool it works. It turns on and makes the chime. It even boots after like twenty years of solitude. Nice! Let’s do something to pass files inside it:
- macOS 10.11
- -> Fileserver
- -> PowerMac through ethernet and NFS
- -> AppleTalk over serial
- -> Mac Classic
After formatting like the tenth floppy, exactly where the internal spring mechanism is released with a -CLACK-… the screen starts becoming bright. Very bright. Too much, there are scanlines, afraid it might start burning out something, I immediately unplugged it.
Successive tests showed that the problem didn’t cause additional damage, and it could actually run fine like this (which is of course unnacceptable and might damage the screen); it must had been a contact problem, as it was triggered in the exact moment I inserted the floppy. Some notable attempts to fix it involved:
- Washing again the motherboard
- Checking all the solder points on the analog board
- Check the transistor on the neck of the CRT
- Verify all the tensions
- Check the brightness wire
- Regulate the dimmers behind the computer (the big ones)
…and nope, nothing at all. I posted the problem on a specialised forum (68k MLA I think), and they immediately tell me it’s a dimmer problem. Weird, I checked that.
Desoldering and bench-testing the component proved that it was perfectly working, the resistence was OK. There is just.. a little spike every while. I remount it, same thing as before.
Larry Pina’s “Dead Mac Scrolls” and other vintage manuals link the issue to solder cracks. Yes, I already checked that.
At this point (and being desperate) I keep the computer on and start turning the dimmer right and left repeatedly, and the screen comes back working! Yes, the difference was that I did it with the computer on.
Hours of stress proved the machine was sturdy and perfectly working. It failed to boot before (some VIAs oxydised maybe), but not anymore.
And it works very well also, I used it for a few days for anything ranging from FileMaker to games and early graphic softwre (it could run Illustrator!). As happy as I was, the next purchase was an ImageWriter II dot matrix printer.
The computer is the last model that inherited the original Macintosh design, it’s a 68k Motorola processor running at a whopping speed of 16Mhz with 4MB of RAM and 80MB of SCSI hard disk (which eventually broke, and after repairing it three times and having it getting stuck over and over, it was replaced with a SCSI2HD card). It was like the best mac of the times, apart from the SE/30 maybe, after this one they started producing the color classic, which had a sucking design.
This classic follows perfectly the original mac design and philosophy: small, easy to use, compact, expensive, minimalistic.