Macintosh Classic « modelrail.otenko

27Aug/230

Macintosh Classic

This was an unexpected find at the local tip shop back home. The price wasn't low, but I couldn't resist. It was powered up on the bench next to the register and was just begging to be bought. Even though it had a RAM error displayed on the screen, I went ahead and threw money at the cashier.

Although it was 'working' at the tip shop, upon powering it up I heard no chime and just got a checkerboard pattern. A quick google had me fearing the worst... had the battery exploded inside? Somehow they had it running on the bench? I also noticed that it came with a serial cable and not the ADB cable, so I couldn't hook up the keyboard.

Hilariously, as I was taking a photo of the checkerboard, the machine flicked past that screen and started loading! It just seemed to be stuck there whilst trying to get the system online. With no ADB cable, I plugged in the mouse and tried to open Hypercard... it froze.

What's on the inside?

I cobbled together a torx T15 screwdriver to get the four screws out of the back. Note that the two screws, either side of the top handle, are a nuisance and you'll need a thin screwdriver with around 12cm of length. Once they're out though, the rear case just slides off. No leaking battery! No chime either when powering on, so the speaker and/or caps on the analog board are toast. The caps on the logic don't look too bad, but they can get replaced anyway.

There's great information all over the web on how to re-cap these. You either need to recap the logic board (more info here) or the analog board. Those later links have a list of all the parts you'll need. I had to do a run to jaycar to stock up on some 10v variants of electrolytics that weren't in the box'o'junk.

The above was the result of re-capping the analogue board, but the machine still displayed erratic behaviour. Sometimes a boot with the RAM extension saying RAM was toast, other times just Illegal Operations half-way through using the system. This machine has maxxed out RAM: 1mb onboard, 1mb soldered on the expansion board and 2x1mb SIMMs. Unfortunately, the 'About' dialog never showed the correct amount of RAM!

I then endeavoured to replace the logic board capacitors. Jaycar didn't have surface-mount tantalums in 47uf, so I went with standard through-hole capacitors and just made them look as-presentable-as-possible.

The machine felt better, but it was still not repaired.

Sound Is Loudest At Volume Setting 2?

I also replaced the capacitors around the audio circuitry and this revived the bong startup sound and others... but... when adjusting the volume, it peaked at '2' and then went quieter as you raised to 7? The actual issue was capacitor residue on the amp IC chip. A good clean with alcohol wipes got the sound back to 100%.

Crash And Burn... Then Sad Mac

Ok, the fun was short-lived, I started receiving erratic startup errors. Usually 00000003 0000FFFF. Fortunately, the internet always has a solution.

Seems the LS174 next to the power plug cops a beating from the omega-3 fish oils from the leaking 47uf 16v self-destructing caps.

The above shot is post-cleaning. Prior to wiping it down (as I did to the audio chip), you couldn't even see the legs. This did get the machine back to stable booting, but the extra 2x1mb SIMMs in the expansion board were still not being recognised! I then realised that there was a broken trace on the bottom-left third leg in.

I couldn't find my transformer winding wire, so I used a resistor leg. All the other pins beeped out, so I booted it up and...

No way! What's next? To hook the keyboard up, I just used an S-Video cable.

It turns out that, although you can use S-Video cables for ADB, you can't safely do it the other way around. ADB cables aren't overly-shielded to prevent interference to video signals.

So pretty in monochrome. The music sounded great also!

Destroying CRTs

Whilst doing all this, I had accidently applied incorrect lateral pressure to the rear of the CRT's input circuit board. This happened to crack the glass stem of the tube, right at the end!

The tip is missing there... it cracked off and the vacuum failed! Fortunately, I was able to find a donor CRT on Facebook Marketplace. Thanks Danny!

The yoke wasn't included, so there was a fair amount of stuffing around to get the picture looking plumb!

What's next?

I have a BlueSCSI in kit form, so I'll build it an get this thing on my local network. I flogged off all of my previous ethernet to localtalk hardware. If you have the space... then hoard stuff... you'll need it all a year later.