29 November 2019

Repairing a Macintosh Classic II

I picked up a Macintosh Classic II for 35 quid - a steal, and a good weekend project to get it fixed up and running.

As expected - nothing but vertical white bars displayed on the screen. Additionally, no startup chime, and the screen was dim - barely visible.

A bit of research showed that the dim screen was due to bad capacitors on the analog board, and the vertical bars due to bad capacitors on the logic board.

Here's what I did to fix it, including a summary of all of the capacitors that can be replaced in the Mac Classic II, and a few gotchas I encountered.

This is not a complete repair guide - most of the process is straight forward, but I wanted to document the exact required capacitors and the small things that get overlooked on other tutorials like removing the CRT anode

Remember - voltage kills, make sure you know what you're touching, and that everything is discharged completely.

Opening the case

Just unscrew it using a hex screwdriver bit and work the rear section of the case away from the front assembly containing the CRT and circuitry - easy.

Here's what is presented after getting the thing open - The logic board is horizontally mounted under the CRT, and the Analog board vertically next to the CRT.

Capacitor Reference - Analog Board

The capacitor types are clearly marked on each capacitor so I haven't bothered to put together a map of which goes where - you can just write it on the board with sharpie as you're removing the old ones :)

The polarity of the caps is already marked on the PCB.

Quantity Type
4 1uf 50V
1 10uf 25V
1 1000uf 10V
2 1000uf 16V
1 1000uf 25V
1 220uf 16V
2 220uf 50V
1 220uf 250V
2 2200uf 10V
1 2200uf 16V
1 4.7uf 250V
1 47uf 16V
2 47uf 25V
1 470uf 10V
3 470uf 25V
1 470uf 50V

Analog Board Capacitor Replacement

The first step is fixing the analog board - if this isn't done first, there will be no way to know whether the logic board repairs worked.

Removing the analog board is a bit of a task. After disconnecting the Logic Board, speaker, hard drive - all of the things with actual connectors - you'll be left with the analog board attached to the CRT.

Before working on the CRT make sure it's discharged! This Mac Classic II has a bleeder resistor and hadn't been powered up for a few days so it was safe to begin work.

To remove it from the CRT, unscrew the black wire running from the analog board from where it is attached at the corner of the CRT tube.

Next, I removed the circuit board from the back of the CRT. It was not obvious how to disconnect this, but you can gently pull on it to reveal it is slotted onto pins on the CRT tube. You can see a bit of glue on the top of mine - it'd long lost it's adhesiveness but it may need removing if it's still sticking.

Next up was removing the anode cap from the CRT. This was the bit I had most trouble with - it wasn't obvious at first how to get it off.

Turns out, there's little claws under the cap which keep it inside the hole in the CRT. Picture putting your fingers inside a roll of sticky tape and bending them - now you can't get them out. This is how the anode is attached. I used a pair of (insulated) pliers to grip the base of the cap and press the claws inwards and wiggling it about until the cap came loose.

The analog board can now be unscrewed and pulled free from the case.

In this case I'm only replacing a few of the capacitors - the ones next to the transformer. The transformer gets hot which reduces their lifespan.

These 9 capacitors were all de-soldered and the board cleaned with Isopropyl Alcohol (Tough stuff to find but does a great job leaving no residue. If you can't find it try a paint supply store).

Here's a full view of the board, cleaned, with the capacitors removed.

And here are the newly installed caps. It looks the same as it did with the old caps, which I guess is a good thing.

Capacitor Reference - Logic Board

The polarity is marked on the board. For the Logic Board I've also included the position of each capacitor as marked on the board.

These are the capacitors for the revision 1 board. The revision 2 board has an additional 4 10uf 16V capacitors.

The black side of the capacitors is the negative terminal.

Position Type
C4 47uf 16V
C5 10uf 16V
C8 10uf 16V
C9 1uf 50V
C11 10uf 16V
C12 10uf 16V
C14 10uf 16V
C15 1uf 50V
C3 47uf 16V
C6 10uf 16V
C7 10uf 16V
C10 10uf 16V
C13 47uf 16V

Logic Board Capacitor Replacement

The logic board just slides out of the case when the cables have been disconnected - no screws!

Remove the RAM and the battery to get them out of the way.

Desoldering the capacitors on the Logic Board is a pain in the arse. There are tiny contacts which they are soldered to on the board, and there's a good chance they've been corroded by the leaking caps. Be careful, you want to try and leave these in-tact as once they're gone, you could be out of luck. Circled below you can see the contact at the top is intact, but the bottome one lifted out with the capacitor. Use as little force as possible to avoid damaging them and working slowly seemed to do the trick.

You can see the trace to the intact and damaged contacts. I will need to find some way to attach these to the new caps, or to fix the contact. This is my first time fixing one of these old macs, so mistakes were made. Hopefully I'll be able to salvage this.

The whole board was then scrubbed with a toothbrush and Isopropyl Alcohol. All residue from the leaky caps needs to be removed as it shorts out the board - even if it's hard to see.

Isopropyl is great stuff as it evaporates completely in minutes, but you need to heed the safety precautions on the bottle as it's volatile stuff.

Now to try and salvage those damaged contacts. I scraped out the bed left behind with a screwdriver, and used a razor to scrape the coatiing from the trace which ran to the contact, exposing the trace/wire within. To that I tried to attach a glob of solder, but it wouldn't stick and just beaded off the PCB. To stop this I placed a tiny bit of wire in the bed to anchor the solder, with the hope that the surface tension of the solder would drag it into the trace before it set hard. Time would tell if that approach worked.

(Yes, you can replace the contacts properly with copper foil and adhesives but this is a weekend job and I don't have any of that on hand)

Here's the recapped board. You can tell it's been recapped, this one was messy. The capacitors are hard to grip with tweezers and the contacts are fragile, and there's plastic connectors in the way.

Now we've finished the job, the Mac Classic II can be put back together and...

I got a boot chime, followed by some sad sounding chimes that are apparently referred to as the 'Death March'.

On a hunch I pulled out the additional RAM sticks and...

Success! It boots despite my dodgy soldering on the Logic Board.

Now that I know that everything is functional, I'll probably go and buy some proper materials to fix those contacts on the Logic Board. I'll probably also replace them with solid-state caps that will never have to be replaced again, to avoid messing with those sensitive damaged contacts again.

Some more RAM is probably also needed now as the built in 2MB is a bit difficult to work with.