Macintosh Classic Logic Board Repair

The Mac Classic range of computers often show a variety of symptoms but which have a common cause of failure, for example:

  • Low volume or no sound.
  • Real time clock not advancing.
  • Power up problems with checkers and stripes etc.
  • No serial or localtalk functionality.

These problems are often caused by:

This is a Surface Mount Electrolytic Capacitor, these commonly fail and leak their chemical residue on the logic board. This residue is conductive and causes a lot of the problems listed above in combination of course with the failure of the capacitor to no longer do its job properly.

This page details the repair of a logic board which has no sound and the Real Time Clock doesn’t advance.

First of all, disassemble the Mac Classic, see Classic II disassembly for how to do this.

The caps on the left hand side are the likely candidates for sound and serial issues.

These caps are also likely candidates for leakage so check around here and decide if you are going to replace these too. In my case I decided not too and just cleaned in this area.

Here I have replaced the 47µf Electrolytic cap C3 with an equivalent Tantalum Bead, upon powering up the sound was back! Replacing a surface mount cap is a bit tricky, you need to heat up the tab on each side of the cap on the logic board and gently prize up the cap. You will find the cap gets very hot during this process and no matter how careful you are you will likely damage one of the pads. If you do then you will need to trace the circuit line back to a point where you can easily solder the replacement cap leg. Be careful to observe the + marking with your replacement cap.

The next step was to replace the other caps and clean the board up.

With the other caps replaced, using PCB cleaner the whole board was covered and the dried off using a hair dryer. This was repeated a few times until all the symptoms had disappeared.


This entry was posted in Vintage and Retro Computing and tagged by Sean. Bookmark the permalink.

About Sean

A self motivated highly experienced software developer and IT professional. Extremely technical, thorough and with excellent problem solving skills. Able to develop on a wide variety of systems and integrate heterogeneous platforms together. A proven track record of project successes from conception through to delivery. In depth knowledge acquired through exposure to a wide variety of technologies.

7 thoughts on “Macintosh Classic Logic Board Repair

  1. Hi, Could you tell us the exact rating / voltage for the caps needed for each position? Also, where could I find the diagram of where each cap is / their location? Thanks

    • Hi, the caps themselves are easy to spot on the board, voltage wise I think from memory they are mainly 16V, though it shouldn’t matter if you replace with higher voltage caps. The voltage is marked pretty clearly on the cap normally after the value and should have a ‘V’ to indicate it is voltage.

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  4. I was wondering if you could tell me the value of the only small electrolytic capacitor near the left side next to the 47uf one on the Mac Classic Logic board it is the only one that is different from the 47uf @ 10v. Mine is worn off and I can not read the value. Thank you kindly in advance.

    Victor De Francisco

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