Amiga Technical Resource

Replacing SMD capacitors on the C= A3640 board

The Commodore A3640 68040 based CPU board is commonly found in many Amiga A4000(T) and A3000T computers.
Due to what appears to have been an error with Commodore's CAD library, many of these boards have been built with three of the polarised SMD aluminium electrolytic capacitors operating in the reverse polarity.

These 22uF capacitors, also known for leaking electrolyte on the main boards of the A600, A1200, A4000 and CD32, often causing audio circuit damage, are even more prone to leaking electrolyte in this situation.

This has been known to cause:

This guide will show what's involved to replace these capacitors to repair a faulty board or as a preventative measure.
This applies to revision 3.0, 3.1 and 3.2 boards.

To do the job you will need:

Disclaimer and caution

While the information on this page has been checked and is correct to the best of my knowledge, there is still the possibility of unintentional errors.
Please report any errors directly to this address so they can be corrected.

As with any repair work to delicate electronic equipment, you risk causing further damage to your system or yourself. I cannot be held responsible for any equipment damage or personal injury.

It is strongly advised that you possess some good common sense and preferably have previous experience in working with electronics before undertaking any repair on your system.
As always, you should take all anti-static precautions when working with semiconductor devices.

A3640 - unmodified

Click image for larger view

  1. Pictured above is the unmodified revision 3.1 A3640 board.
    The first thing to do, is see if your board is affected by the capacitor reversal problem or not.
    Begin by inspecting the capacitors' solder joints and checking for obvious signs of corrosion or leaking residue, which are common signs of leaking capacitors.
    If the joints are not clean and shiny, the capacitors will certainly require replacement.
    The image below shows solder joints on a capacitor which has been leaking (centre). The joints should be smooth and shiny, not dull or dirty looking.

    Leaking SMD electrolytic capacitor

  2. Remove the A3640 board from the computer and use an ohmmeter/multimeter to measure between ground (use the metal body/case/enclosure of the crystal oscillator, U100) and the legs of C105, C106 and C107, highlighted in red above. If you measure less than 1 ohm between ground and the -ve (negative) side of the capacitors (the side with the black stripe), then your board does not require any changes.

  3. If you measure less than 1 ohm between ground (the body/case of oscillator U100) and the +ve (positive) side of C105, C106 and C107, then you need to replace these three devices.

    SMD electrolytic capacitor

  4. To find out how to correctly replace and work with surface mount SMD capacitors, click here.
    Use isopropyl alcohol or methylated spirits to clean away any electrolyte residue from the board.

  5. Replace C105 with a new capacitor, fitted as shown below. Note that the silkscreen printing on the board is also incorrect.

    C105 - incorrect
    C105 - correct
    C105 - incorrect polarity
    C105 - fitted with correct polarity

  6. Replace C106 with a new capacitor, fitted as shown below. Note that the silkscreen printing on the board is also incorrect.

    C106 - incorrect
    C106 - correct
    C106 - incorrect polarity
    C106 - fitted with correct polarity

  7. Replace C107 with a new capacitor, fitted as shown below. Note that the silkscreen printing on the board is also incorrect.

    C107 - incorrect
    C107 - correct
    C107 - incorrect polarity
    C107 - fitted with correct polarity

  8. The finished board should appear as shown below

    A3640 - modified

    Click image for larger view

  9. It would also be worthwhile replacing the other two capacitors on the board at the same time, C102B and C102E. Note that these capacitors are fitted with the correct polarity and will not need to be reversed.

  10. Visually check your soldering and check for solder shorts between the capacitor terminals using the ohmmeter/multimeter. The capacitors are all connected in parallel; therefore if one capacitor has a solder short circuit, the fault will appear across all of the capacitors on the A3640. Do not plug the A3640 into the computer's motherboard if there is a short circuit across any of the capacitors (+5V supply)!

  11. Else if things appear to be normal, there are no short circuits on the +5V supply (you should read between 100 ohms and 10kohms across any of the electrolytic capacitors), then test the board's operation in an A3000/A4000 computer.
    The two motherboard clock source jumpers must both be in the "EXT" position for use with the A3640.

  12. If the computer does not boot (caused by the A3640 not working) then the board may have been damaged as a result of the corrosive electrolyte leaking from the capacitors. The kind of damage usually caused by this is corrosion of the board's vias, as shown by this diagram.

  13. Suggested repair action is to measure the continuity of all vias physically close to the electrolytic capacitors using a multimeter and repair as necessary.
    Board schematics can be downloaded here.

Also see the A3640 reference page for more information on this board.

Questions, comments, errors? Email me.