Amiga A3640 CPU Board Repair

The purpose of the A3640 CPU daughterboard is to provide a replaceable board with the CPU on it. This allows a cost-effective way to increase the power of the Amiga 4000 by plugging in a more powerful CPU, rather than buying a whole new computer. The A3640 is composed of several components. The major one is, of course, the 68040 CPU. There is also the crystal oscillator which provides the master CPU clock frequency. Then there are custom PAL IC's, bus transceivers, and other IC's that help tie everything together on the board. Last but not least, we get to the discrete components like the SIP resistors, ceramic bypass capacitors and five polarized aluminum electrolytic capacitors.

It is these aluminum electrolytic capacitors that are the focus of this article. They are a silver colored package about one-quarter of an inch in diameter, and just as tall. The values used on the A3640 are 22uF at 16 volts. They are connected across 5 volts and ground and are used as bulk DC storage capacitors.

If you look at Figure 1, you will see a basic layout of the A3640 CPU Daughterboard. I have only shown the 68040 and the general location of these capacitors and their reference designators, which are C102B, C102E, C105, C106 & C107.

After examining the circuitry, I discovered some of these aluminum electrolytic capacitors (C105, C106 & C107) were in the circuit backwards!

A datasheet from one manufacturer of capacitors reveals some information about what happens when an electrolytic capacitor is installed backwards (Figure 2).

Figure 2. DC Electrolytic Capacitors are polarized.
When a capacitor is inserted backward in a PC board, the electrical characteristics can deteriorate significantly. There would be the danger of short circuit, the vent could function, the capacitor could overheat and burst into flame because of the abnormal current flowing through the capacitor.

It looked like two of the capacitors (C105 & C106) had leaked their electrolyte, which is a problem as indicated by another note from the same capacitor manufacturer (Figure 3).

Figure 3. Flammability of electrolytic paper and electrolyte.
The main solvent of the electrolyte and the electrolytic paper are flammable. Electrolyte is conductive material, and therefore if electrolyte leaks out onto the PC board and comes into contact with electricity, it can cause corrosion, and/or short circuit between the patterns and/or smoking and/or fire.

This is what finally caused my Amiga to fail. The capacitors that were in the circuit backwards, over time, failed causing the capacitor to vent its' electrolyte. Over a period of time, the electrolyte flowed across a portion of the printed circuit board. Being conductive, it caused circuitry that it came into contact with to turn on (or off). This is equivalent to traffic lights at a busy intersection all turning green at the same time, allowing all of the traffic to go at once, resulting in a major traffic jam.

I would urge anyone who uses an Amiga 4000 with the stock A3640 CPU Daughterboard to verify my findings. Do not take them at face value. The revision of the board installed in my Amiga, as marked in the silkscreen is Rev. 3.1. This problem was also noticed (and corrected) on a Rev. 3.0 board. (Perhaps these problems were discovered and corrected on later revisions of this board?). It would not take very long to pop the cover, and examine the CPU Daughterboard for an oily substance on the board. You may also see corrosion on the IC's near the electrolytic capacitors. Measure the voltages on these electrolytic capacitors with a voltmeter. If they exhibit the symptoms as previously described, then I suggest you take the steps necessary to replace them. If you do not, then it is only a matter of time before your computer fails.