This article contains graphic content dealing with the disassembly and repair of the Amiga 4000 and may be considered too graphic by some readers.
Or put differently: This article requires the Amiga 4000 to be opened to make changes. I make no guarantees of success nor can I be held responsible for any damage incurred while implementing the changes described herein.
Please be sure to read through this document in its' entirety before attempting any repairs.
In my last article, (Amiga A3640 CPU Board Repair), I discussed the problem of the reversed electrolytic capacitors on the Amiga 4000 A3640 Daughterboard. Many of you are probably heaving a big sigh of relief. You own Amiga 500 or Amiga 2000 computers and do not have to worry about this problem since you do not have the CPU Daughterboards in your computers.
Well, before you get too smug, let me mention that while investigating the reversed capacitor problem, I stumbled onto another capacitor issue that has existed for a long time. It exists on the Amiga 4000, the Amiga 500, Amiga 2000 - and these are just the computers that I had schematics for. I'm not a betting person, but if I were, I'd wager that this issue is on the Amiga 3000, 1200, 600 and probably goes back to the grandmother of them all, yes - the Amiga 1000.
Just what is this problem? Is it serious? Can it be corrected?
The problem lies in the audio circuitry, which all Amiga computers possess, and is on the motherboard. (See Figure 1.)
It is not serious, and yes, it can be corrected.
First, an explanation of how the audio output circuitry works.
The audio output amplifier is connected to a bipolar voltage source. That is, it is connected to +12V and -12V. When an amplifier is connected this way, its' output can swing from a positive potential through zero volts to a negative potential.
Normal connections on the output of an amplifier like this are as shown in the figures. It is either a direct connection to the output, or through a non-polarized capacitor.
If an amplifier is connected to a uni-polar supply, it is connected to a source like +12V and Ground. Then the output of this amplifier would swing from Ground to near +12V on the output. With an amplifier like this, we can use a capacitor on the output. The capacitor could be a polarized capacitor as the voltage potential on the capacitors' positive lead would always be positive.
What evidently happened at Commodore was that someone did not realize the affects of designing a polarized capacitor into a circuit like this. The first designer made the mistake, and every other designer after that just copied the circuit which propagated the error. Or, if it was the same designer responsible for all of the Amiga designs, he did not bother to check his previous design.
What was needed was a capacitor coupled output on the output of the Amiga. This is okay. However, the way that it was implemented in the Amiga could use some improvement.
Will this capacitor incorrectly designed into the Amiga damage the computer? Well, the answer to that is - Yes - and - No. You see, electrically, there will probably not be much difference between the polarized and non-polarized capacitors. This is the 'No' part of the answer.
In the Amiga 4000, they are using surface mount (SMT) components, including SMT capacitors. For some reason, I believe that these capacitors may have been stressed during the soldering phase of assembly. Either that, or this particular value of capacitor in the SMT version is a dud. In either case, on my Amiga 4000, the left channel capacitor had leaked its' electrolyte out onto the printed circuit board (PCB), and corrosion had set in on a few of the surrounding components. This is the 'Yes' part of the answer.
I have removed the defective SMT capacitors, cleaned the area surrounding the capacitors, and replaced them with a through hole variety non-polarized capacitor.
I recommend that those of you with at least an Amiga 600, 1200, 3000 or 4000 check your own machines. These are computers that Commodore converted over to SMT production, and are most likely to exhibit this problem.
If you choose to ignore the potential problem, here is what could happen. As the capacitors leak their electrolyte onto the PCB, the electrolyte flows out onto the board covering any circuitry and component pads that it comes into contact with. Over a period of time, the fluid, being corrosive, begins to corrode the things that it has come into contact with. It's just like automotive battery acid, on a smaller, but no less dangerous, scale. If left alone long enough, eventually the corrosive electrolyte will eat through the copper traces, vias, and even integrated circuit (IC) leads. The result will be a major repair job.
The way to correct the design error that exists in our Amiga computers (short of redesigning the audio circuitry), is to use a non-polarized capacitor. When choosing a non-polarized capacitor, use the same value of 22uF, and be sure that the voltage rating is high enough to withstand the largest output from the audio amplifier. I chose a 35 volt capacitor (See Figure 2).
A non-polarized capacitor would cost a bit more and many times a more costly item will not be used in the design - so that a few pennies can be saved.
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