These keyboards have different PCB revisions depending on the year and/or month of production. But in general they have two electrolytic capacitors. Their lifespan should be well off by now. One of the capacitors (33 uF / 16 V) is in the microcontroller reset circuit and the other one (10 uF / 16 V) is probably a local power supply ripple filter.
I decided to replace both of them with solid aluminum polymer capacitors. As I don't have any 33 uF / 16 V capacitor around, I have replaced the first one with a 47 uF / 25 V part instead. It's role is to form a time constant along with a resistor to correctly pulse the microcontroller reset pin. Nothing system critical here. I replaced the other one with a correct capacity and a slightly higher voltage rating.
The PCB is made of an ugly composite epoxy material (CEM) which is very flexible and looks like multiple layers of paper bonded together. I don't know what it's made of but its color is very nostalgic and reminds me of old low-cost hardware.
There is also some glue securing the capacitors in place. I had some bad time removing it with acetone. As it's cold outside I had to do it in my room. But even with the window largely opened, high purity >99% acetone fumes gave me a big headache that would last for a couple of hours. Yeah, like I didn't knew what was going to happen.
There is a piezoelectric effect ceramic resonator of presumably 6 MHz next to the microcontroller. While removing the glue, I have severed the text on the yellow epoxy package of the oscillator. Its function is not affected but it looks ugly. The PCB raster also provides holes for a quartz crystal oscillator. But I had none available in my parts box. And I believe this microcontroller is not that sensible to frequency stability. Or at least the implementation does not require high stability. I think these ceramic oscillators have a frequency tolerance of only about 0.5%. Real quartz crystals can easily attain a 0.001% frequency tolerance. I will skip the replacement of the resonator for now.
Here are some pictures that I took during this operation.
This was a quick repair that will last for a very long time.
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