Microelectronics | BTC 53 Series Keyboard

Synopsis

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.

The BTC keyboard is connected to the StarView SV421 KVM Switch, and it's always powered on; it has to be reliable.

Maintenance

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. Its 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 a hard time removing it with acetone. As it's cold outside, I had to do it in my room. But even with the window largely open, high purity >99% acetone fumes gave me a big headache that would last for a couple of hours. Yeah, like I didn't know 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 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 sensitive 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.

Pictures

Here are some pictures that I took during this operation.

I'm not sure about the wire bridges, but the rest of the components might've been hand placed.

In fact, the overall PCB construction feels cheap.

And even the new capacitors seem out of place for this circuit. But that's OK.

This was a quick repair that will last for a very long time.


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