Friday, November 9, 2012

Fatar SL-880 Repair - replacing crystal with oscillator

This keyboard would not turn on reliably. I did at first attempt a hard reset, pressing UP/DOWN/ENTER while turning on. This had some effect, but it still frequently failed. I also tried the instructions referenced here, which possibly helped but not completely.

I did find that when it was locked up there was no clock pulse around the crystal... however replacing the crystal had no effect. After downloading the datasheet for the micro controller: ST9036, I found that the crystal could be replaced with an oscillator, a device which creates a pulse on its own, eliminating the need for much external circuitry.

I had a 24mhz oscillator around, and connected it to ground and to the 5 volt regulator. I removed the old crystal and the small value caps that were around it (they are not needed with an oscillator) and made sure that the output of the oscillator went to the "osc in" pin on the microcontroller.

Somewhat surprisingly, this worked perfectly!

I glued the oscillator to the board, and the unit has worked fine ever since. It is not a pretty repair, aesthetically, but it does work!

I tested the idea with a breadboard first, connecting the osc output to the microcontroller with alligator clips
The oscillator is similar to the crystal, but needs to be powered with 5 volts, and doesn't need to have capacitors around it

The fixed board has an oscillator glued to it where a crystal once was.