Debugging a micro servo

Recently I bought some VERY cheap micro servo’s. And as the saying goes: “if it looks to good to be true: it probably is”. I bought 2 ‘Micro servo 9g’ servos for 8 Euros including shipment, and on arrival both worked like a charm. After 2 days one of them started spinning around like crazy, and I wondered if I could repair it.

Electronics

The first thing I looked for was trying to get the motor to stop turning while the servo was opened and all the gears were removed, just by turning the potmeter manually. The motor kept on turning. I took a closer look at the electronics:


As you might see (pictures were taken with telephone camera through my son’s magnifier glass) pin 1 isn’t soldered at all! I soldered this pin, and the servo started acting somewhat normally again AFTER receiving it’s first update.

Mechanics

Somehow the gear that attaches to the servo ‘slips’ in this broken servo. At startup it tries to find its center point, but I see the gears turning and turning, although the end stop of the potmeter should prevent the servo from turning further. When I rotate the shaft manually (without gears) I could not force through the end stop.

You get what you pay for

Did I get what I paid for? YES! I do have the servo working somewhat again, and for the price I paid that’s quite a lot. I’m always interested in how these things can be made so cheap. The reasons I’ve seen in this example are a very low cost carbon strip potmeter, a chinese motor driver chip (KC5188) used without the external FETs that are recommended (? or obligatory ? I’ve copy-pasted some text from the datasheet to Google translate to try to understand something of the IC). Mechanically, the grease used looks and feels more like lard than ‘normal’ grease, and in this case the output gear obviously is not attached well to the output shaft. Furthermore, 5 wires are hand-soldered between the motor, potmeter and control electronics, which makes me think WHO can solder 5 wires in such tight space for so little money…..

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