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How to fix a servo with a burned out green circuit board? Answered

I have a Hitec HS -805BB servo that seems to have a burned out component on the green pcb circuit board (the component looks like a H- bridge of some sort). For this reason the servo won't work. I tested the potentiometer, gears, and DC motor inside this servo and they work great. I was wondering if I could somehow get this servo working again since it was really expensive. Could I cut out the current burned out green circuit board and wire the other working components through an arduino? So pretty much use an arduino as a replacement circuit board for the servo. If this is possible how would I wire it and what would the code look like. Here are some links to instructables that I think could help with this problem but am not sure:

If there is any other way to get this servo working again please let me know as this servo is vital in my project and costs a fair amount. Thank you for your help in advanced.


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4 years ago

Ehh, it is possible to fix it if you are absolutely determined and know a bit about how servo's work. I can't find any info on that particular servo. All of these hobby servos (digital or analog) work by accepting an analog PWM signal representing a angle, and the closed loop controller inside will compare the input signal value to the feedback signal from a potentiometer connected to the output arm internally, and turn the motor in the correct direction to try to eliminate the differences. If you want to learn more about control systems and how they work and how to make them, here is a great youtube channel to learn about that stuff. https://www.youtube.com/channel/UCq0imsn84ShAe9PBO... But keep in mind that is a very advanced topic, while the concepts are not super difficult to grasp (if you know how to drive, then you are unknowingly already familiar with closed loop control), to understand the math and theory behind this stuff, you need (or should have) a strong background in calculus and differential equations.

I recall some of those higher end servo manufacturers use secret proprietary circuitry and will do what they can to prevent people from reverse engineering the controller inside. If that chip had the number rubbed off, then you can bet that they don't want you to know what voodoo they are doing.

You could control that servo directly from arduino and a motor controller chip (an H bridge) if you take the feedback from the servo potentiometer and use that so that the arduino knows where the servo arm is and how the motor needs to rotate to go where you want. While that probably will not be easy, it can give you a lot of flexibility on how you control the servo. Alternatively, you could modify the servo and turn it into nothing more than a geared down motor (often incorrectly called a continuos rotation servo mod.)


Answer 4 years ago

Thank you for the
helpful information. My application for this servo was with my robot arm kit
(lynxmotion al5d) for a school competition. I used this specific servo served
as the shoulder motor since it provides a fairly large amount of torque (24.7 kg
cm at 6.0 v). I am controlling this arm with a series of potentiometers so that
each servo moves the exact angle the potentiometer was turned. For this reason
I still need the position feedback from the servo. Referring to the two links
in the question, I think I can wire the existing potentiometer and dc motor
with the H-bridge (like an l293d) and the Arduino as they show in the example
instructable with the 120 kg cm motor. Then I was thinking that I could just
upload the provided code and be on my way. Do you think this would work or
would it be too complicated? I have just under a month to fix up my robot arm
(the basics are already built) before my competition. Also, when I opened up
the servo, some of the numbers and letters on the parts were rubbed off but
many were still on there. I tried googling some of those part numbers but didn’t
turn up anything relevant. When I plugged in the servo at between 4.5v and 6.0v
(while the servo was open), the 4 voltage regulators (at least that’s what they
looked like) on the green circuit board seemed to heat up tremendously despite
the voltage being in the range suggested by the company. The little H-bridge
looking component seemed to have some darkened (blackened) areas along its
pins. Do you think these could be related to the problem?


Answer 4 years ago

If you can, go ahead and attach some wires to that motor inside the servo directly and connect that to 3 to 5 volts, see if you can get that motor to work. That way you know for sure that it isn't also bad. I am certain that motor controller inside the servo is dead based on what you describe.

I wouldn't expect that code to do what you want first try. You will probably need to make modifications especially if you are using a different motor controller (that is interfaced to the arduino differently). That said I don't know much about the arduino code, or what boards you are using. It sounds like you will have some fun troubleshooting if you do go for it, and especially write your own code, you will learn a lot. Considering you only have a month to finish the project, don't spend too long trying to fix it. Maybe a week or so, but if you don't get anywhere with it, just get another servo.