Servos, particularly the cheap ones from Ebay

Tower Pro servos
 Tower Pro are a well regarded Taiwanese manufacturer, see their website at :
 
Home Page
They sell their products via their international distributor for specific regions of the world. However, as far as I know, they don’t sell via Ebay. There are a number of businesses based in China that are selling servos that carry the Tower Pro brand, via Ebay and other sources, at low prices. Carefully examine the images on the real Tower Pro website and compare them. Are these genuine Tower Pro servos? Draw your own conclusions!
 Nevertheless, because they are so cheap, they are useful for projects such as robotics. Personally, I wouldn’t use them in anything that flies, as their reliability is an unknown quantity, particularly regarding motor longevity- but you do get some well proven electronics, a powerful motor and metal gears, pretty good for the money. They have a few problems, though, and can be improved.

Note that there are a number of different variations on the Chinese manufactured servos, mine were purchased in 2018. There doesn't seem to be any difference internally between the MG995 and MG996R servos that I bought, apart from the labels.

 !! Undertake these modifications at your own risk. Some of the work is very fiddly, using very small components. Please don’t blame me if you wreck your servo !!

 1) Dismantling
 Remove the 4 screws holding the case together. Lift off the upper case and remove the gears. Remove case bottom. Using pliers, carefully grip the motor and ease it from the casing. Take care not to squash the motor can!
 2) Gently tilt pcb forwards to gain access to the 2 tabs that secure the square feedback potentiometer into place in the outer case. Ease these tabs apart with a very small screwdriver while gently pressing on the Pot Shaft. The entire assembly should drop free from the case.

3) Note that the 3 chips on the board are:
 a) AA51880 servo controller – see datasheet for full circuit details
 b) DTM4955 Mosfet dual ‘P’ channel
 c) DTM9925 Mosfet dual ‘N’ channel
 The feedback pot is 5K linear (mine measures as 3.6K)
 The 2 Mosfets are arranged in H-bridge configuration to drive the servo motor.
 4) 2 components on the circuit board need replacement, as the ‘dead zone’, as supplied, is too wide, making the servos very unresponsive. Change them to the values specified in the AA51880 chip datasheet.

R3 should be 1k; C4 should be 100nF(0.1uF); both type 603 SMD parts - These 2 components determine the ‘dead zone’ for the servo arm.

No idea what the supplied capacitor is, but the resistor is a 2k2.

Also add a 100nf disc ceramic capacitor across the motor terminals to reduce motor noise and interference



5) The 2 sleeve bearings (brass at the top, nylon at the bottom) on the output shaft should be replaced by 6 x 10 x 2.5mm roller bearings.

 
6) Thanks to Jfitter for the original info – see his post at https://www.rcgroups.com/forums/showthread.php?2485803-Fixup-MG996R-servos

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