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CrustyNoodle
03-31-2006, 11:30 AM
First up some acknowledgements: The incentive to give this a go along with great instructions came from this article: http://www.bakehead.com/minit_mods_page.htm.

Now a bit of background (I'm sure if you did a bit of a Google search you'd find a better explanation than the one I am going to give here but for the purposes of this article this will do).

How a normal servo system works:
A normal RC servo system uses 3 wires to the servos: two of these wires are for power to the motor and the circuit board inside the servo (the servo drive) and the third is for the signal that tells the servo where it should be. The signal to the servo is in the form of a pulse width modulated (PWM) signal (think of it like 0's and 1's - different length 1's tell the servo to move to a different position).
Inside the servo is a shaft encoder (in our case a trim-pot, variable resistor or whatever you'd like to call it). The output from this encoder represents the current position of the servo shaft or horn. The circuit board inside the servo takes the PWM signal from the RX which represents a command to turn the output shaft (horn) to a particular position and compares it with the actual position of the shaft as reported by the encoder. If the shaft is in the wrong position then the motor driver circuit in the servo applies power to the servo motor to move it to the desired position. The difference between the commanded position and the actual position determines how hard the motor is driven to get it back to the commanded position. This principle is basically the same for any servo system regardless of whether it's part of our RC model or on a $1m CNC machine tool (although the encoder, controller and other components differ in their complexity and principals of operation, the general effect is the same).

How the Kyosho 4-wire system works:
Well as you might have guessed it's actually pretty much the same as the above in principal however instead of having the control circuitry inside the servo, the H8 has it inside the combined receiver/ESC unit. As the principal of operation is the same, the controller inside the receiver/ESC needs both a PWM signal to tell it where the servo output shaft should be and the feed back from the encoder to tell it where the shaft actually is so that it can send the correct voltage down the line to the servo motor to drive it to the correct position.
Hence the Kyosho servo requires 2 wires to drive the motor and another 2 wires for the encoder feedback signal and there you have it - a 4-wire servo.

So how do we convert a standard 3-wire servo to do what the 4-wire system expects it to do????
Well it's easy really. As the control circuitry is in the H8's receiver/ESC we basically don't need the circuit board that is inside the 3-wire servo and instead we need to pass the motor power wires and the pot (encoder) wires straight back to the receiver/ESC.

OK so that is the theory so here comes the practice - but you guessed it, I'm too lazy to put the actual mod into words so I'll refer you again to the article that helped me work it all out http://www.bakehead.com/minit_mods_page.htm. If you follow his instructions for the electrical side of things then you can't go wrong, the only difference I found with the converting of a Hitec HS-81MG to work with my MIST was that I had to use the original pot out of the Kyosho servo as the resistance figures for the two servo pots were slightly different. This meant that in order to centre the servo when using the Hitec pot, I needed to have the radio trim almost all the way in one direction. This wasn't a problem however, I just used the wires that connected the Hitec pot to the Hitec circuit board to connect the Kyosho pot to the Kyosho circuit board (it's not really much of a circuit board - just a means of connecting some wires) and put all this inside the Hitec servo. I had to file the sides of the end terminals on the Kyosho pot however to get it to fit into the Hitec but that was about it.

So in the end, the set up consisted of:
Hitec servo (HS-81MG) (I would have preferred the HS-85 BB but couldn't get my hands on one as easily as the HS-81).
Pot from the Kyosho MI servo inside the HS-81 (it's pretty much a direct fit).
Circuit board from the Kyosho MI servo to connect everything up.


You will also need a soldering iron with a fine point and a third hand plus either a solder sucker or some solder wick to remove the old solder and remove the various wires and pins etc.

And finally, the result:
What a difference! I can now get significantly more steering lock under all conditions (sitting on the bench and on the move) so my turning circle has been greatly reduced. The steering speed is also much better (maybe 2 times faster) and the whole system is far more robust and responsive. The only thing I noticed was that there was significant overshoot in the servo before I reinstalled the wheels but under driving conditions I have not noticed it - probably as the forces the servo experiences under load tend to provide a damping effect. The overshoot is probably a result of the significantly faster rotational speed of the new servo and the fact that the Kyosho control system is (possibly) optimised for the smaller and slower Kyosho servo.

The only thing I would have liked to have done differently is to have used a Hitec HS-85 BB servo as the HS-81 that I used does not have proper bearings for the servo output shaft (it just runs against the servo casing). As a result there is a bit of unwanted deflection of the servo horn when loaded.

See this thread for some installation pics: http://www.halfeight.com/forums/showthread.php?t=1828

I'm sure this mod would work for just about any servo as long as the encoder resistances were compatible or the Kyosho encoder could be used.

activeperception
05-05-2006, 11:36 AM
Hi All,

I'm really pissed off of the stock servo but I don't want to spend for new radio and esc (in particular because I should stick with the standard motor because of race rules at our track).

Now my stock servo seems to have lost torque when rotating in one direction.

So I'm thinking to buy a hg-85 and modify it, unfortunately the link with the photo for the mod doesn't work anymore.

Can someone post photos or give me a link with more details about this mod? (it seems that is the same mod required for the mini-t).

Do you suggest that I modify a standard size servo? It will work with the standard receiver/esc?

thanks,
Raf

CrustyNoodle
05-05-2006, 11:58 AM
Try copying and pasting the link in your browser - it works fine for me.

activeperception
05-05-2006, 01:52 PM
I was referring to this link:

http://www.bakehead.com/minit_mods_page.htm

Jaymz
05-05-2006, 09:48 PM
Link seems to work fine for me..... CrustyNoodle is the mod able to be done on the HS-85BB+ aswell? As i just bought one for a really good price, too good to pass up... If i cant what would i need to change to be able to use it in my ST???

activeperception
05-07-2006, 02:47 PM
Link seems to work fine for me..... CrustyNoodle is the mod able to be done on the HS-85BB+ aswell? As i just bought one for a really good price, too good to pass up... If i cant what would i need to change to be able to use it in my ST???


You are right...I don't know why but my browser was farting

CrustyNoodle
05-08-2006, 03:00 AM
Jaymz,

I'd (almost) bet my left one that the internals for the 81 and 85 are exactly the same - certainly the principles will be the same so it's just a matter of identifying the correct wires.

Jaymz
05-08-2006, 08:41 AM
ok thanx mate... i'll have the knife sharpened and waiting if its not the same :P
j/k...

Thanx again