Repair Guide for the Multiplex RHINO digi 4 servo.
(may also work for the Hitec 5745MG servo)

Writen by Jimmy Hansen 20/02-2008
Translated by Simon Brejne

 

 

Intro

This reparation guide works with the servos where the mosfet’s are fried which mean the servo can only turn one way, or not turn at all. Furthermore you will know the defect because of the smell of burned electronics.

The guide implies solder work and exchange of SMD components which can be done with a common soldering iron, but some kind of experience is required. If you won’t do it yourself, then pay off your local electronic nerd with some pizza and a coke ;-)

The required tools are: a peaked soldering iron, solder tin, 2 flat tipped screwdrivers, a peaked cutting nipper, a file, and a hobby knife.

The setup of the guide is a step-by-step with pictures to illustrate how to separate, repair and assemble the servo.

The servo can be repaired for 18 Euro, and between 1 and 2 hours enjoyable work for those who haven’t done it before.

 

Technical explanation of the burned off servo

The RHINO servo has some weak points which mean that it might burn off, even though you haven’t done anything wrong. The weak points are the mosfet’s and the motor The mosfet’s are simply burned off because of a short circuit in the motor. The motor burns off if it´s overloaded, but it might apparently also short-circuit buy common use. Also wornout bushings may cause it to draw too much current.

The motor is a RK-270RH which pulls 1,8 Amp. maximum. The mosfet’s (IRT7389) is able to work with a continuous current at about 4 Amp. The problem is that the motor might short circut, and pull above 1,8 Amp, which will cause the mosfet’s to burn of, because they are not protected against the high current. A 3 Amp fuse would probably had protected the mosfet’s.

 

Necessary parts to do the repair

To repair the servo you will need:

1 pcs. RK-270RH motor [specifications]

2 pcs. MOSFET IC IRF7389 SMD [Datasheet]

 

Let’s get started

 

Repair – separation

Take off the bottom cover by unscrewing the 6 screws.

Solder the motor free with a tin absorber and lift out the print board.

Separate eventually the black middle piece to get better access to the print board.

Take out the motor

Unscrew the cooling plate

At this point you can check whether or not this guide can fix your problem. If the motor pulls more than 2 Amp with 6 Volt under pressure, or if you visually can see that the IC’s are burned (black traces on the cooling board) then move on safely.

Cut of every leg of the burned of mosfet IC (‘s) with the small cutting nipper, and remove the IC’s.

Remove the remaining legs with the soldering iron

 

Repair – exchange of MOSFET

Please notice eventually burned off print boards. You will have to exchange these with thin wire. Solder the wires from the islands to the legs, and try to keep them in top of the print board. If it’s not possible, then wait until the IC’s are soldered in, and then draw the wires to the underside of the print board as mentioned later. Please notice the colours of the solder islands in the diagram. It indicates the same solder island in the top, and under the print. Then it’s easy to find out where the wire from the leg has to be soldered on the underside.

Solder the new mosfet IC’s in. Turn the IC’s correct as showed in the diagram (leg 1 on both site of the IC’s towards the middle), and solder a wire on each leg which needs print tracks.

Screw on the cooling board (remember cooling paste on the IC’s) and check that the wires aren’t stuck. Cut the edge of the print with the hobby knife so that the wires can pass to the underside of the print board.

Solder the wires on the solder island which belong to the respective leg. (shown in the diagram above)

If the black middle piece is dismantled then put it on again. Remember the o-ring.

 

Repair – exchange of the motor

Secure the motor, and file down the axle in top of the cogwheel.

File until the axle and cogwheel are on same level.

Separate the cogwheel from the motor with the two screwdrivers.

Use the end of the screwdriver to knock the cogwheel on the new motor. Make sure the bottom of the motor is supported, to avoid damage to the endbell.

Put some cooling paste on the motor house, and put in the new motor in the servo (+/- has to be right) and solder it. Remember to solder the connection to the motor house.

 

Repair – assembling

Cut of some of the long soldering flaps off the motor, and screw on the bottom plate again, remember the O-ring.

Test the servo. If it doesn’t work, then look at the diagram again, and measure the connections.


Test video of this RHINO servo

Enjoy!