View Full Version : mr-02 40Mhz FM Conversion

2006.04.17, 11:46 AM
Ever since I got my SANWA MX-3S 40MHz FM TX for my Half8 I wanted to convert my mr02 over to 40Mhz. Why? No real reason

apart from the challenge of being able to do so, lol.... The lack of features in the Kyosho Perfex KT5 TX that comes in the

mr-02 RTR set sorta just gave me the ****s and I wanted to be able to use fine tuning features like end point mapping and

other response based mods that my MX-3S has and I didn't want to fork out an arm and a leg to get a Futuba 3PK to do it!

The main challenge in this type of conversion is finding parts that will fit! Not only do you need to find a small enough

ESC, but also a small enough RX that will work with a pistol grip based TX.

In the end I chose the following parts:

1) GWS PICO Standard Servo ($15USD)
2) GWS GW-R4PII40-H 40Mhz FM RX ($17USD)
3) Berg 40Mhz Micro Crystal 40.685Mhz ($7USD)
3) GWS GS-100E Fwd only ESC ($15USD)

I chose the servo 'cause it was one of the cheapest around at the time. Also being a pico sized servo ensured that the PCB

inside it would be small. I basically gutted it, kept the PCB and threw out the rest....

I chose the GWS RX since it was small and on the few I could find that was 40Mhz FM and therefore compatible with my MX-3S.

The Berg micro crystal was selected since it matched a channel on my TX. An important note here, DON'T buy the Japanese

market 40Mhz crystals, their LSB is 40.xx0 not 40.xx5 and most 40Mhz TX's won't do 40.xx0 as I (rudely) found out.....

I chose the GS-100E ESC for several reasons:

- Most popular micro-ESC's won't fit. I checked out the Novak SPY, Robitronic and Associated XPS ESC's, all of them were just too big!!! Yes, I only have FWD (no reverse!!!) but that doesn't really matter to me since I race this chassis against a mate quite often. Also, you need to make sure your Tx can reverse channels and (idealy) have a sub-trim function. Both

these attributes make the running of the GS-100E ESC a lot easier. This ESC also handles hot motors just fine. My test

setup included running a PN Anima motor (33 turns) and the ESC didn't break a sweat.

OK, enough of the BS text, here's some pics. The primary goal at the moment was to makes sure everything fit and worked.

I'm yet to actually run the chassis, time permitting I'll do that soon!

Here's the RX, I removed the gold plated pins for space reasons...


Here's the ESC, bloody small!


Here's a partially assembled pic, showing the RX (with pins removed) and the servo PCB wired in.


Here's a pic showing the ESC installed as well. Notice the ESC and the Servo are both soldered directly to the RX due to

space constraints.


The picture below details the wiring from the feedback potentiometer. Unlike stock mini-z that only has two wires, the GWS

servo had 3 wires going to the feedback pot, so I had to replicate that too!


Here's a "fully assembled" picture showing the locations of all the installed parts. Notice the micro crystal on the right

hand side near the white motor wires....


You wouldn't believe it but it all actually fits underneath the chassis cover, yeah!


So that's it for now, I'll post more once I have done a proper "road test" and we'll see how it all goes....

fingers crossed.....


2006.04.17, 12:17 PM
Great job. Good explanation.

2006.04.17, 01:27 PM
All I can say is Wow!!

2006.04.17, 03:40 PM

very nice work. i am extremely interested to know if the servo board you used is able to control the miniz servo motor and potentiometer. i had done a previous project with an mr-01 chassis, wiring in a hitec hs-55 servo board into the existing miniz servo motor and pot. i found that the steering was unreliable and wouldn't hold. i hope what you have works out, and i'm sure i will pick of one of those servos to use its' pcb, if you get good results. looking forward to your test run report.

2006.04.17, 05:42 PM
thanks fellas! :)

Yes bobbyz, this particular GWS servo PCB will control the mr-02 servo setup just fine. In fact I've noticed the the steering is a LOT stronger!

In the 2nd last pic you will noticed a piece of white heatshrink near the servo PCB, this has a resistor inside it that actually came attached to the servo PCB. I just connected the potetiometer and motor on the mr-02 inplace of the pot and motor on thw GWS servo and everything worked first time! I'm very happy that I didn't have to add or modifiy the value of the resistor to accomodate the different feedback pot. Sometimes you need to do this when using physically different servo pots and motors so what was originally in there....

hope to get some runs in later this afternoon!


2006.04.17, 06:59 PM

great to hear about the servo board. i will definately be trying this servo board on my fm project. the hs-55 pcb has no resistor, and i assume that is the difference. i'm no electrical genius, but it was pretty easy to see how much bigger the mini z servo motor was in relation the to hs-55 servo motor. i didn't want to do trial and error and spend $$ on multiple servos, so i'm very glad to run accross this information. keep up the good work. so have you driven it with everything hooked up yet??

2006.04.17, 11:48 PM
Amazing work, great job ph2t!

2006.04.18, 02:15 AM
post a vid & review when its done... :D

2006.04.27, 08:38 AM

OK, I put it all together and installed the pn anima motor. The ESC has no problems handeling this 33 turn motor in terms of current, the GWS GS-100E ESC is rated at up to 8A.

Unfortunately it didn't run well. If I applied the throttle too hard the whole car would stutter and the steering would go crazy. It seems that the ESC is dumping all the current to the motor and therefore the supply voltage is dipping too low to run the RX and Servo properly.

Given this I tried to add several capacitors to the voltage rail to help smooth this out. In the picture below you can see the ghetto setup of several capacitors in parallel (it's all I had available at the time) totaling approx 900uF.


Even with the caps the ESC would still dump too much voltage. It did lessen the effect, but over time it got worse as the batts started draining.

In the end for the first racetrack test I just put a stock xmod stage 2 armature into an ATOMIC bb can. With this setup there was no issues at all. The car ran smoothly and had no twitches under acceleration....Mind you it was no match for betty's mr02/nelly/S04 combo!

But it didn't stop there.... :)

I bought a PN Saber motor from the ausmicro shop and installed that instead. The PN Saber is a 35 turn motor (ie: 2 turns "less hot" than the PN Anima). It's basically an S04 with a fat commutator.

This motor ran fine in the car. No issues with the ESC dumping at all. Not until the very end of the batt's charge. I tested this and found that when the car did start dumping the current, it was near the end of the batts charge anyway. I drain one set and they only had 22mA left in them, so given that I find this acceptable.

So to be on the safe side and to get the greatest amount of smooth, "non-dumping" runtime I installed a 330uF 6.3V capacitor across the voltage supply on the RX. This capacitor now supply's the RX and Servo directly. It's a balance between how much uF (capacity) I can get vs. the space I have to install it in.


So, with the swap to the Pn Saber motor and the addition of the 330uF capacitor, I can say now that this setup runs really well and I'm very happy with it. I am pretty much limited to the 35 turn motor, as I said, not because the ESC can't handle the current of lower turn motors, but because the ESC doesn't handle the current draw very well in terms of keeping power up for the RX and Servo electronics...

Maybe this is because I am running this setup on the lower limit of 4 cells (4.8V). It is a 4 - 6 cell ESC. I think what's happening is the 5V regulator on the ESC is the cause of the problem. This 5V regulator is the "BEC" portion on the ESC that drives the servo and RX and a constant 4.8 - 5.0V. If the supply into the regulator isn't high enough (ie: just like a 4 cell supply may be) then this "dumping" effect may be the outcome of this limitation. I may eventually put a 6 cell or 2S (7.2V) li-ion combo in just to test....

Still, why stop there???? lol...

I noticed on the ESC the MOSFETs are layed out in a nice, "please modify me" sorta way, lol....

Since this is a FWD only ESC it doesn't have the traditional h-bridge design. The setup is 2 n-chan MOSFETS that are wired in parallel (ie: 2x1) for the main FWD drive and 1 p-chan MOSFET that is used to create the break function.


So I replaced the main FWD drive MOSFETS with some more efficient MOSFETS. :)


Comparing MOSFET specs.

Stock MOSFET (4410) = Rds(on) @ 4.8V = 0.017 Ohms
New MOSFET (IRF7456) = Rds(on) @ 4.8V = 0.0057 Ohms

So the new MOSFET is ONE THIRD the resistance of the stock MOSFET. On paper anyways, but still, the difference in the end result is VERY noticeable...

So with all that now done, here's the curernt setup. Looking a lot cleaner than before..... haha..


Now that's I've had some time to run with this chassis I thought i'd list some pros/cons I've found.


-FM design, more range, no freq clashes with competing Z's.
-Cost of electronics is equal (if not just a bit cheaper) to the cost of a Kyosho PCB.
-ESC can handle hot motors with the need to upgrade the MOSFETS.
-Can support 2S li-ion or 2S li-poly install natively.
-Steering strong, ESC powerband very smooth.
-4 Channels!! (I may do a headlight mod...).


-FWD only. ie: a track only type setup. (I myself don't find this a big issue).
-Tricky to install.
-ESC dumping issue with < 34 turn motors.

I'll try and get some video of this chassis in action soon!


2006.04.27, 11:22 PM
bro, you win. ;) that looks so good.

what if you wired the batteries to the 'power' line of the receiver? would that do any good in terms of getting power to the receiver and servo? (just a wild idea, probably not right).

2006.04.28, 02:15 AM
Very Nice!


2006.05.17, 12:12 AM
got some video of itin action, see here:


it's the red Enzo. you will notice it really pulls up in corners, this is the break function kicking in when the throttle goes to it's centre point. the difference between a nice coast through a corner and braking is very small on the trigger. This is bit of a bugger but otherwise it hammers along pretty fast so I'm happy, lol...


2006.05.17, 12:59 PM
got some video of itin action, see here:


it's the red Enzo. you will notice it really pulls up in corners, this is the break function kicking in when the throttle goes to it's centre point. the difference between a nice coast through a corner and braking is very small on the trigger. This is bit of a bugger but otherwise it hammers along pretty fast so I'm happy, lol...


I know you commented that this could be adjusted via trim, have you tried to adjust it so there is no braking at neutral throttle?

2006.05.17, 06:03 PM
yeah, I have. but just one step inthe digital trim above neutral results in a coasting speed that forces a larger cornering radius than what I want to take. So that's the bugger... I'm looking at adjusting the slew on the channel to allow less change that end of the throtte, the MX-3S allows me to do that :)

But I'm looking at lossening the diff a tiny bit and putting on a wider CF hplate. The thin one that's on there at the moment is allowing too much roll...

there's always more than one way to skin a cat, lol...