mr-02 40Mhz FM Conversion....

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ph2t
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mr-02 40Mhz FM Conversion....

Post by ph2t »

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 shits 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...

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Here's the ESC, bloody small!

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

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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.

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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!

Image


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....

Image


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

Image


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.....


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Post by ph2t »

UPDATE:

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.

Image

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.

Image


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.

Image


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


Image


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..


Image




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


PROS:

-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...).


CONS:

-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!

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Post by SuperFly »

That's pretty dang cool. I really like how it's all internal.

So that's about $80 in parts?
Image

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Post by Clint »

Very nice. I've been doing the same to all my cars. MX-3 and that GWS rx
work out very well together. After using the MX-3, i cant use any stock-ish radio anymore. xmod being the biggest pos.

If you end up wanting reverse, you can probably fit a novak spy in there.
the spy is 90% plastic. take it out of the case and it'll fit in most
anything(including zzmt's). whats nice is that you can power it with
as little as a single lipo..so it should work fine with 4xAAA.

i usually check the hobby shops for broken spy's and offer ~15 bucks.
fet replacement, as you know is cheap and easy. Newark has a good supply of fairchild semiconductor fets.

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Post by ph2t »

thanks fellas... :-)

@superfly, actually mate it's quite cheap, at approx $60USD for the parts (not inc shipping, but that should be even cheaper for you.).

I got the RX and Crystal from www.aircraft-world.com. I got the pico servo from www.rcmart.com but you can most likely pick it up either locally or on ebay US for cheap. I got the GWS esc from a local mob in melbourne, but again, in the US they should be everywhere. The capacitor cost like a buck, nothing much at all.

You don't need to upgrade the fets on the ESC, the upgrade only increases the (already quite solid) performance.

@clint, thanks for the info man.. Originally I was looking at 1:18scale ESCs, the Associated XPS is too big, the Robitronic I couldn't get anywhere and the Novak SPY was out of stock at rcmart.... It's good to know that once it's decased it is smaller. I wasn't to keen on a $60 gamble, lol.... Even then it would have to sit up the front on top of the chassis infront of the oil damper mount (where nelly usually goes....). I may still do that with an xmod EVO conversion....

I still like the fact that it's all internal. You can just see the micro RX crystal behind the antenna post in the pics, lol....

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Post by ph2t »

OK, the plane ESC was a good first mod, but not having reverse and the fact that the brake kicked in when I would come back to neutral on the TX was giving me the shits. IF you saw the mr02 video I posted not long ago you will see my car (the enzo) kick around the corners in a knee jerk sort of fashion. This was due to the brake issue.

So in the end I upgraded to a real car ESC. But this had it's challenges. A lot of the micro ESCs on the market are two PCB's sandwiched together in a (relatively) compact case. The problem here (when it's decased) is that it has a very "tall" profile making it impossible to fit within the mini-z chassis.

Both the Novak SPY ESC and the AE Micro ESC (comes on the RC18T/B/MT cars) are very tall in their profile and I would of needed to of mounted them high up front which I didn't want to do.

So I had my eye on the little known Robitronic ESC.

Image

This bugger is small. Only a single PCB size and compact dimensions made this ESC the one for me!

Here are the specs from the Robitronic website.

• Input Voltage: 4 - 8 cells
• Forward, intelligent proportional brake and reverse operation
• "Smart detection" circuit for brake and reverse control
• Pre-wired with micro-style connectors for battery and motor
• Direct soldering tabs for easy rewiring
• Dimensions (of case): 22.7 x 22.5 x 5.9 mm
• Max. Current: 10 Amp
• Weight: 11g

Here it is decased.

Image

It wasn't hard to attach. I just soldered the 3 wire signal loop from the RX to the ESC and soldered the power and white motor wires as well..

The key here was to orientate it the right way up so I had access to the programming button..

Here's some more pics showing the detail.

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Image

And of course, it's all in the chassis!

Image


So I'm happy with it. Testing will happen today at betty's track and we'll see how she goes....


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Post by Clint »

nice pickup. seems to be 29.99 euros for that esc from the manufacturer. but everyone is selling it for 41-50. :-? looks like they pop up on ebay germany every so often.

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Post by ph2t »

yeah, they're a bugger to get... I got mine from ebay whilst looking for something else, I was lucky....

I think rcmart sold them but they don't anymore:

http://www.rcmart.com/catalog/advanced_ ... ce_range=0

bugger....

I'm going to give it a good run today and depending on how it goes, I may replace the MOSFETS on the ESC with my own....

I have stock MOSFET issues, lol....
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Post by color0 »

With only 10A capability on tap, ph2t, you'd have to replace them; I don't think you've ever NOT blown stock FETs, lol...

Still looks clean as ever, though you've now got a bigarse hole in the back. :lol:
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