2wd Mod Brushed MR-03 Mini-Z Build!

[lambdapriest]

Hey gang, I posted a thread over in the Mini & Micro RC subforum - I really want to build up a BRUSHED Mini-Z to run in 2wd Mod - and I think I've talked myself into it.

I want to build a MR-03 (or maybe PN 2.5w-based) Mini-Z to run in 2wd Mod, but I want to use a brushed motor. I love the sound and driving feel of brushed motors, and I think it would be fun to put on my engineering hat and try to do something a little unconventional - run in 2wd Mod without a chassis car. I'm certain that I won't be able to get the car to handle as well as the cars of the people that actually run in 2wd Mod, but it'll be fun to see the looks on their faces when I beat 'em on the back straight - even if I can't hang anywhere else!

Here's my approach: Starting with an MR-03, I want to upgrade the electronics so that I can run a lower-turn brushed motor on 2s without blowing the MR-03 board. Having a nicer receiver with better responsiveness and higher resolution will help, too.

Because I love them, I might build it up as a PN 2.5w - but with the general idea that it's definitely not a chassis car - just a brick of plastic with a built-in servo and a giant motor.

I already have:

• A donor MR-03
• A FlySky micro receiver
• A 2s-capable brushed ESC
• A big brushed motor (PN 43t that's probably going to blow the FETs on my MR-03 board any day now)

So I need:

• A way to use the FlySky receiver with the built-in five-wire servo
• A PN 2.5w (probably)
• A bigger brushed motor (PN makes a 31t!)

My first big challenge is going to be to drive the onboard servo from the FlySky receiver. To achieve that, I've ordered some cheap sacrificial micro servos from Amazon. Based on my research, I should be able to solder the Mini-Z servo motor and "position sensor" (it's just a potentiometer!) to a 3-wire servo controller board, and then I'll be off to the races.

I was going to cut up the existing MR-03 board, or buy another one - but they're $40, and I don't like wasting usable parts or cutting things up if I don't have to - so I found the servo motor and the potentiometer at A-Main, shipped, for under half the cost of a new MR-03 board. I've also ordered some 30-gauge wire (should be right, I think?) to hook everything up.

Since I have several other projects that are WIP, I'm in no awful hurry to get this built, so I didn't get rush shipping on the servo motor or potentiometer. They should be here next week; if they arrive before I leave for a week to travel with my wife, I'll try to solder everything up - or maybe, I'll just desolder the servo motor and potentiometer from the MR-03 board that I already have, secure in the knowledge that I have replacements on the way; depends on how impatient I'm feeling, and how much free time I have.

At any rate - I'll keep you all posted! I've never done a build thread before, but I can already tell that this is going to be a lot of fun!

 

[WoodiE]

Looking forward to seeing this build come alive.

 

[lambdapriest]

@WoodiE - thank you so much! The potentiometer was supposed to come today, but it's stuck in Detroit. It might arrive tomorrow - if it does, I might have a little time to start tearing servos apart tomorrow evening or Friday morning. The servo motor arrived yesterday. I'm getting ready to take a trip with Mrs. Lambdapriest, but I'd love to have something actually going on this build before we leave... fingers crossed!

 

[lambdapriest]

I tried to get started on this before we left, but my first attempt at disassembling the servo and soldering everything on was a total bust; so over the last few days, since we got back, I tried again!

So as mentioned earlier in the thread - the first and biggest challenge for this build was always going to be getting a 3-wire servo to work with a Mini-Z MR-03 or PN 2.5w chassis, since their “servo” is built into the chassis and has no housing, and since the 5 wires (two motor wires and 3 potentiometer wires) are soldered directly to the MR-03 controller board.

I picked up the official Kyosho brushed servo motor and Kyosho potentiometer (linked in OP).

The first order of business was disassembling the servo. On my first attempt, I was able to desolder the motor and potentiometer from the board; however, the motor terminals were soldered in to the board, and removing them left a gaping hole, and I was unable to actually get the new motor wire to mate up to the board (probably a skill issue - I’m still pretty new to soldering). Since I still had 3 more servos from that 4-pack (which cost about $9), I decided to start fresh.

The second servo came apart nicely; but this time, I tried to disassemble the motor in such a way so that I left the terminals as “tabs“ sticking up from the board. I had to use a micro Dremel to cut away some pieces of the motor, but eventually it came apart. To make my own life easier, I also left some of the potentiometer lead material connected to the board, so that I’d have an easy piece of metal to tin.

It was about this time that I started getting concerned that the potentiometer on the donor servo had a different range of resistance than the Kyosho servo. I did a little research and figured out how test it with my multitester, and I was in luck - they’re both rated at 5 ohms! It was pretty easy to solder everything up; I put some heatshrink tubing over both sides of the potentiometer wires, just to be safe. I’m not sure if 5 ohms is the standard for servo position sensor potentiometers, or if I just got lucky, or what; but I’m grateful that this one “just worked out”.

I plugged the servo in to the receiver on my 21.5 buggy and fired it up - and while I’m not 100% certain that it’s perfect, the servo motor responds to input from both the transmitter and the potentiometer. While I was traveling, I ordered a PN 2.5w chassis (part #900100) and a PN 31t motor (part #133931) - and I grabbed one of PN’s brass 4th servo gears (part #MR3056B) as well, because everyone seems to like them and I’m going to be in there anyway.

My next step is going to be to pull the electronics out of the donor MR-03 Readyset and install the servo motor and potentiometer, to see if they work correctly in the car. If we’re all clear there, then I’ll be in great shape; and if not, well, I’m going to have to get creative with my Dremel and figure out a way to mount a micro servo on the PN 2.5w.

 

[lambdapriest]

Worked on this again for a little while this morning.

I tore the donor MR-03 apart and tried to install the servo motor and potentiometer. Everything installed into the chassis just fine - but when I connected the servo to my receiver and turned everything on, the servo motor went wild and immediately ate the 4th gear.

Fortunately, I ordered a pair of brass 4th gears (linked above), so I popped one in and powered everything back up. Again, the motor started up at full power.

If I was very, very careful to center the potentiometer, it would stop moving! But as soon as the potentiometer turned even the slightest, the motor would run at full power.

It was then that I realized that I had probably soldered the servo motor on backwards - so when the potentiometer turned to the left, the servo's controller board tried to turn the servo back to the right, to center it back out - creating an endless feedback loop.

I desoldered and re-soldered the motor with the wires swapped; but when I was putting the motor back into the chassis, I accidentally sheared off one of the leads, at the can.

Undeterred, I used a hobby knife to carve out the hole with the wire stub - pulled the end of the can off of the motor - tinned the wire - slid it back through the hole - and soldered it back to the brush.

And then promptly sheared it off again. (Skill issue.) Pulled the end of the can off. Tinned. Carved out hole. Soldered again.

Finally got it all re-assembled, and... it worked! Almost. Kind of. Now, when I turn the wheel to the right, the servo turns to the right; when I turn to the left, the servo turns to the left. But the action is slow and weak, and sort of... it's like it's cutting out. The servo's gear train doesn't seem to be binding - I think I may have damaged the brushes when I was trying to solder the wire back to them.

My club has the aftermarket PN servo motor for sale for $5.99... I might run up there in a little bit and pick up a fresh one. If this works - I might try to actually build up the PN 2.5w and get the car running, since my club's having some Mini-Z racing this weekend. Fingers crossed, breath held.

 

[DavidB1126]

We be needing some running video once your done! Or next time at the track.

 

[lambdapriest]

Oh, without a doubt... plenty of close-up photos of the thing, too. But I'm getting ahead of myself - I've got some work to do!

 

[J’sCrawler]

I can't wait to see this