Here’s a funny little side road I went down over the course of the 2020-2021 holiday season when nothing was open, nothing was shipping even if it were open, and I had otherwise nothing build-wise going on. It was a good trip back to the days when I did these little hacky things more often, building and making something without really a purpose except to see it happen because it needs to happen. Even better that I ended up with a mobile, displayable simulacrum of the current trendy 80s-90s nostalgia that has fully engulfed pop tech culture.
It all started during one of my random van adventures (of course) up to North Georgia. Having passed a lot of what I call “knick knack stores” on the way to and from the Blue Ridge and Smokies, I decided one weekend to just spend my time perusing their wares.
You know what I mean – the “antique markets” and “junk stores” that pepper state highways and county roads, well outside of metro areas. I’ve some times collected vintage tools or neat mechanical things from them.
But this little place on the side of Highway 515 was special. That’s because when I walked in, this thing was just sitting right there on the main display counter:
What is that!? Well first of all, it’s a Dr. Inferno, Jr. After that, it’s a Tomy Omnibot, one of the outfalls of the 1980s robot toy craze after the first microcontrollers became economical enough to put in (at least high end) toys. This was, it seems, an over $1000 (in modern dollars) toy back in the day, and it had all the 80s computer trappings – a tape drive that you could record movements onto and play back, for instance.
There was no remote or other documentation or original packaging to go with it. Without the paired remote, there wasn’t really anything it could do besides “Be Collectible” which isn’t really my thing. I thought “Well, I already have several 80s vans, so why not add an 80s robot to the mix”? The thought popped into my head then that it would be fun to drive around at shows and meets, like #RadwoodBait but for robots.
And so this is the story of how I turned this Omnibot and a vintage AM radio into a contemporary dance duo that never actually was.
There’s really two independent parts to this story: The AM radio conversion, and the buildout of the robot itself. I feel like the radio conversion is actually the neater technical contribution, but nonetheless, here’s the index:
1. Repairing and Upfitting the Omnibot
Fixing up the Omnibot
I heard lap belts in the back rows are no longer the recommended way to keep your children safe in the car.
Anyways, besides the Omnibot, I also picked up a few old knick-knacky tools, including the pictured brass hammer for the Master of Benches.
The robot itself was in good shape – it had one of the arm guide links broken off at a plastic boss, and the wheels were missing small chunks of tread. Nothing major exterior-wise besides a few blemishes. The drivetrain took a little bit of wiggling to become ‘unstuck’, frozen grease and grunge if I had to guess. Much like my M.O. with vans, if it’s vaguely robot shaped at all, I can probably get it to drive.
The back lid pops off with a screw to reveal the battery bays. The main battery for the drive system and lights was a 6V sealed lead acid battery (which is long dead). The ‘computer’, timer, and tape deck were seemingly operated from 3 volts of AA batteries, the terminals of which were also long corroded off.
It’s a bit gross everywhere, like it clearly sat in a basement/attic with bugs for years. At each juncture I took the opportunity to try and get the detritus out with some bleach wipes and rubbing alcohol.
This thing actually pops apart very quickly once the six or so screws holding the body together are removed. There’s a LOT of what appears to be hand-wiring inside. I can imagine the assembly line for this thing being a jig holding all the body pieces in position as technicians install the wiring, and then the whole thing is closed up.
More corroded wiring and bug detritus abounded inside the shell. How’d y’all even get inside?!
To my dismay, the arms were not motorized. They’re all held in place by friction washers backed by springs. I was at least expecting the shoulder to be a lifting axis or something, as that’s all you’d really need to make the thing more animatronic. Alas!
Maybe this can be a future upgrade path if I choose to dive in further – adding an arm axis that never was!
The shoulder joint in particular is a mounted on an insert in the main body and has a set of O-rings to give it some more friction over just the spring loaded washer.
Two follower linkages keep the forearms parallel; one of these, as you can see, broke off its stud. It didn’t affect the operation all that much because there were three others, especially the one on its same side. I’ll likely just epoxy this boss piece back on.
As I was taking all of these joints apart, I was also cleaning up any worn plastic debris (powder) and then lubricating each joint with silicone grease as I was reassembling it. The grease didn’t make each joint looser so much as less grindy and prone to squealing.
This is what each arm looks like once disassembled. The claws are also unpowered, and just have a open and close lever that snaps the ends apart or together. I could see this end having a small servo embedded in the wrist joint to pull on this lever, if I went the full animatronic route.
I decided that the most immediately helpful replacement for the 6V lead acid battery was a 6.6V bank of A123 cells! Conveniently, two cells in a row was basically the same width and height, so this was a no brainer.
I whipped up a quick battery pack using the original plug cut off from the lead-acid battery, and a repurposed 3 pin servo connector as a balance tap connection so I can throw it on one of my 2S dedicated mini-chargers.
To get the bot base driving independently of the onboard controller, I decided to embed two of my leftover Vex 29 ESCs into the motor power path. This meant I had to tap power from the main board, and it was actually pretty nontrivial to find where.
Unlike most new PCBs, there’s no obvious “power bus” because the components are all through-hole and the power has to snake around to get places, using jumpers. I had to follow the original power input wire around on the board until I found where on the multi-position switch terminals it ended up.
Now when I turn the robot on, my newly appended JST battery lead gets power.
I had these de-husked Vex 29s left over from beetleweights of long ago, either Colsonbot or the first Stance Stance Revolution. They’re still my go-to for “Random small bidirectional DC motor driver that doesn’t need more than 12 volts and like 2 or 3 amps”.
The wiring was deceptively simple here. Because the receiver needed power as well, I just stuck the battery connector through a servo connector conversion into the receiver. The Vex ESCs are supposed to receive power and signal through that same connector. It’s only 6.6 volts anyhow!
(I’ve done the exact same thing for Stance Stance and Colsonbot, mind you. The receiver’s power trace is thick and short enough that on the order of single digit amps isn’t a problem)
I’ve closed the robot itself back up now after polishing the dome a little to get rid of blemishes.
For extra cheekiness, I gently peeled the original label off the lead-acid battery and then applied it to my newly made pack!
Converting the Futaba Conquest T4NL Transmitter
Here’s what I think is the bigger meat and thicker potatoes of this little holiday boredom project. A long (long) time ago (in a MIT far far away, at least now it is!) I collected a box of cruft which had a Futaba Conquest T4NL transmitter in it. It hailed from the era of “boxy and chrome” transmitters, and I always figured one day I’d shove a 2.4Ghz radio into it as a restomod. This was probably at least in the 2010-2011 timeframe, and it traveled with me through my multiple great cruft moves.
Well I think finally it’s time to do it! The idea came to me immediately when I brought the Omnibot home.
Like the Omnibot, it was in good mechanical and cosmetic shape, even if the batteries were long missing.
The radio of choice I was going to swap in is the “Microzone MC6C“. It’s one of many lower-end market 2.4Ghz basic model radios hailing from the vast ungoverned high seas of Jack Ma. I originally picked up two last year when I was putting Trashcopter together, since I wanted to see what the evolution of cheap radios has been like over the past roughly 10 years since I’ve owned two HobbyKing T6Av2s (which are still working, mind you, and exist for sale still!)
The upside of this MC6C radio, which made me select it over just getting another T6A/CT6B model to use as the transplant, was that it had mixing and servo reversing switches right on the front panel, something the FlySky/Hobbyking radio lack.
I’ve always been annoyed that you had to use a computer and USB programming cable along with a horrible VB6 application (or an open source one like DigitalRadio) just to do servo reversing – like mixes I can understand, but that’s some serious cost cutting to not even put reversing switches in. I’m hoping that maybe my exposé about these will help push them a little more mainstream in the robot community as a starter radio or a “class set”, like we used the HobbyKing radios for back in the 2.007 days.
So I went ahead and cracked open the spare one. There sure isn’t much going on inside the MC6C compared to the old Futaba. Miniaturization! Push the problem into software!
On the left side of the Futaba unit is the “RF Module”, so to speak. The larger motherboard on the right performs the sticks to servo PPM encoding. That encoding is done by a dedicated IC, the OKI MSL9362RS, specifically made for 4 channels of digital proportional RC PPM encoding. The combined PPM signal is modulated with the 72 or 75mhz carrier that’s generated by the RF module, and off we go to the races.
Before I started unsoldering things, I decided to do a little poking and prodding with the MC6C. See, there were unpopulated connectors inside on the main board, and unpopulated connectors mean “expandable features”. I figured I would see if the radio could be capable of more. By following traces, I determined that the unpopulated connector was an unimplemented Channel 7, and that it was 3-position (or potentiometer) capable, even. This was confirmed by putting a servo on the receiver’s Channel 7 as I fiddled the contact with tweezers.
I determined at this point that my plan of attack was to go a lot more in depth than the usual “Old Radio Revival” which seems to largely entail putting a modern radio’s RF module’s PPM input on the PPM output stream of the old radio. That would limit me to 4 channels only because of the fixed-function ASIC handling the encoding.
With the T4NL’s chassis containing a whole lot more holes than it actually had buttons and switches (as the same chassis would be used for several different models, such as 6, 7, and 8 channel ones, with dual rate settings or other functions), I decided to do it the hard way and fully embed the now 7-channel MC6C control board and transplant all the switches over, to make a retro 7 channel radio!
The problem to solve now was to decypher the wiring of the potentiometers that make up the joysticks. There was an additional complication, too. The MC6C is a modern computer radio and has digital button trims (click the button, move the servo center a little, save it in flash). The Futaba T4NL had analog trim potentiometers.
Separate ones, even. Most cheap radios that still have analog trims just have the potentiometer body on the trim lever so you can manipulate it separately, but it’s the same pot. This thing, however, had the separate potentiometers on each channel feeding into an analog voltage summation circuit on the motherboard.
The dual row of resistors on the right near where the wire to board connector comes in is the voltage summation. Between the three resistors in each junction are four pads, making the four channel voltages. Through some poking and probing, I discovered values and voltage levels for this circuit while under operation:
I dunno who else would do this, but if I’ve learned anything from this website, it’s that someone else will. So here’s the handy dandy wiring guide I’ve made! It’s interesting to note that the servo reverser switches just flip the polarity of power supply to the channel in question.
The next step is going to be deconstructing the T4NL’s joystick circuit and making it compatible with the MC6C inputs.
So here’s the wiring cleaned up into something that’s spliceable. I’ve made a few wire jumps to power the pots from the same wire, for one.
The next step was to discretize the voltage summation circuit of the mainboard onto the joystick’s trim potentiometers. They now feed the main pot via a high-value resistor.
What this does is let the trimpots influence the voltage output, but not by that much because the main pots have a much lower resistance and dominate the output range.
Through some trial and error, I found that a 3.4K resistor piping the trimpot into the main potentiometer gave me about 20% adjustment range in the signal, which is more than enough as a “Trim” function.
So the “hard electricity bits” of this conversion are actually done. By this point, I had 4 channels of joysticks which had known voltage output ranges.
The next steps were largely mechanical integration. I decided to take a hint from the T4NL case and stick the servo reversing switches out the back of the transmitter (Those 4 little slots were their locations). This area would interfere with the sticks, though, so I decided to hang them out of the battery compartment since I was planning on using a much smaller lithium battery.
The cut was made with a Dremel and a regular carbide burr bit in multiple passes to slowly whittle the line down further each time, until it broke through.
The MC6C reverser switch board used to be attached to the main board using some rigid standoff pins. Because of their move to the back of the radio, I replaced these pins with some jumper wire so they can make a 180 degree turn and fold over.
A little 3D printer nugget will hold the top of the MC6C board level, and its bottom will sit against the former telescoping antenna mount.
I also made two little nuggets to space the servo reverser board apart from the battery compartment’s rear panel. It turns out the switches stuck out really far into the battery compartment if I didn’t. This spacing gets them basically flush with the underside, enough to still manipulate.
See? As another cheeky relabeling, I removed the servo/mixer panel sticker from the front and glued it to the backside.
At this point, I had the transmitter powered on for power testing. Everything seemed to check out, so it was safe to proceed with adding the additional channel switches in. However, what I found was that the servo centers and travels were way off. Obviously, there’s not going to be any guarantee the potentiometers are going to be the same value, or in the same position at all between two radios separated by around 2.5 decades.
Here’s what the final integration looks like on the inside. I ended up removing the board spacers for the servo reverser board and letting the switches stick out again, after finding that it interfered with closing the case all the way. I transferred the switches from the MC6C case, and added a third 3-position switch. Maybe in the future this would be my flight mode select switch if I made some kind of retro-drone.
My last “Hmmmmm” moment was trying to sort out the stick calibration problem. What would I do if I were the enterprising managing designer/engineer of a board that had to take four analog inputs from potentiometers which weren’t guaranteed to be 1. in the exact same center position once mounted or 2. give the exact same range of travel, and 3. can’t be tested and calibrated individually for assembly/expediency reasons?
I’d make the final assembled device capable of running a calibration. My more expensive radios have this as a function, selectable from the questionably designed UI of the LCD screen. But what are the chances the MC6C just has a “Hold this button or close this jumper to run a stick calibration”?
100%, really. I noted a set of mystery jumpers when I first took the thing apart. At the end of the day, you want the factory calibration method to be accessible and easy to use for the assembly workers. A single big 0.1″ jumper is pretty damn out of place given everything else on this board is small surface-mount components.
So why not. I plopped the jumper in and booted the thing up. Immediately, it went into a flashing and beeping mode, which is obviously special. I swirled all the sticks and switches around a few times and left the sticks and trims centered, then powered it off and removed the jumper.
And that was it. All 4 channels now gave the same outputs for my test servo.
To really sell the look, I reached out on the robot combat Facebook groups to see if anyone had in the depths of their cruft piles a “rubber duck” antenna for use with these old 72/75mhz systems. It turns out someone did, and it was glorious. It looks extremely period correct when off, but of course once I hit the power switch, the orange and blue LEDs of the MC6C control board shine through the old battery meter.
Of course, this antenna isn’t connected to anything on the inside. The MC6C has a small internal PCB antenna, which I dropped behind the mainboard once it was mounted in the T4NL chassis in the same orientation. On my other MC6C (used on Trashcopter) I actually unsoldered it and added an external WiFi antenna using an RP-SMA fitting.
And that’s how we got this seriously #RadwoodBait photo at the January “Not cars and definitely not coffee” show. I’ll just say that the Omnibot is excruciatingly slow outdoors – it’s a fine speed for a house toy, I mean, but it took a good long while to get anywhere out here in the parking lot of the mall.
Plans for this thing? I’m not sure If I actually want to go back and try to engineer a movable shoulder joint. While it’ll be neat and all, this is a very reversible hack and it’s nice to just have. This project will, for the foreseeable future, just live in the house and be a thing I can pull out and demo. When the Radwood events come back around, I’ll bring it by!