Grotto 2: Retro

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Last update 20 OCT 2015

If you’re a dedicated RC antique collector, do not look at the following! These are the results of  hot-rodding old RC equipment to be legal and flyable. What’s the point of keeping RC history at home in a glass case when you could be out at the flying field enjoying it with your friends?

Futaba Front

This is an anodized Futaba F7xx that was given to me by Mike D’Amico new-in-box. This type was similar to the F710, but the PC board is definitely different. You’d never know it was a Futaba except for the PC board and some very small lettering inside the meter.Futaba Back

I left the encoder stock but did replace it’s polystyrene capacitors. The transmit chain and oscillator were removed except for a modulator transistor that I needed as an inverter. Originally I was using an FMA transmit module, but it went for a Burton, fortunately while the plane was on the ground. RF power is now provided by a 50 MHz Futaba TK module, but this required that I slice a rather large hole in the PC board. Oh, well! At least it’s 100% Futaba.

This Tx looks and flys great!

One thing interesting about this system was that the entire charger was built into the transmitter, including a transformer. The transformer is important to those of us that may grab a system being charged while in bare feet on terrazzo. Some transmitters from the “anodized” and “vinyl” eras  didn’t isolate the charger power line from the case. Yikes!


This is a picture inside the radio compartment of my Dave Platt Executor that I’ve been using to fly the above radio. The servos are Futaba FPS2’s. That’s not a typo, it really is Futaba Proportional Servo type Two. Dual linear outputs, reasonably quick and strong. The amps were replaced with some from Futaba S148’s to be compatable with modern receivers. If you decide to duplicate this, you will also need a 47K damping resistor in series with the arm of the feedback pot to settle the jitters.

Back in the late 60’s when these radios first started appearing, Kraft was king and Futaba’s reputation wasn’t very good. Rumors circulated that modelers were crashing due to poor output power and antenna dead spots. I remember flying these systems with no problems, though. Futaba even put out magazine ads touting their technical expertise to counter the rumors. Even today, though, if you look at the guts of these radios the quality is evident. With their low street cost, the Japanese would ultimately break the American RC market.

You should be able to find one of these on ebay for $50 to $100 if you want to duplicate this conversion.

MRC-Futaba AdMRC-Futaba advert

Futaba FactoryFutaba factory 1970

Let’s not shortchange Kraft! Here is a beautiful ’68 vintage vinyl Gold Metal transmitter like I had in the early ‘70s. Of note is the removable antenna with the military-style connector. This is also a 50MHz Futaba TK module conversion. The oscillator transistor was reused as an inverter. All other RF chain components were removed, including the meter circuit. The meter now acts only as a voltmeter in all of my conversions, as I had second  harmonic problems once when I left the RF detector circuit in.

Kraft Front

Phil Kraft

Phil Kraft w/Gold Metal guided Kwik Fli 3

Kraft Interior

You can see the Futaba module just above the left hand stick assembly. The in-Tx charger (the diode, big brown resistor hanging on the connector and a light bulb mounted on top of the battery case) still works and charges the modern NIMH pack hidden in the black plastic case.

Big, sturdy and simple, this Tx also flys great and attracts comments at the flying field. I got this one off ebay with good working KPS-9a servos for less than $50.


Here's an update! This is the same old '68 Kraft transmitter, but as you can see the original printed circuit board has been completely removed. The module is similar, in this case a Hitec dial-a-crash, but held in with velcro so it can be easily swapped out. The batteries are no longer nicads and have been replaced with a 1500mAh LIFE battery from Hobby King. It fits nicely in the original Kraft battery magazine. The encoder is completely new and is in first production beta testing as of this date. Please contact me if you would like to order one.


Pictured above is a newer Series Eighty Kraft, again with 1500mAh LIFE batteries and the new encoder. In this case, however, I chose to nibble out the back panel to fit a 2.4gHz TM-8 module. On the inside I used 1/8" fiberglas to make a shelf for the module at just the right height so that the module snaps into the back panel. Note the shelf assembly is held on the top by two screws used in the corners of the stick assemblies. The bottom end of the shelf has a leg (not visible) that rests near the bottom of the Nobel power switch. This leg provides support, but only attaches to the shelf and not the chassis.

Note also that the module contact for the antenna has a white wire connected to the original antenna base. That's just in case I want to snap in a VHF module and use the old long antenna.

Here's some more information about the encoder:

This encoder was developed for use as a retrofit encoder for vintage transmitters, as an encoder for control-line transmitters, or a basis for robotics transmitters.


Eight channels, six proportional plus two switched

Reversing on all channels

End-point adjustment on all channels

Direct hardware connection to Futaba style FP-TP style modules including TM-8, Hitec, Corona, etc.

Center and stick range calibration in-situ. No diddling with stick pots!

Center alignment to trim to 1.5ms

Stick pots interface with servo pigtails, allowing easy control order adjustment

Simple onboard programming, no need to connect to an external PC.

Dimensions 3" x 1.5"

Programmed microcontrollers are available separately for integration into custom circuits

Email me if you would like one.

Micro Avionics Front
Micro-Avionics was a big name in the middle sixties. Using mostly Orbit mechanics for their sticks and servos, they were a class act. Look at this beautiful anodized green transmitter vintage 1966. They really tried to minimize and smooth the appearance…strikingly stark by today’s standards.Micro Avionics Side

The hot-rodding is the same as the aforementioned Kraft, except that the inverter was not needed at the encoder to RF module interface. Note that this Micro Avionics has a positive ground!!! Despite this, the encoder is built with NPN transistors suspended between the case positive and -9.6V.

I remember seeing local legend Bill Youmans (later known in heli circles) flying this type transmitter in the sixties. He commented to a bystander that he had startup problems with the Tx encoder until all the capacitors were replaced. Likewise I have replaced the encoder capacitors with new monolithic or polystyrene throughout.

Here’s a couple of pictures of the Orbit stick assembly from the same Micro Avionics transmitter. I had to remove it to replace the pots, which appear to have been replaced at least once before. Obviously, it has seen a lot of use. I don’t know a lot about pots, but elected to replace them with some Clarostat conductive plastic pots (Jameco part#13995CL).  This interesting design used gears to reverse the stick direction to match the trims.

Orbit Stick FrontOrbit Stick Back
Doug SprengDoug Spreng w/Micro Avionics guided Thunderstormer

Flight testing:

All three of these transmitters were flown controlling the same test aircraft, an ARF Eagle with an LA40 for power. Servos were modern BB Futabas.


Futaba F700- A very pleasant, modern feeling transmitter, especially insofar as the balance. The angled antenna is very comfortable for me compared to the Kraft and Micro Avionics. The throttle ratchet is a little coarse, but acceptable. The sticks feel nice, but have a little center play typical of the time. Trims were the most comfortable to use of the three transmitters. This transmitter also has provision for the use of a neckstrap, unheard of in the US at the time, but now many of us have learned to love. I would have no problems using this as a daily driver. Interesting note: The instructions show the sticks to be used in Mode 3, my personal preference.

Kraft Gold Metal- Nice, but a little overbalanced with the 27MHz antenna. No overt neckstrap option, but I have seen people rig something off of the bottom screw of the noble switch. Stick assemblies feel similar to the Futaba copies, but the trims are a little smallish...this could be either good or bad under contest conditions. All-in-all a positive experience. Easy to clean and use daily, this is a nice addition to my flying antique collection. It's easy to argue that this was the start of modern economic RC.

Micro Avionics- This is a big sturdy box with long sticks. Some people have problems holding it. And this was way before anyone was considering neckstraps or trays. Maybe a strap loop could be rigged to the toggle switch, but this could be a bit dangerous. Flying the Orbit sticks leaves me with the feeling that I've been stirring concrete. They are monstrously stiff and I'm at this time convinced that they are stock. Not only are they heavy, but they leave the impression of being insensitive. Perhaps this was because of the stiffness. Often, I would find myself in a pilot-induced oscillation on final approach as I tried to transmit delicate commands using brute finger strength. The trims are overly sensitive and difficult to get set. This is perhaps the most miserable proportional transmitter I have ever used in my entire RC career. A flight is really fatiguing. You really can't appreciate how far things have come until you take a trip with this time machine!

Tech Details:

Here's a simplified block diagram of the conversion. The meter resistor shown is just what I found from experience. You may want to vary it a little to get the desired deflection.
Once you've located a point on your encoder where you get the pulse train shown, you can begin to wire up the RF module. Your pulse train will likely be upside down, and you will have to invert it to get the signal shown. The high level may be from TTL to's not too critical.  You can order  part number 517-647-07-36 from  Mouser and cut it down for a nice connector to the module. It's not a bad idea to add a little bypassing right at the connector power leads, say 0.1MF.
FYI, the positive pulses are fixed at about 350ns, the time between each rising edge and the following one is the pulse width for each channel (try 1.0 to 2.0 ms. centered at 1.5ms.) and the frame repeats at about 16ms.
The TK module is the more powerful Futaba module, but I've used the other one successfully, too.
A word about legalities. I'm told that you can't do this with a 72MHz module as they were type accepted in the Futaba cases only. This conversion is shown for ham band operation only. Once you have wired everything up, get access to a spectrum analyser and make sure the power output looks good and the harmonics and sidebands are 60dB or more down! I've received brand new modules that don't make spec, so don't skip this step!
Block Diagram

Circuit Details for Kraft:

Kraft Gold Metal and early seventies transmitters are easy to obtain on the used market as so many were sold. They were a bit pricey at the time, $400 for a four channel when the Japanese products were beginning to appear for half that. But time has shown that they are mechanically sturdy and generally outlast their contemporaries. There is no reason that a well-kept Kraft might not be flying in the next century if our hobby exists then. If you run across a PCS, get it! The circuitry is pretty much identical and it is great fun to show up at the field with a chocolate colored transmitter. My point here is that I am offering more detail on these particular transmitters because your chances of success (and satisfaction) are highest with them, IMHO.

The Kraft encoder (like most of the era) consists of a master clock that kicks off a chain of one-shots. The timing of each one-shot is controlled by a fixed capacitor and variable resistance, the latter being a mix of control pot, alignment pot and possibly trim pot. Each one-shot has an output that is high until it is fired and then it goes low for the RC period. The outputs are all ANDed together by tying them to the cathodes of diodes that have a common anode. At that point, the pulse train is complete, but fed into the circuit shown below for some final shaping.

Your first task in the conversion is to make sure the common diode point is connected to the 1K as shown. You may have a buddy-box circuit connected at this point with a .05 capacitor in series with the 1K. You can bypass all that junk by removing the capacitor and connecting the 1K directly to the diode AND. Note: You have the option of skipping this step and leaving the buddy box circuit intact.

Next, get rid of the RF chain. It's the circuitry not shown to the right of the first diagram below. At the very least, remove the crystal, the nearby transistor, variable coils and/or variable capacitors and any transistor with a heat sink. Generally speaking Kraft tends to demark the RF section in a literal "strip" that is easy to identify. I usually work my way down the strip shearing off components with a pair of dykes. The big trimcaps are best unsoldered.

Kraft Encoder

At this point, the circuit board should be cleared of everything but encoder. If you wish, you may cable up your power, turn it on, and scope a few points of interest. You will find that the output of the shaping circuit above is inverted to what a Futaba module wants. I have provided a circuit below that will solve this problem. You should have little challenge finding a clear spot on the board to construct this inverter circuit. If desired, you may put it on a separate postage-stamp-sized board elsewhere in the transmitter. The point in the first diagram titled "to RF chain" connects to "Kraft Encoder Output" in the second diagram. Resistor values in the circuit are probably not that critical, so feel free to try what's in your junkbox, but you may want to keep the ratio of the two base resistors close to that shown. I like MPS-A05s a lot so that's what I show here, but any garden-variety NPN should work fine.


Other Notes:

Keep the antenna wire as short as possible, say 2-3" from the module to antenna base.

Make sure the encoder board and module have good case grounds.

I like to epoxy a 100K multi-turn trimpot to the side of the meter and place it in series with the 9.6V bus (see block diagram). Adjust as required.

Kraft single-stick transmitters have a black wire coming from the rudder pot that attaches to the common +9.6V bus that runs to all the other control pots via red wires. Weird, huh?

Note: I have documentation for some old RC equipment as listed below. Email me if you would like a copy. Also, if you have any documentation for other RC equipment (particularly pre-1970) , I'd appreciate a copy!

Note #2:  Response to my documentation CD has been great. My friends have supplied me with  many additional  files  for  antique RC equipment, so the list continues to grow.

Note #3:  Most all of my documentation has now been shared with Tom Mavracic and may be found online at

Documentation available:
....Digital 1-8 receiver complete manual.
....Half-A 2 channel receiver schematic.
....Bantam Midget servo complete manual.
....Silver Seven Transmitter schematic w/narrowband mods.
....Multi servo instruction sheet
....Transmite schematic with suggested modifications
....Transmite instruction sheet
....Cardinal receiver schematic.
....Pulser instruction sheet w/schematic and suggested modifications
....3VTR receiver instruction sheet
....Actuator instruction sheet
EK Logictrol
Fly-tronics Engineering
....FAIL-SAFE various hookup drawings
....Midas reed receiver power and servo wiring
....Digital 5 manual w/schematics
....Matador Warranty
Glass City
....Unimatic instruction sheet
....Transmitter schematic.
....GDA 1205-5 servo schematic
Hobby Specialties
....GM Nicad instruction sheet
....GM Geni servo instruction sheet
....KP-5 Sport Series complete service manual.
....Gold Medal Series complete service manual
Micro Avionics
....4,6,8,or 10 Channel relay receiver schematic (tube).
....10 receiver schematic (all transistor).
....10 simultaneous tone transmitter schematic (all tube).
....2-10 relayless receiver schematic.
....4 channel transmitter (all tube).
....10 channel transistorized manual w/schematics & hookup OS,Controlaire,World
....SH100 to OS S103 Single servo hookup
....World S11,S12,S16,S17 servo schematics.
....World Mk4,Mk5 Rx & Tx schematics.

....Competition 10 receiver schematic.
....Article "Lil Bandit" homebrew reed transmitter.
Space Control

If you think this kind of stuff is cool, check out this great Japanese museum!

Here's another link to a Japanese site with a lot of nice photos.

But the very best one is HERE

Another interesting site is Joe Martin's autobiography on the Sherline website. Joe was involved in the manufacturing of RC in the sixties.

Our antique RC engineers anxiously await your challenges.

Reed Project
Here's some new pictures of my F&M Matador with the 2nd generation encoder. A number of my encoders have been installed in Orbit, Controlaire, and F&M vintage transmitters, including a couple of Krafts. Production of my synthetic reed boards has ceased as of this date, however I will continue to provide information for interested parties. A similar board is available from Phil Green of the UK. He can be contacted through his website

Winner of the 2007 VRCS Walt Good Technical Achievement Award!

Non-Proportional Encoder Specifications (General)
Power input:                 9.6V nominal switched power
                                    9.6V nominal from master trainer Tx
Power output                9.6V nominal to transmitter module
Modulation Outputs:            Directly compatible with Futaba,    
                                    1.0ms-2.0ms with 1.5ms center
Meter output:                Linear voltage, user adjustable by potentiometer
Recommended Tx module:             Futaba FP-TP-FM or        
                                                        FP-TK-FM on 50mHz
Recommended TX antenna length:   54”
Trainer connection:            Slave only via Futaba six pin DIN
Serial input:                   9600 baud. Programming use only.
Size:                             2.5” x 3.0”  63.5mm x 76.2mm

Non-Proportional Encoder Specifications (Reed mode)
Modulation Inputs:  One to six self-centering switches, SPDT
Retrofit Tx type:     Up to 12 channel reed
Digital Channels:             Six
Receiver output:             1: Aileron
                                    2: Elevator
                                    3: Motor
                                    4: Rudder
                                    5: Elevator Trim
                                    6: Aileron Trim
Trims:                           Elevator and Aileron dipswitch selectable
                                    as standalone channels or 20% mix
Recommended Rx:       To match Tx module, four to six channels
Servos speed:               Four speeds adjustable by dipswitch

Non-Proportional Encoder Specifications (Single Channel mode)
Modulation Inputs:  One spring action push button, SPST
Retrofit Tx type:     Single channel manually keyed
Digital Channels:             Four
Receiver output:          1: Rudder
                                    2: Elevator
                                    3: Motor
                                    4: Direct output
Trims:                           None
Recommended Rx:       To match Tx module, four channels typical
Synthesis model:            Two Cascaded Bonner Varicomps with
                                    three-tooth quick-blip motor escapement

Single Channel Notes
With reliability in mind, many single channel retro fliers have resorted to using some level of hybridization (if not outright substitution) with modern components. This is no doubt the way to go to guarantee safe flying. For the button-pushing crowd I offer the following details so you can design your own system, depending of course on the contents of your junkbox.
Here's a diagram to get you started.
In the middle you will see a rather conventional radio link. This could be a 2.4gHz transmit module and receiver, perhaps. Another possibility would be to use something like the module I used in the proportional conversions shown above along with a matching receiver, maybe on 50MHz. The first one of these I built used an actual 27MHz vintage transmitter with the tone modulation circuit modified to accept the digital encoder and give digital AM output. If you decide to take a similar route with 27MHz, I recommend the ACE TR403A BEC Narrow Band receiver. At one time you could get a full-range Futaba 27MHz AM receiver (R-114H) or the HPI RF-2 which was identical. Good luck on finding these now! The Ace is presently in production and I've had good luck with it to the edge of vision. In addition, you can plug a 2S Lipo directly into it and it will deliver 5V to your servos.
For the encoder, an NE5044 or similar IC will suffice. Another possibility would be to wire up a discrete transistor encoder using a vintage schematic. There are also some other experimental circuits available if you do a web search. The main idea, though, is to make the first channel change from a narrow to wide pulse with the push of your control button. If you can manage it, making the second channel change from wide to narrow might be a handy thing to do at the same time. Something like a PIC microcontroller is a shoe-in for this minor task and that's the route I chose. Actually I used an ATMEGA168. At any rate, a few lines of code is a whole lot easier than a bunch of wiring, for sure. Here's the guts of my code:

If you've made it this far, the rest is easy. All you really need to finish the job is a servo amp. Hopefully you have a discard and don't have to sacrifice a good servo. Remove the amp noting the connections for the pot and motor. Measure the pot resistance between the ends....I'll bet it's 5 KOhms (Maybe not, but I haven't seen one different yet). You will need to replace the pot with two resistors roughly half that value. I've used a pair of 2.2K as well as a pair of 2.7K and both worked fine.
At this point you may want to plug the system together for a quick test before adding the interface components to the escapement. Use a digital voltmeter to probe the voltage at the motor connections. Find the direction that gives a negative voltage until the transmitter button is pressed and the voltage goes positive. You will want to add the diode to the side where your positive voltmeter lead is now connected. I used a 1N4148 for the diode, but almost anything will do. The value for the capacitor is pretty sloppy, but you want at least a 6V one and large enough capacity that the escapement doesn't buzz when engaging. Be sure and strain relief your amplifier wires and provide a non-conductive covering of some sort. One more thing: If you happened to complete the servo amp mod without checking the motor polarity, you'll know why I suggested reversing the pulsewith on channel two!
We've had great fun flying with this system and I hope you find these hints helpful!

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