‘’MODEL RADIO CONTROL ELECTRONICS’’
A historical collection of circuitry and technical data
By Terry Tippett and David Caudrey
THE NOTES KICK OFF WITH FAULT FINDING THE MICRON RECEIVER KITS:-
General If assembled correctly; all four receivers in the micron range should perform correctly at switch-on. There is little or no variation between range (sensitivity) and other characteristics of correctly assembled versions of the same type. (It is even difficult or impossible to pick out a ‘good one’ to keep for yourself).
The FET receiver generally will show slightly more range to complete loss of signal compared to the other versions, (probably owing to increased front end stage gain.)
Coils that have been cut wrong and mounted the wrong way round. IF coils may have had the centre pin cut too short. The winding loops around this pin and if cut too short will cut the winding. (Check primary pins for continuity using low ohms on meter)
Sometimes capacitors get mixed up and you may find a 47p and a 47uf in the wrong places or similar.
Have a sample PCB to hand or do a ‘pencil rubbing’ of the boards before construction, as it is possible for customers to join two small copper lands with solder so that it looks correct as one land. (Particularly mini Rx)
Look carefully at the 104 caps, either the yellow type or the blue type as after soldering, the leg can become detached from the side of the capacitor, shown by a crack around the outside edge of the cap. This fault only occurs if the capacitors have been mounted very close to the board where the thermal shock of soldering is increased. If in doubt another 104 cap can be touched to the bottom of the PCB, across the suspect cap, during test to see if the problem clears.
Look for the obvious, as many times, IC’s are put in the wrong boards or the wrong way round. Check that only Futaba, Fleet, Multiplex, JR, or GWS, receiver crystals are being used in the receiver.
Check that only the transmitter manufacturers crystal is being used in the transmitter. (use of a different make of Xtal will almost certainly result in an ‘off frequency’ transmission.
The decoders of the receivers rarely produce problems providing component values are correct. Very, very rarely a significant static shock (type that stings your finger when closing the car door) can knock out the Cmos chip and this is shown by scope input readings to the chip (normally clock @ 4volts & reset ramp of around 3 volts) being clamped at below 1 volt. Indicating that the chip inputs have gone low impedance and the chip needs replacing.
Often customers use flux on the boards when soldering which unfortunately has an acid content and therefore adds many unwanted resistors to the circuit. This condition can be detected visually with residue on the boards. The only possible cure is to clean the residue from both boards using a toothbrush soaked in methylated spirits but often the flux has impregnated the board and satisfactory operation cannot be regained and the receiver is not recoverable. Replacement is the only answer.
Receivers that work but show low range this mostly points to the antenna input bits. Happily there are few of these parts involved. (The 159 coil, the 27p capacitor, the antenna input cap and the capacitor feeding pin16 input of the 3361 chip. Often the flex antenna can be shorted to ground with a solder whisker on the PCB or a stray wire from the flex antenna remaining on the board surface and touching the metal coil cover. When cleared with a model knife, normal range is restored. Perhaps the wrong capacitor has been inserted across the 159 coil. Is the coil the correct way round? If the 159 coil responds to tuning, then the lack of range could be further on in the circuit.
If the front end is checked out and OK then a possible lack of range could be found in the filter section of the receiver. ‘The filter section’ has its input from pin3 of the 3361… filters the 10KHz spot frequency…feeding it back into the 3361 pin 5. The filters involved vary with the receiver type. The standard and Comp receiver use a transistor between the filters with associated resistors/capacitor. The filters rarely go wrong but the transistor can be inserted with its legs wrong and associated resistor values need checking. The transistor gives around 10/12dB gain when fitted correctly. If you have an oscilloscope, the following can be checked. With the Tx on the bench, with about 25 cm of aerial, pin 3 or the base of the transistor will show around 0.1 volt of mixer output. If the transistor is working OK then there will be 1/1.5 volts of IF at its collector. (as seen on the scope). It is worth mentioning that the 3361 works well without this extra gain as in the mini receiver.
The amp in the mini Rx is also used to increase white noise of the whole circuit so that with the transmitter switched off, there is a pile of noise activity at pin9, which bombards the 4015 decoder to keep its servo outputs quiet. Note: - the mini receiver works differently and without the IF amp there is a much reduced noise level at its 3361 pin9. This lower noise helps to keep the 4017 decoder servo outputs quiet when the Tx is switched off.
Voltage levels around the circuit. I must admit that I do not have any record of voltage levels. I often made sure that the receiver board was getting 4volts supply from the decoder board (or slightly more,) but beyond that always used the scope to prod around during faultfinding.
Remember that after trying to get a receiver with a fault working, the coils could be well out of correct setting. This does not matter for the 159-antenna coil, as the receiver will still work at close range at any possible setting of this coil.
The setting of the IF coil however is critical to a quarter of a turn to get any response at all from the servos. Resetting visually as compared with a working receiver or a new replacement coil is a useful start.
The ‘Transmitter Power Meter’ kit available from micron is sensitive enough to detect the oscillation of a receiver Xtal stage if its antenna is held very close to the crystal. Also but not so convenient maybe, a spectrum analyser will pick up the receiver crystal stage by simply holding the input probe close to the Xtal.
jittering or servo noise using the Micron FET receiver.
Although range and general performance of early dual conversion micron
receivers seemed OK, reports from some parts of the
MICRON TRANSMITTER GENERAL NOTES:-
The very first ‘Micron’ transmitter circuitry that I assembled was actually fitted into a ‘redundant’ commercial transmitter case and sticks. The transmitter had developed a fault that was not repairable but the hardware was still excellent including sticks, switches, meter, antenna etc. Some ingenuity was necessary to secure the two Micron printed circuit boards in place but the end result made an excellent transmitter working on the 35MHz band!
Transmitter electronics kits are not available from Micron now. The seven-channel circuitry changed in recent years owing to the obsolescence of the dedicated ‘Motorola’ R/C coder chip. The replacement coder is interchangeable with the earlier type and now uses ‘bog-standard’ easy to get electronic parts, which are readily available from most hobby electronics shops.
There is little point in commenting on the earlier circuitry as inevitably most problems involved the special IC, which is now unobtainable. The only practical remedy for repair of these is replacement with the later version coder board.
The photo to the left is now 38 years old! It shows
one of the first
Having assembled several of the later coder boards and seen other peoples efforts, the faults found were as follows: -
Blown 4017 IC. This is usually caused by incorrect battery wiring giving a reverse input voltage! The other circuitry survives but it is worth replacing the 22uF. Usually a new 4017 solves the problem but the battery wiring must be checked before switching on again.
Note that the coder circuit will run without the IC plugged in at around 1 KHz which can be seen at the yellow output wire on a scope, or even heard using a crystal earpiece. This test shows that most of the circuit is functioning except the IC. If there is no life, check component position, in particular the correct positioning of the transistor legs into the board.
Working but a channel(s) is missing this fault can often be traced to an incorrect setting of one of the joystick pots. All the stick pots must be pre-set so that their wiper is at mid position, when the sticks and in-flight trims are at a centre position. This can be checked using a multimeter.
It is also possible that one of the crimped connectors of the plug-in flylead from the stick, has not located correctly in the plastic shell and as a result, the crimp has pushed out of the top of the shell. Relocating the crimp, making sure that the small plastic fingers of the shell are pushed in to secure the crimp, is usually a cure.
Another possibility is a blown diode (usually caused by accidental shorting of the board to the edge of the metal case, during testing and adjustment). Often this can be confirmed using a multimeter on low Ohms setting across each diode in turn (there are 10 of them!), to find the ‘odd one out’, followed by replacement.
Coder board quality, I have seen more than one coder Printed board now which was not up to the usual crisp copper etch that is normally seen. On these boards it was necessary to carefully inspect the copper lands and cut through with a model knife, the several bits that not intended to be joined! So look carefully with light behind the board.
Section is a smaller board that feeds the antenna and like the
coder board, if assembled correctly, does function at switch-on. The outputs of
this board has been passed by the ERA (Electrical Research Association) for
Not working at all this points to resistors or capacitors in the wrong places. Remember if you find one wrong then there will be another where that one should have been! Look for coils that have had the wrong pins snipped. These will need replacement. Check the transistor legs are going into the correct holes on the board. Check that only Micron or Futaba crystals are being used and ‘Tx’ is indicated on the crystal tab. Try another crystal in case the one fitted is duff.
Reverse Polarity fault. This always shows itself as a burned brown/black 100R resistor in front of the output transistor. Unfortunately both the output transistor and RFC will need replacing. The oscillator coil always survives, as does all of the other circuitry.
THE MICRON DUAL CONVERSION R/C RECEIVER
The fundamental advantage of Micron’s receiver front end is acknowledged in the RSGB ‘Radio Communications Handbook’, 5.16. As a result, the receiver Jfet does handle strong out of band transmissions particularly well.
Microns use of the FET is interesting in that some of the known disadvantages of this device have been addressed.
JFET’s, used in mixer stages, do like, a high oscillator drive to work well. Unfortunately JFET’s also have poor isolation of the oscillator frequency and this results in the oscillator frequency being transmitted via the receiver antenna! Although this transmission could still be termed as ‘flea-power’; Just imagine thousands of such receivers on a good flying day, all transmitting on a frequency that has nothing to do with radio controlled model aircraft! The interference to other users of the radio spectrum would be at risk and it is important that R/C receiver emission is kept to an absolute minimum.
The Micron FET
receiver uses dual conversion
Another problem with JFETs was the divergence of characteristics from one device to another but technology has advanced and JFET characteristics are now much more controlled, with even selected versions of the same device available.
The Micron FET receiver circuit diagram comes next and surprisingly, its almost as simple as the Mini receiver that they do, except for the two transistors added on at the front! I will try and run through the circuit as best as I can without causing too much pain for the reader!
The 35MHz parent transmitter signal is picked up by the 85cm flex antenna. (Length is not critical). This excites L1, producing a 35MHz signal input to the BF244A (gate). The 27p/4.7uH trap grounds the 13.5MHz image frequency and the 24.3MHz oscillator leak through via the 15p cap. Meanwhile the 24.3MHz plug-in crystal oscillator circuit output is injected via the 0.1 cap to the BF244A source terminal and mixing of the two frequencies occurs, producing a 10.7MHz output at the BF244A output. There are several other frequencies produced by mixing but the 10.7MHz crystal filter rejects these.
The selected 10.7MHz signal is passed on to pin 16 of the Motorola 3361 chip. Mixing takes place for the second time using the on-board 10.245MHz Xtal oscillator. This produces a 455KHz signal at pin 3.
This signal is filtered by the 10KHz filter (CFU455HT) and then amplified in the chip, with the FM content being detected at pin 9.
The 4k7 and .022 cap at pin 9 get rid of white noise on the output signal, leaving rounded signal pulses (from the transmitter) of about 0.5v peak to peak. Note L2 needs adjusting to achieve this. Pin 12 is an input to a squaring amp with outputs at pins 13 & 14. These two outputs (4v pp) are used to clock the standard Cmos counter chip, giving up to 8 servo outputs. The 2N3904 provides an extremely servo noise free supply of around 4volts to the whole receiver. The ‘image frequency’ rejection of this receiver is around 60dB which means that transmitted signals on the 13.6MHz band (image band) would have to be a million times stronger to cause a significant interference problem. This compares with ‘normal’ single conversion receiver image rejection figures of around 10dB, allowing 34MHz band signals to cause havoc when only 11 times stronger! The 34MHz band is for ‘Ministry of Defence’ use and has been little (if any) used over recent years.
A HOT TIP !
If you use one of the cycle pump type de-soldering tools, try pushing a short length of silicone fuel tubing on to the nozzle end so that just a couple of millimetres protrudes from the tip. The resulting ‘soft end’ seals around the solder joint better as the tool is used and also reduces the recoil kick back. The silicone tube is also unaffected by the solder iron heat!
MODEL CONTROL TRANSMITTER OUTPUT TESTER CIRCUIT
This next circuit lends itself not only for home checking but also club and quick model shop checks. The circuit checks for correct power output of any 35 or 40 MHz radio control transmitter is shown. These things are called ‘field strength meters’ and are a standard piece of electronic equipment in the service workshop to check the output power of R/C transmitters. ‘Field Strength Meters’ (as they are called) are usually based around a reasonable size sensitive 50uA moving coil panel meter. These are now listed (Farnell) between £20 and £30 each (before circuitry!). This circuit is based around the National Semiconductor LM661CN Cmos quad op-amp IC. The circuitry components should cost no more than £4.00! and it has greater sensitivity than the standard meter type. Transmitter output strength is shown by four Superbright red light emitting diodes. A correctly functioning R/C transmitter, will illuminate three to four LEDs at a distance of 10 metres away. Adjusting the length of the short telescopic aerial will allow all LEDs to operate at a shorter distance for indoor checking. With occasional use, a four AA alkaline battery lasts over a year (even occasionally leaving the thing switched on)
The OA47 diode seems to work best but more difficult to get. L1 needs to be initially adjusted to illuminate the maximum number of LEDs at a range of 10 metres or so. Once set that’s it. The Toko coil used is no longer manufactured but many are still in the pipeline and there are alternatives. Remember, if you set L1 using a 35MHz Tx then the unit will only check other 35MHz transmitters. If 40MHz Txs are to be checked, set L1 using a 40MHz Tx. L1/C1 form a tuned circuit at 35MHz. A 35MHz Tx will excite this coil and cause a resonance of L1. D1 detects this and a little current flows at 35 million times a second! into C2. This increases the voltage across C2 (slightly) in proportion to the power of the transmitter signal. The LMC660CN is a Cmos op-amp and has little effect on the input circuit. The op-amps are arranged as voltage comparators using the potential divider R1-R5. The resistor values are selected to give a 3dB step between op-amps flipping on. (each one showing twice the transmitter power output) So with a weak signal, IC1D output will illuminate LED4. As the received signal gets stronger, the remaining LEDs will illuminate in turn, until all four are illuminated.
An excellent practical layout of this circuit using Veroboard and some up to date components can be found at www.pm.keirle.com/
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