Radio Glitching and Troubleshooting
There aren’t too many things more annoying than to have an RC vehicle with a malfunctioning radio system. A common term for un-commanded vehicle operation is called a “glitch”. Glitches can be caused by interference or malfunctioning radio equipment. The purpose of this article is to detail the problems that cause glitching and how to solve them. There is no one answer to solve all radio problems, but once you know the causes of glitches, your troubleshooting ability will go up.
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Radio Equipment
The basic radio equipment required for a Savage is comprised of a Transmitter, Receiver, Steering Servo, Throttle/Brake Servo, Fail-Safe, ON/OFF switch, battery and sometimes a third servo if the Reverse Module is installed.
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Figure 1 - Radio Equipment |
Interference
Radiated and Conducted EMI
The signal from your transmitter to your receiver may be interfered with by either radiated or conducted interference. Radiated interference is usually picked up by the receiver antenna or the vehicle wiring. Conducted interference can be thought of as voltage spikes on the battery power lines, and are usually caused by electro-mechanical devices, such as electric motors, connected to the same power source. Electro-Magnetic-Interference (EMI) is used to describe all types of interference.
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Figure 2 - Nasty Voltage Spikes |
Radiated EMI may consist of electric fields, magnetic fields or both. Shielding can be used to keep electric fields from causing EMI, but will not work well with magnetic fields. Alternate techniques, such as using twisted-shield wire pairs must be used to keep EMI from magnetic fields at bay. Problem magnetic fields are usually caused by high currents and are not normally a problem with RC models. Automotive radios are usually manufactured with metal enclosures to keep the electric fields at bay. Although not a common solution for an EMI problem, one can try wrapping the receiver with aluminum foil. The best result is when the foil is electrically connected to the battery ground.
Sources of Radiated EMI can be other electronic equipment such as other RC transmitters, CB and Ham radios, and emergency VHF radio equipment. Equipment. Metal-to-metal is a common source of EMI. Interestingly, power line interference may be a contributing factor, but at 60Hz, with little in the way of higher frequency harmonics, it‘s outside the range of your radio system. If there are metal towers or fencing nearby, these can be a problem.
Metal to Metal Contact
A very common cause of glitching is often caused by metal-to-metal contact, such as moving or vibrating brake rods, running engines, and sloppy servo metal gears. This can be from a direct static discharge, such as when you touch a carburetor mixture screw with a screwdriver. This type of glitching will not affect operation of the vehicle. Two metal parts intermittently contacting each other also can cause a static discharge, but for a different reason as they are normally at the same potential. One theory suggests that the friction of rubbing two pieces of metal together generates broadband electrical noise due to electrostatic induction and the triboelectric effect. Two materials only need to come into contact and then separate for electrons to be exchanged. After coming into contact, electrons move from one material to the other to equalize the overall charge. When separated, some of the bonded atoms have a tendency to keep extra electrons, and some a tendency to give them away and the imbalance causes corona discharge, similar to television and radio interference during lightening storms.
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Figure 3 - Metal-to-Metal Contact |
Look for metal rubbing when you operate the controls and see if glitching occurs at the same time. Brake and throttle rods can be insulated from where they contact metal with heat-shrink tubing. Servos can sometimes generate EMI if the gear lash is too high from an accident or wear, and the metal-to-metal contact of the gear teeth can causes problems.
When a nitro engine is running, many mechanical parts vibrate. Usually it isn’t the engine itself that is generating the EMI, but the vibration that is causing other parts to touch intermittently. Make sure everything is tightened down and then do a visual inspection of the vehicle with the engine running to see if you can identify the problem components. Glow-plugs that are close to failure can sometimes cause problems, but it’s rare. Loose bearings can also cause problems, but it’s even more rare. Locating the EMI source due to a running engine is trial and error.
If a metal-to-metal EMI problem occurs with the Savage far away, it merely indicates that the signal-to-noise ratio has degraded to the point the problem becomes noticeable as the metal-to-metal EMI is weak to begin with.
Low Batteries and Fail-safes
When battery voltage becomes low enough erratic receiver operation can occur. The problem can appear to be very intermittent,, For example, the increase in current drain when the steering servo operates causes the battery voltage to drop further, and throttle servo misbehaves. Once the steering is at neutral, the battery voltage rises and the throttle then seems to operate correctly. This can become more apparent when the steering servo is upgraded to a higher torque unit.
Under load, the higher internal resistance of an alkaline battery causes the voltage to drop a lot. This effect is not as bad when using NiMh batteries For example, under high current draw, the NiMh may drop from 1.2 to 1.1, while the alkaline may drop from 1.5 to less than 1 volt. Although the substitution of alkaline batteries with NiMh AA’s in the stock battery holder will be a significant improvement, a dedicated rechargeable 6 volt (5 cell) NiMh pack is the best choice to power your on-board electronics. Although NiMh’s lose power over storage (even short term....i.e. it gets worse daily), NiCd’s are not recommended as the AA size in particular, is susceptible to internal shorts.
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Figure 4 - Rechargeable Battery Pack |
Fail-Safes are designed to cut the throttle and apply the brakes for a number of different conditions. One of these is when the Fail-Safe detects low battery voltage. Low battery voltage can occur not only when the batteries are low, but even with well charged batteries as the voltage can temporarily drop due to servo load, such as jammed steering servo. A test drive with the fail-safe removed will reveal what is taking place.
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Figure 5 - Fail Safe |
Wiring
Wire Routing
Re-routing servo, power and antenna wires may help resolve glitch issues. Try rerouting your electrical wires, especially if you have them bundled together and tied with Ty-wraps. Keep battery and servo wiring away from the antenna wire. Further separation of the battery wiring from servo wiring may help, and further separation of throttle and steering servo wires may be required.
It is easy to crush or tear wiring, especially servo wiring, when doing routine maintenance or repairs. Visually check for crushed or torn wiring.
Ferrite rings act as an RF choke and may be used to suppress noise spikes on wiring. Simply route the wiring through the ring several times. Make sure you use rings with an inside diameter large enough for the connector to pass through. Filter capacitors are also used and merely connect to the battery power lines. Commercial ones are available and can be plugged into a spare servo slot on the receiver.
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Figure 6 - Ferrite Ring |
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Figure 7 - Capacitor Filter |
Connections
All of the on-board wiring connections are subject to dirt and moisture. Take them apart, clean them as best you can with alcohol or contact cleaner spray and re-connect them. Using a little WD40 on connectors can help prevent corrosion.
Always pull on the connector when doing maintenance, not the wires. Pulling on the wires to remove a connector can cause an internal break in the wires that can cause erratic operation.
Water
The ingress of water into the receiver and/or servos can cause all types of radio malfunctions. Dismantling of the receiver or servos is required. Cleaning with alcohol helps absorb water from the circuit boards. Spray them, with WD40 to help prevent corrosion. Methods to help prevent water ingress include sealing servo cases with RTV, using grease around the servo output shafts and installing receivers in balloons.
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Figure 8 - Don't Get Your Electronics Wet |
Proximity and Range
When everything works fine up close, but as you move away problems start to occur, the radio signal is getting weak compared to the ambient noise. External sources of EMI, metal-to-metal contact are common causes, however sometimes the radio equipment needs tuning. Tuning is performed by electronic technicians with access to expensive test equipment and results in matching of the transmitter and receiver for maximum range.
Sometimes the servos will chatter when you are very close to the vehicle. This can be caused by the transmitter signal being so strong, it swamps the receiver input electronics and chattering results. This should not be confused with glitching and is normal for many radios. It is not a concern as everything should work fine once the distance between the transmitter and receiver exceeds approximately 2 feet.
Antennas
Antenna problems are usually associated with range issues, not glitching, however a few details on antennas is warranted as sudden loss of control can be similar to glitching.
Antennas are vertically polarized and point to the sky when in use. Do not point your transmitter antenna towards the vehicle or orient the receiver antenna wire in a horizontal positions, such as inside the vehicle.
Make sure the transmitter antenna is not loose and is fully extended. Corrosion should be checked for on the antenna mounting base and cleaned up. A transmitter antenna length is critical as if the tuned length is incorrect, the transmitter will lose a lot of it's output power. The loss is basically reflected back into the transmitter and damage can result. Modern RC transmitters are designed to be less susceptible to this damage and range checks often require operation at 100 feet with the antenna collapsed. For full range, always extend the transmitter antenna fully.
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Figure 9 - Do a Range Check Before Driving |
The receiver antenna length should not be altered and as long an antenna tube as possible should be used. The excess antenna wire must not be cut and should be coiled up inside the receiver box as far away as possible form other radio wiring. Coiling up the excess retains the electrical tuning length, but the gain of the antenna is reduced.
Receiver antenna length is almost always 1 meter (39.7 inches). This results in a nice ¼ wavelength antenna for 75 Mhz radios. For other frequency radios, impedance matching circuits internal to the receiver adjust for the theoretical mismatch. A receiver antenna length isn't as critical as some people think. It mostly feeds a tuned input circuit and the "Q" of the antenna wire is low, as well as the gain. Making it the correct "tuned length" improves the Signal to Noise ratio, so deviating from that only reduces the operating range a bit. If in doubt, make it a bit longer. The S/N may not improve, but at least a signal is getting to the receiver.
To replace a receiver antenna, the best method is to open the receiver and solder on a new ire of the correct length. A soldered splice will work, but once soldered and insulated with heat shrink, it may be impossible to insert the wire in the antenna tube.
AM, FM and Frequency Bands
The susceptibility of glitching from interference is highest with AM radio equipment. This equipment operates around 27 MHz and is close in frequency to CB radios. FM radios are less susceptible due to their higher operating frequency at 75MHz and type of modulation. Spread spectrum GHz radios are the best due to their very high frequency range being outside the band of typical interference as well as their frequency hopping design.
AM radios operating on 27Mhz may be statically more susceptible to outside interference as there is only 6 channels available. FM radios at 75MHz have 30 channels, so there is less chance of interference form other hobbyists.
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Figure 10 |
Crystals
Crystals are sold in sets with one for the transmitter and one for the receiver. Although they look and are marked the same, they are not interchangeable. When there is no response from a radio, double check the correct crystals are installed.
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Figure 11 - Crystal Set |
When a crystal fails, you should not get any response from your radio equipment, however intermittent operation can occur. Crystals are similar to a light-bulb filament in their construction. The crystal is connected inside to two wires and is suspended inside the case....just like the filament in a light-bulb. One wire can sometimes break away if subjected to a shock. Due to handling and orientation, the crystal can sometime temporarily make contact and this intermittent operation can appear as glitching..
To check the transmitter crystal, tap the case and move it in different orientations while operating the controls and observing the vehicle for a response. To check the receiver, remove it from the vehicle, but don't disconnect any wiring. Turn on the transmitter and receiver and operate the transmitter controls while moving the receiver in different orientations. Sometimes if you hold it just the right way, the system will start working again as the crystal makes temporary contact. If this happens, you know for sure you need crystals. If it doesn't respond, you still may need crystals, but it's a bit of a test.
Faulty Equipment and Trouble-Shooting
Although it seems obvious, the first thing to check is the batteries. Make sure they are fully charged and check the transmitter light(s) for brightness. Some batteries build up a slight invisible chemical film on the terminals . A bit of Scotchbrite or fine emery paper can be used to remove this film. Battery holders or compartments present a different problem as they are often plated copper and removal of the plating by this type of cleaning can expose the copper and result in corrosion. Being very careful, fine scotchbrite soaked with metal cleaner usually works satisfactorily. The good news is these types of contact terminals rarely suffer from a chemical film build-up.
Make sure your TX antenna is fully extended and your receiver antenna is not broken and that the excess is not coiled up near servo or battery wires. Keep the transmitter a few feet away from the vehicle during testing.
Glitching can be caused by defective transmitters, receivers, servos, etc. The most common fault is poor solder joints made during manufacturing. Mechanical wear on parts such as ON/OFF switches, steering and throttle control potentiometers and servo feedback potentiometers are another common problem. Faulty crystals, broken servo internals, bad connections, and batteries long past their useful service life, are other issues. Some of these problems can be difficult to diagnose unless you have spare components that you can temporarily swap and see if the problem goes away.
Does the problem only exist when the engine is running? If so, you will have to identify the cause. Look for loose parts and metal-to-metal contact.
If the problem occurs all the time, the first thing to do is find out which item is causing the problem. The easiest way is by substituting known electronics, one at a time. If spare components aren’t available, you can at least try swapping your servos around....so throttle controls steering and vice versa. See what happens? If your steering control knob now operates the throttle, but didn’t when connected normally, you know the steering servo is bad.
Also try one servo connected to the receiver at a time. A binding servo can cause the battery voltage to drop and it seems neither servo responds. In this case, try disconnecting the electrical connector for the steering servo and see if the throttle servo starts working.
Do you use a failsafe? Is it set properly? Does everything work fine with the Failsafe removed?
Servos and receivers can be dismantled. Look for green corrosion on circuit boards and clean it off. Look for broken wires, especially broken servo leads. Rinse with alcohol and spray lightly with WD40.
A somewhat common problem are ON/OFF switches that can cause weird things to happen, so check them out as well. One easy way to eliminate the ON/OFF switch, is to wire the battery directly to the receiver and see if there's an improvement.
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Figure 12 - After a Successful Troubleshooting Day |
