GARAGE DOOR OPENERS--Electronic Test & Alignment Techniques (from early 1970s)


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There are lots of garage door opener systems. The typical one, however, is broken down into five sections (Fig. 106). One is the power supply which is the house current, 117 volts AC. This rather obvious section of the system is mentioned first because it is the first source of trouble in a large percentage of cases. If a short circuit develops in the wiring of the motor, or the door should jam along the tracks, too much current can be drawn from the fuse or circuitbreaker and the system cuts off.

On a rare occasion the fuse, circuit breaker or actual house current input circuit could be defective, but usually when the fuse goes, an overload is occurring in the door opener circuits. Test the door opener carefully ; it is causing an overload.

The other four sections of the system are the receiver, the motor and machinery, the transmitter, and the pushbutton circuit mounted in the garage. The transmitter cannot possibly blow a house fuse since it is not connected by wire to the system.

The trouble is usually in the motor and mechanical section. After changing the fuse, try the motor with the manual pushbutton in the garage. If the motor starts jamming, shut off the fuse or circuitbreaker immediately. The trouble is in the motor and machinery.


A mechanical checkout is recommended first. However, it must be done to decide whether the trouble is mechanical or electric. Disconnect the mechanism from the door and work the door the old fashioned way-by hand! You'll feel if the door is running on its tracks smoothly or not. If it is, reconnect the mechanism. The trouble is not in the door.

Should the door run rough, stick or jam, find out why. Perhaps the runners need lubrication or some foreign object is jamming it. At any rate, figure out the mechanical difficulty.

Watch any gears, pulleys and other parts .

Fig. 106. The five sect ions of a garage door opener are the transmitter, receiver, motor, wall button and power supply.


The main part of the opener is the motor. Its operation is the final result of all the electronic activity. The motor is quite tiny, rarely more than a quarter horsepower and usually less. The motor is activated by a capacitor. A capacitor-start motor comes up to full speed almost at once and can reverse itself easily upon electronic directions.

Capacitor-start motors have two windings. One is in the circuit all the time and is called the "running winding." The other, which is in series with the start capacitor and the switch, is cal led the "starting winding." As the double-pole double-throw switch is reversed, the starting winding is reversed. This DPDT reversing switch is mounted on the mechanism so that when the door reaches the end of the track, either open or shut, the switch is thrown and is ready for the next movement of the door in the opposite direction.

A centrifugal switch is also in series with the DPDT and capacitor. It closes the circuit each time to start a cycle, either open or shut. The DPDT simply stops the cycle and readies itself for the next cycle.

When the motor circuit is indicated as shorted because of fuse or circuitbreaker failure, the simple circuit can be tested for a short circuit with an ohmmeter (Fig. 107). The capacitor could short, the run winding could short and the start winding could short. A simple ohmmeter test will reveal it.

No. 177 : MOTOR OPEN-CIRCUIT TEST The motor circuit, due to the large current drawn through it during quick starts, could burn open. The starting capacitor could open up. The best test is to try a new one. The DPDT switch could open up, too. If that happened, the centrifugal switch would not be able to start the motor. The centrifugal switch could also open. The motor itself sometimes burns up and stops functioning. Here, again, the best test is a new motor.


If the door will open and not shut, or shut and not open, the prime suspect is the DPDT reversing switch. One side of it has probably opened.

Fig. 107. A capacitor-start motor uses a nonpolarized filter and centrifugal switch in the start winding.


Once it is decided that the trouble is not in the power supply or the motor-mechanism area, three sections are left : the push

button circuit on the garage wall, the receiver and the transmitter. Push the button on the wall. Does the door open and shut normally? If it does, the trouble is in the receiver or transmitter. Should the door not open or shut normally, then the pushbutton circuit is indicated. It is a simple wiring job that connects to the motor and gear box. Check it.


If you have a spare known good transmitter handy, this is the best test. Try to get the garage door to go up and down with the spare. If it will, the old transmitter is definitely defective. If it won't, the receiver needs further testing.

Unfortunately, a spare transmitter is not always available. Also, the amount of power that comes out of the transmitter is very tiny. You'll have to rig up a form of field strength meter. This is done with some wire and a tiny neon bulb. Wind a coil on a pencil that will respond to the approximate frequency of the transmitter. Attach the neon tester to the coil.

Then, place the little field strength indicator against the transmitter antenna. Try the transmitter (Fig. 108). If the neon lights up, RF is coming out of the transmitter and it is probably good ; the receiver must have problems. I say, probably, because some door openers have a modulated RF transmission. The RF could be good and the modulation missing or incorrect. This would be a rare occurrence, however.


For the particular transmitter you are testing, you'll need the schematic and service notes from the manufacturer. These transmitters are quite similar to a remote control model radio transmitter, except for one thing. The FCC has a garage door opener regulation which states that these transmitters can send out only a short timed burst of RF energy and then must shut off, even if the transmitter button is held down. This is to prevent any of these transmitters from sending out RF that could possibly interfere with aircraft radio communications. Door opener transmitters cannot be left on. The pulse can last for only one second out of every 30-second period. When the transmitter button is pushed, there is a one-second RF modulated pulse that quickly shuts itself off, even if the button is held down. If you want to try another pulse, you must wait 30 seconds. The unit won 't operate for that length of time.

The receivers are designed to receive the one-second pulse. It's actually, from an electronic point of view, a large segment of time, one million microseconds. The pulse goes through the receiver, the modulation is detected and the audio modulation is fed to a small relay instead of a speaker, like a radio. When the small relay closes it causes the larger relay to close and the motor starts. The motor pulls the door up or down and the reversing switch opens the circuit at the end of the track. The door is then ready for a run in the other direction.

Fig. 108. A transmitter output test can be conducted with a tuned winding attached to a neon.

The transmitter, usually transistorized, has an oscillator circuit, quite often crystal controlled if the frequency trans

mitted is in the high range, for instance 465 MHz. On the other hand, the frequency could be down in a low range ; for in

stance, 41 kHz. In the high range, the code signal to trigger the relay modulates the RF. In the low range, the code signal turns the RF off and on by switching in the transmitter.

Whatever the procedure, the receiver is constructed to respond to its own transmitter and none other. Otherwise, a next door neighbor could get your garage door open with his transmitter. An unlimited number of transmission systems can be designed by selective coding. A selective filter picks out a code even if it is a signal such as 250 Hz and other transmitters are sending out 260 and 240 Hz. Another consideration during design is freedom from noise. Noise could possibly open or shut the garage door unless it is carefully designed out. At the high frequencies, ordinary electrical static and lightning flashes have no effect. In the 41 -kHz range, noise is a problem, so they use what is called a double coding system. The door won't open unless both codes from a transmission are received. The chances of an accidental noise burst generating both codes at the same time is almost nonexistent.

Another type of transmission can be right down in the audio range between 2000 and 10,000 Hz. This is a transmitted RF signal and is completely free of noise or other trans

missions. The FCC has no restrictions at this low range. The main limitation is the size of antennas ; they have to be large.

You can work on any of the transmitters legally, since the higher frequency ones have a low enough power to be under the legal restrictions and the low-frequency ones are so low there are no restrictions.

A typical low-powered transmitter has one transistor in an oscillator configuration, with the frequency controlled by a crystal. Or it could be two transistors in a multivibrator arrangement. The factory service notes will tell you which one it is and what voltages should be where. You can't isolate the trouble any further than this. Once it is decided that the transmitter is defective, routine DC voltage and resistance tests are next. Some transmitters have a plug-in decoding device so the same transmitter can send out different modulations. The only practical test is to try a new plug-in unit.


The receiver is quite like an ordinary radio, except that it is fixed tuned and variably tuned. Also, its output goes to the relays and not to a speaker. Old-time garage door openers used one tube in a super-regenerative circuit. Then, 3- and 4 tube receivers appeared. Finally, the transistorized receiver was made.

The receivers are designed to be on continually. The tube circuits cost a few pennies a month to operate, but the transistor circuits cost practically nothing. Both tube and transistor circuits are designed with cutoff bias applied.

Therefore, no current, or very little, passes through the tube or transistor during the standby operation. When a pulse arrives, it overcomes the bias and some current is drawn for an instant. Since this occurs just a very few times a day, it is not even measurable in pennies.

The signal is picked up by the antenna, passed through an RF amplifier and then injected into a detector. The detector output is the modulation or code, which is converted into a DC signal that fires the relay transistor. The motor opens or shuts the door. When the transmitter is deemed good, and the pushbutton on the garage wall makes the door open and shut normally, the receiver is, by elimination, considered defective.


Should the garage door opener work, but not at a distance, the receiver has weakened. The door may open when you pull up next to it, but not when you transmit a pulse as you round the end of the driveway. If it is a tube receiver, test the tubes. A weak tube could be the trouble. In a transistorized receiver, the transistors also could be weak.

What's happening is that the transmission is not overcoming the bias on the amplifiers from any distance. This could be due to lowered plate or collector voltages or raised cathode-control grid or emitter-base bias. Approach the testing as if you had weak audio in a radio. According to the service notes, test the DC resistances and voltages as well as the tuning of the RF transformers. No. 184 : RELAY NOT CONTACTING TEST

The relay has the job of opening and closing, period. If you transmit a pulse and the relay is activated, that is, there is a click, then the receiver is working. Should the motor still not start after the click, the trouble could be in the relay contacts. Either they are not touching, or are so pitted or corroded they pass no current even when they do make contact.

There are the two relays (Fig. 109). One is the receiver relay and the other is the heavy-duty motor relay. The receiver relay triggers the motor relay.

Fig. 109. Typically, a motor relay in a garage door opener is a heavy duty type. It's opera ted by a tiny circuit relay.

The test is easy. Take a small piece of wood or a pencil and press the relay shut. Even though the contacts are corroded the motor will start if the relay is the trouble. Cleaning or replacement will fix it. Test both relays this way.


The relay has spring tension applied to the armature. The amount of tension is carefully designed. The strength of the magnetic field, under normal operating conditions, is enough to overcome the tension and cause the relay to close.

When the magnetic field gets weak, it will still attract the relay armature, but as the spring bends and builds up tension, the amount of magnetic flux is not enough to pull it all the way.

The spring, as a result, pulls back the armature quickly. The net result is a chattering relay. Usually, a weak output signal from the receiver is the reason for the chattering relay. Go back to the receiver and test it for weak amplification.


A major problem that occurs with garage door openers is unwanted operation. The door goes up and down by itself. This can be quite annoying and under some circumstances hazardous. Some kind of signal is triggering the receiver and

it is not coming from the transmitter. If there is a gain control on the receiver, try resetting it a t a lower threshold. If the gain control is cranked all the way up, the receiver gets overly sensitive in the same way a radio speaker blasts too loud when the volume control is turned all the way up. The more stages of amplification in the receiver, the more sensitive the receiver. Try different settings of the gain control, keeping it set as low as possible, of course.

If necessary, adjust the gain until it is almost off. You might even have to pull your car right up to the door in order to trigger the relay ; however, that is preferable to the door going up and down out of control.


If you can pinpoint the cause of the unwanted operation as being due to noise, in addition to desensitizing the receiver as above, it may be necessary to make the receiver more frequency sensitive. Perhaps you'll find the garage is near a high-voltage line or the receiver antenna is close to some other AC line. If so, you'll have to improve the signal-to-noise ratio at the receiver input or improve the peak alignment inside the receiver so it responds to a more narrow band of frequencies.

Make sure that the transmitting antenna is not enclosed under the metal hood of the car. If it can be attached to the car radio antenna, all well and good. If there is a tuning capacitor in the antenna circuit, try different settings until you get one that is the most sensitive. Park the car at the bottom of the drive and start triggering the door. If your distance is about 100 to 200 feet, you'll find that at extreme settings of the capacitor, the door won't open. At the correctly tuned setting, the door opens and closes beautifully. When you have the gain as low as possible and the antenna transmission as strong as possible, the improved signal-to-noise ratio will enable the receiver to respond correctly.


If your garage door is going up and down and you can pin the reason for the unwanted operation on a nearby transmitter, there is probably something wrong with your receiver and not the transmitter, especially if you need the gain control turned all the way up in order for your car transmitter to activate the garage door.

After testing all the tubes, transistors, DC voltages and resistances, if it's still happening, the receiver probably needs alignment. The receiver is a small radio type circuit with TRF type coupling between stages. With the manufacturer's service notes and your radio alignment equipment, align the RF transformers between the stages. At the correct peak alignment the unwanted operation should cease.


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