Cavity Tube Tester
Price: $595.00
Designed and built by: MBT Electronics
| King | 021-0038-00 | |
| Narco | 63002-0001 | |
| Collins | 277-0446-030 | |
| GE | C2080A | C-2080A |
| Richardson | PC2080A | PC-2080A |
Applicable Transponder Types:
- KT-76
- KT-76A
- KT-78
- KT-78A
- AT-50A
- AT-150
- AT-150A
- RT-359A
- RT-459A
Many other types also contain similar cavity tubes. EDO-AIR, Genave, Bendix, etc. These can be tested with this equipment.
This equipment is designed to test the transmitter tube used in most general aviation transponders. This tube, known as a “cavity”, is a self contained power oscillator using a single pencil tube inside a metal tuned circuit. The oscillator is modulated to form narrow pulses of RF energy at 1090 MHZ. The power output is 250 watts in a typical transponder. These cavity units are rugged and reliable, but have a finite life measured in hundreds of hours. They have several possible failure modes.
When bench repairing a transponder, the condition of the cavity is one of the most important considerations, because the replacement cost is high. The technician should know whether the cavity is good or bad before repairing other faults in the transponder. Unfortunately, in order to verify the cavity condition using the typical avionics bench test equipment, the transponder must be functional! It is a waste of time to repair the transponder’s power supply or receiver only to discover that the cavity tube is weak. The transponder may not be economical to repair if it has multiple problems, including the cavity tube. The customer needs to know the probable life left in the cavity before authorizing other repairs.
This test equipment was designed to enable the technician to test the cavity tube in different situations and determine the likely usefulness of the tube. The tube can be tested in an otherwise completely dead transponder. Operating voltages from the test set can be substituted into a partially functional transponder to activate the cavity and facilitate troubleshooting. A loose tube can, also, be tested independent of the transponder.
Tests can be performed on the cavity which are impossible using typical avionics test equipment. By controlling the filament voltage and plate voltage and viewing the resultant output pulse, the emission can be judged. Low emission indicates a poor or damaged cathode. The tube may work in the transponder with full filament voltage, but will soon fail. This ability to predict imminent failure of the tube is very useful. When a transponder is in the shop, for any reason, the tube can be checked. Sometimes tubes become intermittent due to loose parts inside. By operating the tube while tapping it on the bench, at different angles, and viewing the output pulse, this condition can be detected.
Most avionics shops have a shelf full of used avionics in unknown condition. Some of the transponders may have serviceable cavity tubes which could be used to repair other units. Why are these tubes not used? Until now it has been too much work to test unknown cavity tubes. This test equipment makes a quick screening of cavity tubes possible.
The pictures show the exterior and interior views of the cavity tube tester. It should be evident that the unit is constructed with high quality components. The enclosure is metal and the connectors are the best available. The controls are self contained pot-and-knob units with metal knobs. No cheap plastic push-on knobs are used. The circuit board is industrial quality with plated-through holes. All the components are standard, off-the-shelf, and available. The circuit board is mounted with metal brackets. The intention is to make it “tool box tough”.
These cavity tubes were pulled out of junk transponders. Some of these are probably good. How can you tell which ones are useable? Install them one at a time in a working transponder and bench test it? This is very time consuming. Some of these tubes are shorted and would damage the transponder power supply or modulator. It would take many hours to test these tubes this way. What about the quality of the tube? How much life is left? This is difficult to judge in the transponder, because it is operating at full filament and plate voltage. You cannot manipulate the pulse width and voltages.
All of these transponder cavity tubes were evaluated with the tester and labeled. Some are good and could be used. Five or ten minutes with the tester and you know the condition of a tube. If it is working, you can tell if it is “strong” or near the end of its life. If the tube is bad, you can tell what is wrong with it. Who cares what is wrong with it? You can get clues about what might be wrong with the transponder that it came from. For example, if the filament is open, better investigate before installing a new tube! If the tube tests “good” with the tester, most likely it will be serviceable in a transponder.
The tester requires a minimum of 12 VDC, but works best with about 15 VDC. The input is protected from reverse polarity by a diode in series with the input. The filament power supply is a dissipation regulator. It requires the same current from the input source as the filament draws. The filament in a cavity tube draws about 500 milli-amps, but the starting current is much higher. The tester limits current on the filament supply to about 800 milli-amps. The current limiter allows for accidental shorts without damaging the power supply.
The power source must be well regulated and provide a peak current of at least 1 A. If the voltage droops, the tester will not work correctly. The typical avionics bench test power source will work well. It is nice to have an isolated, independent, power source. The illustrated power supply is a switching regulator rated at 1.6 A. at 15 VDC. This unit is recommended. Several cheap wall transformer units were tested and found to have poor voltage regulation.
The tester is pictured connected to a cavity tube for testing. The clip-lead cables connect to the cavity tube and plug into the ground jack, filament jack, plate jack, and cathode jack, on the tester. The knobs at the top of the tester control the output of the jacks. The filament voltage is adjustable from 5 VDC to 7 VDC. The plate voltage is adjustable from about 100 VDC to 250 VDC. The cathode output is a negative going modulation pulse, adjustable from zero to about 20 us. The coax connection to the cavity carries the output pulse to the RF input jack on the tester. The RF input jack is a 50 ohm load. A detector circuit associated with the RF load provides a video output which is viewed on the oscilloscope. A typical cavity tube will generate enough power to provide a video pulse of about 1 VDC at the scope. The output of the current pulse jack is a negative going pulse representing the instantaneous plate current during the modulation pulse. This is viewed on the second trace of the scope and compared to the video pulse on the first trace. The output of the sync jack is a positive pulse which is used to trigger the scope.
The oscilloscope displays are pictured for a typical cavity tube under test. Note that the input to the scope must be terminated with 50 ohm loads. The pulses will not be true without the proper load. The top trace is the video pulse. The second trace is the current pulse. With 250 VDC, a good cavity tube will oscillate and generate a sharp leading edge video and current pulse. The top of the pulse is square and no droop is visible.
A good cavity tube will begin oscillation with about 120 volts on the plate. If the tube is weak, it will require more voltage before oscillation begins. When the voltage is not adequate, the start of oscillation is delayed, and the leading edge is not sharp. The oscillation does not start for a few micro-seconds. Note that the current steps up to an intermediate value as soon as the modulation pulse begins. This is due to the forward bias of the tube. When the modulation pulse brings the cathode to ground level, the tube draws plate current. When oscillation begins, the current greatly increases. With marginal voltage, the oscillation hesitates and then “pops on”. With more than adequate voltage, the oscillation begins instantly, but there is still a visible delay between the current pulse and the beginning of oscillation.
Testing Cavity Tubes
In the transponder with the normal voltages, the cavity tube will generate adequate power until the end of its life. It will then seem to die very suddenly. Actually, the emission from the cathode diminishes over time. It is not evident during a bench test, because the only parameter tested is the power output. For the bench test technician, this is frustrating. When testing the transponder, there is no way to vary any of the operating conditions of the cavity tube. Often a transponder tests ok on the bench, but fails soon afterward due to weak power output.
The cavity tube tester allows the operating conditions of the tube to be varied and the results viewed on the scope. The cavity tube is operated with very low plate voltage so that subtle losses in efficiency can be easily noticed. A weak tube may oscillate with 1000 V on the plate, but not with 200 V on the plate. With these minimal voltages, the output is directly proportional to the voltage. A weak tube stands out. After testing a number of new or good tubes, a weak one is very obvious.
The plate voltage at which the oscillation begins and the power out, is an indication of emission quality. In a vacuum tube, the emission is generated by a hot cathode and the filament voltage determines the temperature of the cathode. In a new tube, the rated voltage heats the cathode more than necessary, but after many hours, the cathode becomes worn out and barely generates the necessary emission. At this point, the tube still works normally, but is near the end of its useful life. When the emission drops too low, the power output begins to lessen. At this point, the failure is noticed. If the filament voltage is raised a little, the tube will regain function, for awhile! In the early days of television, this trick was used by repair technicians to “brighten” failing picture tubes.
When testing a cavity tube, these facts are used to judge the quality of the tube. The filament voltage is reduced until the oscillation cannot be sustained. A good tube will show good oscillation with as low as 5 V on the normally 6 V filaments. A low filament voltage with normal output is an indication of a tube with ample emission. If the output varies with changes in filament voltage near the rated voltage, the tube is at the end of its life. It may seem to work normally in the transponder, but will soon fail.
The plate voltage and filament voltage are interactive to a certain extent. During testing, different combinations of voltages can be applied. A good tube will oscillate with 6 V on the filament and 120 V on the plate, but with 4.6 V on the filament, 200 V on the plate is necessary. If a standard plate voltage of 200 V is used during testing, the filament voltage necessary for oscillation gives an indication of tube emission. This is a very sensitive test of overall condition.
Low emission is the most likely problem with a cavity tube, but other problems also occur. A high voltage short is possible. In this case, the plate terminal is simply shorted to ground. Unhappily, the tube inside is probably good! This often results in damage to the transponder power supply. When testing a transponder, the cavity tube tester can be used to assess the cavity tube before repairs are begun on the power supply. If both the tube and the power supply are bad, the transponder may be uneconomical to repair.
A short from cathode to filament is possible. This causes continuous output and also overloads the power supply. The technician finds the power supply burned up. Better test the cavity before beginning repair!
A loose part inside the cavity is a possible failure. This causes intermittent operation and is hard to detect in the transponder. The cavity tube tester can detect this condition. Tapping on the cavity while monitoring the waveform, will reveal any changes in output level or frequency. Remove the cavity from the transponder and connect it to the tester. Tap on the cavity from all directions until the problem is verified. This is much better than beating the whole transponder on the bench!
Using the tester as a signal substitution source
The cavity tube tester can be used as a source of certain signals when troubleshooting a transponder. This will save some time and speed up the evaluation of a transponder before repair.
If the filament dropping resistor is open, use the internal filament supply to energize the cavity filament. If there is a short, which may have caused the resistor failure, it will become evident without damage. The current limited power supply in the cavity tester will not cause damage.
If the modulator system is not functional, use the internal modulation source in the cavity tube tester. Disconnect the cathode connection from the transponder cavity and connect the cathode jack to the cavity. Set the pulse width for minimum width (ccw). Energize the transponder and adjust the pulse width control for a narrow pulse (1 us, for example). View the resulting transponder output on the shop test equipment. (Note: Do not use the load and video detector contained in the cavity tube tester. The transponder output power will be too high). Sync the scope from the cavity tube tester. Maintain a narrow pulse width to avoid excess duty cycle.
If the high voltage power supply is not functioning, use the plate voltage source in the cavity tube tester. Disconnect the high voltage connection to the cavity and connect the plate voltage jack to the cavity. Adjust the plate voltage pot for maximum. The power output will be very low (about 10 watts). Some test equipment may not respond to power levels this low. By viewing the transponder receiver video test point, you can tell if the transponder is replying. A shorted cavity tube may be the cause of the power supply failure. This will be evident when the substitute voltage is applied. The cavity tube tester has current limiting on the plate supply. No damage will be caused.
Conclusion
The cavity tube tester has many possible uses for the avionics bench technician. The high cost of replacement cavity tubes is a fact. The high cost of removing a transponder after failure is a fact. This tester will save money three ways. First, it allows the technician to be sure about the condition of the cavity tube before beginning repair on a customer’s transponder. Second, it allows the technician to predict approaching failure in a transponder due to cavity tube failure. Third, it makes possible fast screening of unknown cavity tubes in trade-in or salvage transponders.