Tonight I am going to perform a simple repair on a 70 KVp 7 mA Dental X-Ray control, Model 770 made by Gendex Corp in November 1997. This x-ray unit was one of the best designed of the all the units I have worked on over the years. The fact that after 18 years it is still working and relevant to current dental x-ray techniques attests to this.
In the course of this repair I am going to describe the technology of the 1997 era units. Bear in mind that many improvements have taken place since then. For one thing, a lot of the dental x-rays taken today are sensed on digital sensors and actual film is no longer a common medium. This has required a change in the design of the dental x-ray units to optimized the benefits of the digital medium. This x-ray was designed with enough flexibility that it still works well with the newer digital sensors.
This unit failed when a wall actuation button became shorted out. This caused the unit to experience a long duration voltage to a couple of key resistors. These resistors are sized to handle the voltage that was applied but only within a duty cycle of seconds. The shorted switch caused the seconds to turn into minutes and the resistors continued heating until one of them fused and the other one carbonized. I have seen this same problem several times in the past and there is never any collateral damage except the resistors themselves. Here is a picture of the block schematic for the unit and a couple pictures of the damaged parts.
This is a poor picture, the bad resistor is along side the small black relay. A better picture of the replacement resistor is further down in the blog.
The middle 30 Watt resistor in the stack has a black band of thermal damage.
I have always been impressed with the simplicity of the design of the Gendex 770. The Head ( the part that actually produces the x-rays) contains the x-ray tube, and a transformer that produces the high voltage 70,000 volts and the voltage to heat the filament of the tube. The transformer, the tube and its support structure are immersed in special oil that serves as an insulator and a coolant for the components. Here is a picture of a dental x-ray tube.
The Anode is at the bottom and the two wires at the top go to the filament that serves as the Cathode.
When the proper voltage , about 108 volts AC, is applied to the two wires to the transformer of the x-ray head the filament heats up and the 70 KVp between the anode and the cathode accelerates electrons from the heated cathode to impact on the tungsten target of the anode. When electrons of the tungsten atoms fall back to normal energy levels they give off photons of x-ray energy. The purpose of the control we are repairing today is to provide a stable AC 108 volts to the head and to provide a way to time and control the duration of the applied voltage. Surprisingly there is no attempt to rectify the voltage to the x-ray tube. Since we are applying 108 volts AC the x-ray tube is actually being subjected to 70,000 volts AC between its anode and cathode. At its basic level the x-ray tube is a diode. It will conduct when the cathode is negative with respect to the anode and block the current when the reverse polarity is present. The result is that this dental x-ray produces a stream of 1/120 second pulses rather than a continuous beam. The control also times out a warm up period for the x-ray head. This consists of 22 pulses or 22/60 th of a second at which time a reduced voltage is applied to the head. This allows the filament to warm but doesn't produce signifcant x-rays. After the warm up period of 22 pulses the full power of the control is applied for the number of pulses selected on the control. At the end when I test the repaired unit I have made a video and you will be able to see this warm up and full power in action. Typically the duration of the full power x-rays will be from 1 pulse to 10 pulses ( 1/60 to 1/6 second ).
Here are pictures of the replaced resistors:
The 2 Watt 3.3 K resistor has been replaced
The middle resistor in this stack, a 30 W 15 Ohm, is the new replacement. This is the resistance that is put in series with the power to the x-ray head while it is in the warm up, 22 pulses, period. While the voltage applied produces more current than the resistor can handle continuous duty the 1/3 second of the warm up is too short to damage it.
Now having replaced the bad parts it is time to test the control to see if it is working properly. For my purposes I will not be testing it with an x-ray head. I will be testing it using a couple of 100 Watt light bulbs. I have connected the 100 Watt bulbs in parallel with the output that usually goes to the x-ray head. I have also jury-rigged a momentary push button to the trigger circuit of the control so I can trigger an exposure. This works well as I do not have to be concerned with ionizing radiation and I have only been asked to repair the control. Son Matt will hook up the head and test and perform calibration of the x-ray before he sells the machine. Here is what my test setup looks like:
Finally here is a short video of the actual test including my not so great moderator voice. If you have questions about dental x-ray units you can post them in the comments and I will try to answer them.
John
.
Top Comments