I have spent the last 30 years repairing the electro-mechanical equipment that is used by Dentists to repair and replace teeth. While I am officially retired at this time this doesn't stop my sons Mike and Matt, who currently own and run the companies, from giving me something to do once in a while. Last weekend Mike brought me a Cadco Dry Processor II that wasn't working properly.
Step one in the repair process: Understand what the unit is suppose to do.
This device is used to condition injectable impression material. For some of their tooth restoration procedures the dentist must make a very high quality impression of the patient's teeth and the area that they have prepared for a new crown. The precision material that they use must be the correct temperature and consistency for the process to work. This device accepts up to 8 cartridges of the material. When the conditioner is first turned on it raises the temperature of the metal block with the 8 holes to 65 degrees C. This is the idling temperature and the material is not ready to be used at this point. Just before the material is going to be needed the dentist pushes the BOIL button. This puts the processor into a cycle that raises the temperature of the of the metal block to 100 degrees C. The block is held at 100 degrees C for a few minutes and then cooled by a small internal fan back to 65 degrees C. At this point the READY light comes on and the injectable impression material is ready for use.
The complaint was that the unit would turn on and warm up to the 65 degrees C but when the BOIL button is pushed it fails to reach 100 degrees C and the READY light never comes on.
Step Two in the repair process: Try to understand the hardware that is being used to accomplish the goals of processing the material.
I opened the unit up and this is what I found on the inside:
Since the unit was partially working I plugged it in and began probing the circuit. The 8 Pin DIP just above the blue transformer is a TS272C CMOS Dual Op Amp. Exploration would show that this Op Amp was sensing the resistance of a Thermistor that was attached to the metal block. The thermistor was in fact tied to the non-inverting inputs of both sections of the dual Op Amp. One side of Op Amp was controlling the temperature regulation at the 65 degree C level and the other section controlled the temperature regulation at the 100 degree C level. Depending on which part of the cycle the unit was in the outputs from the Op Amps were selectively driving an SCR that was delivering half cycle line voltage to the small heater element embedded in the metal aluminum block. My measurements showed that Vdd for the dual Op Amp IC was 11 volts, The reference voltage for the 65 degree Op Amp was 5.6 volts and the reference voltage for the 100 degree Op Amp was 8.2 volts. Voltage from the Thermistor varied from 2.6 volts at room temperature to 8.2 volts at the 100 degree level.
This is the back of the Aluminum metal block. The wires in the middle go to a small heater that is driven by the SCR. The wires on the right edge attach to a Microtherm L10 130 degree C switch that is being used for protection in case the SCR were to short out and cause the unit to over heat. The Microtherm switch was hooked in series with the mains power line to the unit. The yellow and black wire further to the right side are going to the thermistor that is mounted to the side of the metal block.
Here is a better view of the position of the thermistor.
Step Three in the repair process: Start running experiments.
To this point the unit seemed to be working properly so I pushed the BOIL button and continued to monitor the upward progress of the temperature of the metal block with a digital thermometer stuck in one of the cartridge holes. The temperature approached 80 degrees C and then the Yellow LED which indicated it was in the boil mode went out and the temperature began to fall back.
Here is a closeup of the board. The white trimmer to the right of the Op Amp is used to calibrate the 65 degree reference voltage. You can see the thermistor leads entering through the hole in the board. To the far left we see part of the SCR and the PNP transistor that was being used to drive it. At this point I ran several more attempts to get the unit to cycle to 100 degrees and each one failed around 80 degrees C. Just for the fun of it I turned the board over to inspect the solder job. Experience told me that this was not likely the cause of my problem but it never hurts to check the solder joints for crystallization and besides it would give me some time to think.
The solder job was not impressive but I could not find anything wrong that would need resoldering and no indication of any other type of failure. The time spent inspecting the board with a loupe had not been totally wasted though. The brain had done its job and I now had a suspicion that my problem might lie with the Thermal protection switch. For a quick test I jumpered around the protection switch and pushed the BOIL button. I continued taking measurements of temperature on the metal block and voltages on Op Amp as the temperature climbed, 80C, 90C, and finally 100 degrees C. At this point the yellow LED began to flash and the temperature maintained for about five minutes. Then the little fan under the metal block came on and quickly brought the blocks temperature back to 65 degrees C and with a beep from a piezo speaker the READY LIGHT came on. Success!
I have really enjoyed fixing dental equipment over the years as no two problems are ever quite the same. For example while this is the first time I have fixed a Cadco Dry Processor II it had within it many things that triggered memories of other units and other experiences. The small thermal cut off switch that was causing the problem would have been simple to find if it had failed completely or even opened for a time long enough to detect but it insisted on opening just enough to spike the line and abort the cycle in the area of 80 degrees C then, quickly closing the circuit after the spike and acting as if nothing was wrong. Its marginal behavior made it a challenge and a pleasure to find.
John
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