Monday, November 9, 2009

Repair of KS94 Temperature Controller manufactured by PMA

I recently repaired a KS94 Temperature Controller manufactured by PMA. The power supply in the KS94 had failed. The Switch-Mode-Power-Supply's Switching Transistor was shorted. Because the transistor was shorted, I removed the Switching Transformer to make sure it was still good. With my DMM set to resistance mode, I found that the primary winding was shorted to another winding. I drew a schematic of the SMPS to determine if this was a correct function of the Switching Transformer. It was not. The primary winding shorted to the secondary winding. This Switching Transformer was custom manufactured for this application and was not available for purchase. The Switching Transformer was a sealed epoxy unit, so rewinding it would be near impossible without destroying it in the process of disassembly.

The owner of the KS94 Temperature Controller needed it back quickly for an urgent production run, and a new KS94 unit was weeks away if purchased. Thus, another solution had to be worked out.

Ron Abts had me work out the necessary voltage requirements for the KS94 Temperature Controller. Then he went to our parts department and pulled two power supplies off the shelf. The one power supply would produce the proper voltages for the CPU Logic Circuits and the Analog Circuits. The second power supply would make the required isolated voltages for the 4-20mA output. I installed the power supplies, and tested the KS94 Temperature Controller good. The PT-100 Temperature Sensor Circuit worked and the 4-20mA output tracked the Temperature Sensor's changes good.

There are three output relays in this KS94 Temperature Controller. I tested them as part of the KS94 tests. I found them to have high contact resistance, so I replaced them as well.

The KS94 Temperature Controller is in the customer's machine and working.

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