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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22627
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I just replaced both the main board and defrost board in a

Customer Question

i just replaced both the main board and defrost board in a rtu model#50TCQA06A2A5A0A0 it ran fine with normal operation amps on both indoor and outdoor fan motors as well as rla on compressor. 35 min later the defrost board burnt out again in same spot. now the blue wire that should have 24 volts to pull in the compressor contactor has only 2.5 volts so i disconnected that wire from the contactor and put a jumper wire from y1 to the compressor contactor that has me cooling but what would cause my fps to burn out and if i traced the blue wire right on the main board it goes to my hps. please help
Submitted: 1 year ago.
Category: HVAC
Expert:  BillyHvac replied 1 year ago.


have you ohmed the hps to ground when the pressure rises?

Customer: replied 1 year ago.
No sir I didn't I'll be back at that job in the morning. Other than checking ohms is there any more advice to go off of.
Expert:  BillyHvac replied 1 year ago.

The defrost board is just a logic board with inputs and outputs...there is no load.

Thus if the board is burning out the short is external to the board but overheating a component. First you can install 3 amp fuse in the line you are concerned with...

Then I would check the hps itself. If it ran 35 min it is not a direct short but a can eliminate the hps for testing but it would also be interesting to see if it relates to pressure as the hps gets more pressure on it.

The culprit will be of course one of the safetys..