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Discussion Starter #1
I installed one of these new 2-stage variable speed units over the weekend. It has been working beautifully until this afternoon. When it would not start, I found that the fuse on the control board was blown. Of course when I replace it, it just blows again.

Any ideas how to isolate this to make sure the control board is the problem, and not something downstream?
 

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Boards grounding out some where, double check the wiring to the w1-w2 connection and make sure that they are correct on the thermostat, if not the fuse will blow.

BJD
 

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Discussion Starter #3
Well, it was working fine wired the way it was for three days. Now, it still blows the fuse with no wires connected to the thermostat. A simple DC ohm check at the fuse to the 24 V common gives about 1.6K, so there is no dead short--until the power is applied.

Could it be that the microprocessor is turning on some relay at power up that is connecting a short on the 24V? I would think that nothing would get activated at power up, especially with the thermostat disconnected.
 

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Yea I would say something went haywire there in the board, hope you still have the paper work for it.

BJD
 

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Discussion Starter #5
I ordered a new board yesterday, but I still didn't feel confident about it, so this morning I pulled the board out of the furnace and started doing a little signal tracing with the ohmeter. I believe that I found the problem:

In addition to just driving the thermostat, this board does actually rectify the 24 VAC to make a little power supply for something. The diode for the half wave rectifier is dead shorted. That means the 330 uF holding capacitor is essentially straight across the 24 VAC. At 60 cycles it looks like about an 8 ohm impedance and so about 3 Amps AC current will flow. Since the board has a 3A fuse, it all makes too much sense.

When the new board comes on Tuesday, I'll let everyone know if I was right.
 

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I would guess that you work in the field of this type design, as I have never even attempted to repair a board. However it sounds like you may have something there.

Good Luck
BJD
 
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