Blower Output Stuck at 100%

I just finished building a second HM v4.3 board and the blower output is stuck at 100%. Everything else is working fine. The HM is available on the network, all controls on the HM work, and the servo opens as it should when the output % is increased manually. I know that the damper and fan (a Micro Damper) work because they operate correctly on the first HM that I built.

After reading over the forum I found this post by Ralph Trimble which provided a troubleshooting procedure that I tried: Ralph's Post

I changed the HM to operate in pulse mode rather than voltage mode and tested the voltages on the BS170 MOSFET located at Q1 with the following results:

• When the blower was set to 0%, I got the following voltages on the indicated pins of the BS170: center pin (gate) = 0V; left pin (source) = 0V; right pin (drain) =.02V.

• When the blower was set to 100%, I got the following voltages on the indicated pins of the BS170: center pin (gate) = 3.25V; left pin (source) = 0V; right pin (drain) =0V.

I also tested the voltages on the FQU11P06TU MOSFET located at Q3 and got the following results:

• When the blower was set to 0%, I got the following voltages on the indicated pins of the FQU11P06TU: center pin (drain) = 12.09V; left pin (source) = .02V; right pin (gate) =12.09V.

• When the blower was set to 100%, I got the following voltages on the indicated pins of the FQU11P06TU: center pin (drain) = 12.09V; left pin (source) = .02V; right pin (gate) =12.09V. (No changes in voltage from 0% blower setting).

All of these voltages were tested with the black lead of my voltmeter on the ground pin at the power supply and the red lead of my voltmeter on the indicated MOSFET pin.

My electronics knowledge is basically high school level, so I was hoping that someone here could help me troubleshoot this HM as I am building it for a friend and really want to get it working. I assume there is a problem with the BS170 at Q1 but don't know how to confirm it. Any help is greatly appreciated!

Thanks

David Dismuke

Sounds like your problem is with Q1 to me, make sure it has the right part number printed on it. Make sure the solder joints are clean, also check solder on R14. Look for stray solder around Q1 and on the lead that goes from it to Q3 for shorts to ground... the entire board has a ground plane over it (should be easy enough to see with the naked eye) so make sure there isn't a large blob of solder somewhere that can short on the ground plane. If you overheated Q1 soldering it could have fried, you could try to replacing it, or if your solder skills are up to it you could pull the bs170 from the buzzer circuit and try it on the blower.
Posting closeup pics of the board can be helpful in cases like this, maybe somebody can spot a problem for you.

Here is a picture of the board. My soldering skills probably need some improvement, but the first board I built worked perfectly and this one is good other than the issue with the blower circuit.

Picture of the board

I checked Q1 and it definitely is a BS170. Unless someone sees something else that looks like a problem, I am going to order another BS170 and try replacing the one at Q1. I don't know what else to do at this point.