I have the same fan (Honeywell HO-5500RE) and I'm also missing the 5 VDC rail. I injected 5 V via an USB charger and the fan started working again. I know a little bit about electronics and also designed and build some PCBs but this 5 V power supply looks funky to me (looks like they saved every penny possible). Like mentioned before the 5 V are connected somehow to mains voltage on one side? I also check D1, 2 and 3 which are fine and R1, 2 and 3. I changed R3 for two 1 Mohm resistors in series to replace the old 2 Mohm resistor which only showed me 1 Mohm on the multimeter. Other then that I don't know what to check. Maybe someone can help or explain to me how the 5 v are created? Nothing looks burned or blown.
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