Power Supply Visual Inspection
First visually inspect the board. Most problems are visible such as a melted transformer winding or a burnt component such as the surge suppressors RV1 RV2.
Check the thin white ceramic resistor R3, the corner wire can break away. This can cause the collector plates to get power but not the ionizer wires.
Continuous cell sparking (for months) can burn out components or wiring. Resonant sparking causes elevated voltage swings that can break down the insulation on the high voltage wires. Check for scorching.
Often the fuse blows when the switching FET Q1 shorts, we believe from line spikes. Check the fuse continuity. A blown fuse typically reads 4.4M ohm because of circuitry in parallel.
The fuse is soldered in and not replaceable. If there is an internal short the fuse blows so to prevent any components from flaring up. Replacing a blown fuse will not fix the problem.
Honeywell does not release the circuit board schematics. Service tech's want to replace the complete board and be confident. Honeywell does not sell the discrete components. We do not do component level repair of the boards.
Replacing the complete board with a new one with a 2 year Honeywell warranty would be the most reliable repair.