Where To Go From Now
Now that the out-of-bounds 5VSB output is reined in, we still have the power-up failure issue to fix. Time to revisit that EC4 capacitor.
I hooked up my multimeter to measure voltage across EC4's leads again, plugged the power supply in, turned it on and still got 0V. Something must have blown somewhere between the auxiliary transformer winding, the capacitor, its rectifier diode and whatever else is left. Time for some more reverse-engineering.
Remember when I said the AUX2 output seemed oddly convoluted? Now this is a classic flyback output: an output winding with one of its ends connected directly to the output side's reference point, the other end connected to a rectifier diode, an output filter capacitor and a zener diode across the capacitor to limit the maximum voltage across it. ZD4 is that scorching-hot 18V zener mentioned earlier and R29 must be a current-limiting resistor for ZD4 and, by extension, EC4.
Why is R29's value unknown?
Because its markings are partially burnt off and what remains of them does not seem to make sense based on the previous schematic. Read one way, it it seems to be 5?6, and read in the opposite direction, it would be 9?5. On my multimeter, the resistor reads open, and a value over 100MΩ would not make any sense. A blown resistor in series with the transformer winding would definitely explain why the main outputs still refuse to turn on after changing most of the caps: no current through that resistor means no charge going into EC4 and no power for the APFC and PWM chips.
What do you think the burnt character is supposed to be? The value needs to be sufficiently low to let enough current through to power the main PWM circuit but not so low as to cause the zener diode to become searing hot. My best guess is that the missing middle character must be an R, since that would match what is left of it and 5.6Ω (5R6) makes reasonable sense for this location in the circuit. The only SMD resistors with plausibly adequate value I have in stock are 10Ω in 0805 package. Let's hope the 1208 package on the PCB was selected by ChannelWell more for its qualities as a potential jumper or excess inventory than for power dissipation. Since the 1208 resistor is glued on the PCB, I will have to solder my replacement 10Ω on top of it.
Soldering a 0805 on top of a 1208 is fairly easy: create a small solder puddle on the pads at both ends of the 1208, position your 0805 part on top of it, hold it there with either a fine-tipped tool or glue, dip your soldering iron tip in the puddle, drag it over the 1208 and 0805 terminals to create a solder bridge, repeat on the other terminal, done. If the bridge keeps breaking up, add some more solder.