Exceltent! That was very helpful.
It looks like your switch went bad in the explosion. Unfortunately, you’ll have to take the pcb out of the chassis to replace the switch. The best way to remove the bad switch is to clip the off the 3 leads and heat up the 2 front posts at the same time until you can pull the switch out of the pcb…be careful, it gets hot!
I’m glad to see that none of the switch contacts are connected to ground. But also please check the round pad on P4 to see if that is connected to ground….hopefully it’s not.
Next, you’ll have to scrape off all the carbon from the burned spot. It’s easy to do with a wire brush, but if you’re careful you can do it with a razor blade or other scraping device. You should notice that there was a trace on the pcb going from the round pad of P4 to the middle pin of the switch. Surrounding the trace on either side is the pcb ground plane (a layer of copper covering most of the board that is connected to ground). Both the trace and the ground plane are usually covered by a layer of blue paint. In the photo i have scraped off the paint on the trace and some of the ground plane so you can see the copper underneath.
The explosion was most likely caused by a short between the trace and the ground plane. Possibly a drop of solder or small piece of wire was bridging the trace and the ground plane. In scraping off the carbon it is important to make sure that there are no tiny little drops of melted copper hanging around that could possibly short it out again. You’ll have to replace this trace with a jumper wire…you can use one of the wires you cut from the transformers. cut it to length and strip off only the amount of insulation necessary to solder the ends to the middle pin of the switch and the round pad of P4 (tin both ends first). make sure there is not strands of wire or anything that could bridge to the ground plane or to other pads near by.
When you re-wire the pcb back in the chassis make sure the transformer wires are not touching anything on the solder side of the board.