Soot is carbon particles, which is electrically conductive, just like tiny metal shavings.
Resistors used to be made of carbon, but now are made of more stable materials, like metal oxide or carbon film.
Soot all over the PC board is like throwing a handful of powdered metal onto it. It’s a wonder it didn’t catch fire.
Blowing it out with an air hose probably got most of the stuff out, but a soft bristle brush, like a paint brush, in addition to air blast would have been more effective.
I used to work for Tektronix field service and when we got in oscilloscopes from the field for repair, they were usually filthy.
The first thing I would do is WASH THEM INSIDE AND OUT WITH SOAP AND WATER.
That’s right soap and hot water!
And a brush!
After a thorough rinsing with clear water, blowing out all the excess water with an air hose, I’d put it in a oven overnight at 130°F so it would be thoroughly dry the next morning.
Never had a problem that was caused by dirt or other contaminates......................
Wow.
I had no idea.
They not only cut power to the house, they ripped the meter off the wall so after the battery ran down is probably why it didn’t go up in flames.
Hubby once fixed a printer by giving it a shower.
Said it’d be fine after it dried out but I was totally “Yeah. You plug it in, I’ll just watch from across the room”
;)