Based on the experience I gathered in repair cafes:
Some old power switches generate an arc (spark?) when flipping them. Cleaning their insides with an "electrical contact cleaner" spray can sometimes help.
Defective electrochemical capacitors do NOT always swell or leak. If it is worth it swap them all for new ones (We sometimes desolder what we think are good ones from dead appliances to be re-used).
All domestic appliances around here (France) have an "anti surge" (?) capacitor soldered just after the power cable and power switch. If I understand it correctly, its job is to prevent a "parasitic" current spike to travel back on the domestic electric network when the equipment is switched on. That's the one I would replace in priority to see if it fixes your problem.
Disclaimer : I have no degree in electronics / electrical installations. Take what is above with a grain a salt as I might be dead wrong.
Always unplug the appliance from the mains before tinkering and discharge ALL the capacitors which are on the 110 V /220 V side of the electronics before touching the PCB on which they are soldered.
Good luck with your repair. It would be nice and helpful if you have a chance to post some pictures :)