Further to -:
It may not be the relay themselves I've known relays to give problems at their soldered pins, two scenarios here, one bad soldering and the relay causing 'shock waves' loosening the soldering around the pins.
As pointed out be very careful, it's not the main potential but the charge on the capacitors – switch-mode power supplies, aka. 'switchers', rectifies the mains to dc, so we have 240 x 1.42 = 340V DC appx. on the smoothing capacitors – they can stay charges, short them out before touching WITH THE UNIT UNPLUGGED, I either use a 470 Ohm 1Watt resistor or a small 240V lamp bulb, out of a fridge say.
An inspection of the relays will do no harm…. but there are time when you need a test meter and times when it needs to be powered up with the PCB accessible from underneath.
Its the volt that jolts and the mills that kill…. a mill is 0.001 of an amp – you only need a couple or so across 'ticker'
Dave.