hi
fixed one of these yesterday!
needed new 330r resistor that feeds opto LED, new opto (used MOC3010) and new triac, used a BT137-600 to replace original SC141D. Worth checking the two big are still OK.
BE careful of the high voltages - i connected a household lamp across the caps to discharge, check with meter to ensure they have discharged before working on.

bob

hello
the opto is the little 6-pin dil package, probably not very easy to check, best to just replace using a socket if possible. The triac is the other side of the two big watty resistors and you should be able to check that with your meter in diode mode, most likely your find a pair of legs shorted. The 330r resistor you can easily check with your meter, not sure why my one went open circuit!!
Also reflow header pins as these often crack causing intermittent problems
bob
l