Elex518

Based from datasheet of L5973D, the pin5 should get 1v2, replacing R1 to 10K will get pin5 to 0v8 only and can possible stress the regulator and blown. Recalculate the voltage to pin5 that it can get 1v2. If you want use 10K for R1 and regulate 5V, your R2 should be 31K (based from my calculation). Check here for more repair tips --> https://elex518.blogspot.com/

Use 2.4K resistor...I found it right here https://www.planetanalog.com/author.asp?section_id=3065&doc_id=562953 Check here for more repair tips --> https://elex518.blogspot.com/

Simplest way is to add 1 ohm high voltage resistor between the wire...measure the voltage drop across resistor....use the formula I = V/R to calculate the current. Check here for more repair tips --> https://elex518.blogspot.com/

No need to add load when measuring transformer output. I suggest you check that power supply capacity in Voltage DC and Current. The find a replacement, so many now in the market with cheapest prices. But you should have a good background in electronics. Check here for more repair tips --> https://elex518.blogspot.com/

search for SSR...mech relay will always have arching due to high voltage. Check here for more repair tips --> https://elex518.blogspot.com/

The simplest way: for input protect just add varistor...for output and load just add a crowbar over-voltage protection circuit. Check here for more repair tips --> https://elex518.blogspot.com/