Looking at the pictures I would say that R1 and C3 power the board. The silkscreen on the PCB for the zener diode is incorrect. A 1n4742a is a 12v part, not a 1n4740a 10v part. Might have something to do with it. The 100k resistor next to R1 looks to be getting really warm... enough to change the color of the paint on it.

Other than the zener diode, the two MOC3063 optocouplers, the LM324 op-amp, 78L06 voltage regulator, and it's 220uf filter capacitor there isn't too much stuff on that board that should blow up. One thing though is the electrolytic capacitor. That is a Chang, one of the cheapest of the cheap caps. Seeing it on that board would indicate that it is made by some low cost fly by night PCB manufacturer in China wherein the origin of a lot of components is rather dubious. I actually wonder if those MOC3063 opto's are in fact real.

If it were me I would rebuild one of my dead boards by removing the LM78L06 and zener diode and replacing them with machined pin headers. This way you can easily drop in all new semiconductors, or as I call it "take the shotgun approach". Also get rid of the Chang electrolytic capacitor and replace it with a 105C rated Nichicon VZ series or equivalent. Should cost $3-5 for parts minus shipping costs. If replacing "everything" on the board fixes it, then you can start putting back old components until you figure out what has died. Not really the most efficient way of troubleshooting but saves you from having to poke around on a live board connected to mains.

If the repaired board dies again in another month or so, and it is the same component that originally let out the magic smoke then you have a design flaw.

Could you give me a component list of things that I should replace and a good source to find quality components. Like the big blue capacitor on the left I don't know much about it. It has no part number on it. And are u saying I need a 10v 1N4740A instead of the 42A 12v.and what are machine pin headers? I guess I would like to have a complete recommend part list of quality components and where to order them. Thanks. There's just so many options when I Google them it gets confusing

 

When you have replaced the boards, do the replacements come with a new reed switch? Looking at the link MaxHeadRoom gave shows that a reed switch senses flow. If for some reason it isn't doing its thing, the unit would stay in standby.

 

If there is corrosion or calcium build up in the inlet pipe where the waterflow sensor is. It can eraticly jam the magnetic plunger & cause problems.

 

When you have replaced the boards, do the replacements come with a new reed switch? Looking at the link MaxHeadRoom gave shows that a reed switch senses flow. If for some reason it isn't doing its thing, the unit would stay in standby.

It doesn't come with board. It checked OK with meter. I knocked breaker with water off read open with water on read zero ohms