Were going to start with the polarized components theses only function in the correct orientation. The way to determine the way the go is simple, look for the band around the diode in theses case it is black. Match the band the the symbol on the pcb that also shows a band.
D4 and D5 are Zener diodes theses controls the final voltage that the regulator circuit puts out if you get above 5.1v here is where you want to look for bad solder joints, or components the wrong way around.
Now components without orientation, they can be placed ether direction. R22 is a 10 ohm resistor that forms an RC filter with C2. Basically it cleans up some of the noise the regulator can toss out.
If you get no voltage at all out of the regulator this is a good component to check. ITs the final components before the voltage gets to the zeners and to the rest of the controller.
This zerocross detection circuit, R15 is a 1 mega ohm resistor, again orientation dosen't matter here ether direction will work.
If you get random dimming look no further than this resistor. Note: you might want to double check this components because if you put the wrong component here you can fry the micro controller.
R3-14 are 4.7 killo ohm resistors theses connect the atmega to the SCR board. Don't forget R16 this is used to pull the reset bit high so the micro will turn on.
If your micro is nto doing anything look at R16. If you have a broken channel with and channel indicator led that is not lit look here.
Theses are another critical set of resistors these setup the biasing of the transistors in the regulator. R17 R18 & R29 are 120 killo Ohm resistors.
Theses are another critical set of resistors these setup the biasing of the transistors in the regulator. R19 is a 3 killo ohm resistor
Theses are another critical set of resistors these setup the biasing of the transistors in the regulator. R21 is a 22 killo ohm resistor.