looking for some trouble shooting tips or advise on a welding machine pc board.
there are three componates that are obveouly burnt up. they are Q5,D18,and Q8 in the electrical drawing. i have replaced the three items and was looking for advice on what may have caused them to fail. and any other devise that would be suspect to replace.
(not obvesoly burnt on board. thanks. page 39 of 56 on pdf file.

 

I couldn't quite follow the schematic to figure out where RC6 goes, but i'd say its safe to assume that 120/240 ended up on that line somhow and thats why the diode, fet and transistor burnt out. U1 is probably OK as long as the main VCC on that board didn't get overvolted, depending on the value of R86, i'd guess its a standard PWM controller and the internal transistors are good to 250/500ma/35 volts. Same goes for A1, the voltage divider will protect it depending on the chip and the value of the resistor, if its a 494 or a 3525 pwm chip and an lm393 comparator i'd replace them anyway. Shouldn't need to replace anything else, as if 120/240 hit the vcc rail, the board would have been vaporised.

 

thank you for looking at the issue for me. RC6 is a plug in connector on the pc board.
PIN OUT FOR RC6 =
PC6-1 = WIRE FEED DC MOTOR + LEAD.
PC6-6 = WIRE FEED DC MOTOR (-) LEAD
PC6 -3 SHIELDING GAS SOLINOD
PC6 -8 24V FROM TRANSFORMER
PC6-7 24V FROM TRANSFORMER

 

It appears that the pnp transistor provides the full current to to motor, with the fet being an additional turn off circuit, in which case if they are both on, that's no good, but that would not burn out the diode... it is safe to turn on, if you want to play it safe, stick a 20-40 ohm resistor in series with the 24 volt ac feed, the motor should still have enough power to verify operation, if instead the transistor and fet get warm, then i think they are both on, and you should look at replacing the PWM controler, or look into what is turning the FET on.

 

thank you for the infomation and your time. the welder is now working again!!

 

If it burns up again connect a .1uf cap and a 2.2/3.3 ohm resistor in series, across D24, on the board with minimal lead length, this is also known as a "snubber"

Glad to hear it works.