Full Wave SCR/Diode bridge failure?

[tarogers]

I have a Kohler 6 KW AC generator on my boat that now shuts down immediately after startup with an over voltage error code. The voltage regulator consists of a printed circuit board driving a full wave SCR/diode Bridge, driving the generator field.
 The SCR/diode bridge passes all of the ohm meter tests with the possible anomaly of the gate to cathode resistance on one SCR is 3 ohms and the other is 30 ohms.  Using an external 9 volt battery and bulb both SCR’s block and then fire with a gate signal.
I  reconnected the bridge to the generator, but left both gate leads off. I fired up the generator and still got an over voltage shut down. I then removed one AC input lead from the bridge and got an under-voltage trip upon startup.
This leads me to believe that one(or both) SCR’s are self triggering causing over excitation of the field and the over-voltage shutdown, though they seemed okay with the low voltage DC test.
Since the bridge itself costs about $60. I thought I would get a second opinion from The Forum before proceeding.
Thanks,
Tom

[coromonadalix]

I would remove the scr assembly and test it ... since you have the basics on how to test scrs,  you test them non triggered to check against leakage, normally and polarity inverted,  after that step you trigger them and check them again, the best tester i had in the past was applying ac and dc voltages and read it with an oscilloscope, i could see leaking diode and src's,  when we test in DC it may be okay, but i've seen in the past some leakage in AC   

Once in a while in our board cleaner we have to change some opto coupled src bridge, they simply go bad, the only trick we have is to get an stronger spec in current to overcome this phenomenon we use normally 50 amps ones, but now we install 75 amps, and they cost almost 100$ a piece.

On google you have :
http://www.circuitstoday.com/how-to-test-an-scr
https://sciencing.com/test-scr-ohmmeter-6681788.html
https://www.androiderode.com/how-to-check-a-scr-with-digital-multimeter/
http://www.completepowerelectronics.com/how-to-test-scr-with-multimeter/



You can try to find your scr(s) part number(s) to see their specs ?

Unless you have the scr drivers who can go bad ?  maybe you have some diacs / optocouplers  etc ... for the gate drive ?

[tarogers]

Thanks for the thorough reply.  I will remove the bridge this weekend and further bench test it.
One curious point is Kohlers test instruction:

"6. Connect the ohmmeter from G1 to (+) on the SCR module.  You should read low resistance in both directions.  Repeat for G2."

I would expect to see a diode like set of readings for a gate to cathode test.
One SCR reads 3 ohms in both directions, and the other 30 ohms in both directions.

I'll verify when I get it home.