I have an old VFD type device that has six of these units. It did work, one day something in it popped and it died. Visual/smell test lead nowhere.
The device has a rectified DC supply in it which feeds to the drive circuit that has 6 MJ10000 transistors on small heatsinks.
The issue was that once you tried to apply output power the small internal breaker/switch would trip. I removed the DC supply to the transistors and the issue stopped. This leads me to think one or more transistors failed.

I'd like to test these transistors, but not sure what the best way would be besides maybe ohm test and I'm not sure what a reasonable value would be or proper polarity lead to transistor pin for testing.

 

I just used these directions to try and test them. http://www.ehow.com/how_8046802_test-darlington-transistors.html
At least using these in the diode test mode all 6 of them seem to be toast.
3 seem to test fine until do the reverse bias they fail that. The other 3 fail the normal test and pass reverse bias. To me, this shows either these directions are not the best or the small pop I heard was in fact all 6 of these failing at once.0

Based on some failing reverse bias it seems possible it may have damaged the small transistors and/or optoisolators driving these although there is a diode between the base and the driver parts on each.

What do you think of my test results and chances of more extensive damage from these failing?

 

1. Did you remove them from the circuit before you tested them?

2. Do you have a "diode" setting on your multimeter, or did you use the "lowest" ohms setting.

3. What were some of the numbers that you read. What was pass, what was fail?

 

Yes I removed them from circuit. I used diode range. I was getting 100 or less some cases reverse bias also failed.

I ran across this too elsewhere:
{That's a power Darlington with an internal diode and two internal resistors. I'd just test them individually (if I couldn’t test them in-circuit). There should be a difference if one or more is bad.

From the emitter to collector, there is a protection diode. With high on the emitter and low on the collector a meter should read a forward diode junction. It should read open the other way around.

From the base to the emitter, there are two base to emitter junctions, but there is also about 115 Ohms in parallel with that. It will read low both ways (unless testing with high current, which I don’t think is necessary).

From the base to the collector, the path goes through the base to collector junction of the first transistor, but there is also the two resistors and protection diode that are in parallel with it. There should be a reading forward and it should be open reversed.

Again, I’d compare the readings to a known good one.}

I didn't see a noticeable issue with the second test mentioned in this, but tests 1 and 3 both showed 3 bad transistors.

Looking at pricing for identical replacement and cheapest place seems to be newerk at 6.90 each or about $45 total. This was a small PLC trainer had builtin 1-2A breaker, the motor that killed it wasn't that big either which makes me think there could be more problems in the unit or running on 120 not 208 killed it as the uses a 208V transformer to power the logic, etc. I have a better rated more versatile Allen Bradly PLC I can tinker with instead so this thing may just be scraped.