I am currently working on a Samlex SEC-1235 switch mode power supply and I have no DC voltage output. It should be in the area of 13.6 VDC. I am to the point that I believe I have a bad switching transistor. I have tested it like a bipolar but it does not make sense to me. Is there another way to test them? The two readings are different from each other which would confirm my suspicions but would like to know more about them.

Is that perchance using MOSFETS?

 

There is no load hooked up to this unit. I have traced voltage to the rectified side of the bridge rectifier and found 345 VDC, which puzzles me but nonetheless, it's there. I seem to lose it going through these switching transistors but unsure how to test. They do have different readings on diode test and one looks like it has a small pinhole where smoke escaped but hard to tell with the naked eye. I am going to put it under magnification tomorrow to see if I am correct. I will post the pics.

Seems high for a DC bus voltage. Blown rectifier (superimposed AC)? What do the caps look like? Swollen?

 

No swollen caps. Going to go back through earlier part of circuit and check for rectifier problems etc. Will post back

 

Good idea bwilliams60. Ohm out the input rectifier. Don't forget, you may see some strange readings due to a free-wheeling diode, depending on the make. The rectifier should have a diagram to assist you.

 

Thank you Ian, appreciate the feedback and the lesson. Slowly learning, very slowwwwly.

Since these transistors are blown, I like cause and effect so what might have been the cause of their destruction and what else should I be looking at?

Does anybody have a good source for these transistors? What is the Chinese company that everyone on here talks about?

Sometimes its the chip - last one I encountered the chip was heat discoloured, but you can't rely on that (but it was a KA7500B!).

At the other end, I've seen underrated MOVs (only 150V each) across the 2 reservoir caps - one fails S/C and unbalances the 1/2Vcc point, which blows the first transistor - that fails S/C and blows away what's left of the dead MOV and destroys the other transistor as well.

High ESR failure electrolytics are the most usual culprit - you should inspect for bulged tops as a matter of course, whatever reason you opened it up for.

Any electrolytic can cause trouble, and the tiny ones don't usually show up like bulged tops, I'd take a look at the pair of 10uF in the base circuits of the 2SC1815 driver transistors - if they're on the way out there won't be a clean squarewave driving the power transistors; high switching losses with overheating etc.

Another possibility is the voltage sensing divider resistors, they can go high in value, the preset probably has a sealed adjuster and may be best left till you've eliminated everything else - but a pot can develop an intermittent wiper if the quality is a bit iffy.