Horizontal output question

Emitter to collector with the anode on the emitter if I recall correctly.

Many horizontal output transistors have a built in B-E resistor of 30 ohms or so. A dead short means it is bad.

David

Thanks. I did check and I read 36 ohms between B and E. But as I said, doing a diode check, I read dead short both directions. Shouldn't a resistance reading only be able to be taken with common lead on emitter if transistor (NPN) is good?

The damper diode is connected between C and E (which usually is ground), with its anode connected to E and its cathode to the transistors C. So if the diode is still good and your transistor's C-E path is not shorted, then in diode test mode you should only get a reading with (+) of your DMM connected to the transistor's E and (-) touching its C. In respect of the 36Ohm base resistor, this value is too low for your DMM's diode test to tell you anything reliable. But still reading the 36 Ohm means your B-E path is not shorted, which is rather unlikely anyway. They usually short C-E, or quite occasionally C-B.

From what you wrote so far, I'd conclude that the transistor is still OK. But you could do a quick test with a 12V source. f.e. car battery, a 10W light bulb (dome- or number plate light), a few connector wires with crocodile clips and a resistor some 220 Ohm to limit the base current. If you can turn on and off the light by touching releasing the base with the resistor, the LOT should be fine. Also reverse polarity on C-E to test the diode. In this case the bulb should be on constantly.

I did a similiar test. Only without light bulb. Properly biased the trans. It worked. Removed the base voltage, and it still conducted with the same forward voltage across the emitter and collector. My conclusion, since I can't turn off the transistor by removing base voltage, it's shot. By the way, I did have to replace another NPN in the SMPS of this same chassis. The Base and Emitter were shorted. The Emitter to Collector checked good. Thank you to all that responded. I wanted to be sure of what I already suspected, just the damper diode made me question if my test results were accurate. If ya'll agree with my findings, I am going to go ahead and order the replacement HOT. Thanks again group.

I always feel safer shorting base to emitter to shut off a BJT.

That B-E resistor really should be low enough to do that.

--- sam | Sci.Electronics.Repair FAQ:

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True, but when I don't rely on extensive knowledge of the circuit under test, I find fewer ways to outsmart myself. :)