U5000 Hot Question (potential between Emitter and Base?)

[ChrisCopper]

Does anyone have a u5000 sitting around to check this , or does anyone know the answer.? if i test the 23sc3686 U5000 HOT in circuit, it reads dead short across the Emitter and Base, when I remove the HOT , it tests as it should, OL between Emitter and base. When I test the PADS of the Emitter and Base with the HOT out, there is a 0.002 reading in diode mode accross those pads.

is this normal? Im thinking not, but I cant find anything dead shorted in its path.

anyone see this?

 

[mecha]

you're supposed to test them out of circuit. the D707 (I think that's the designation) part next to it is a damper diode that sits inline with the collector and emitter I think is how it's run. so it'll never test correctly in circuit. remove from circuit, red probe to base pin, then black probe to collector and then emitter. if you have anything but junction drops to either, it's bad. it can be good one way and fail the other, still bad.

 

[M K L]

This is one of those things that keep baffling noobs. There must be dozens of posts like this. It's the same for every monitor and easy to understand.

The base of the HOT (Q704) is coupled (often directly, in this case through a coil L704 which has a very low, practically negligible resistance) to one end of the secondary side of the horizontal drive transformer (T701). The secondary side, being a coil/winding, has a very low resistance. The other end of the secondary side is coupled to ground. Hence the resistance from the base of the HOT to ground is very low and easily mistaken for a short. And since the emitter of the HOT is coupled to ground, you'll always measure a very low resistance across base and emitter when the HOT is in circuit*. It's totally normal and not a short.

* HOTs that have an internal damper diode (e.g. K7000) will show a relatively low resistance (40-50 ohm) across base and emitter when out of circuit because of a built-in resistor in the base/emitter junction.

 

[ChrisCopper]

ok, so you should see a low resistence between emitter and collector when the hot is IN and OUT of circuit? this is normal also?

i get the relationship between the base and ground. i just dont get the 0.002 reading in diode mode accross the emitter and collector both IN and OUT of circuit. your saying this is normal?

 

[ChrisCopper]

you're supposed to test them out of circuit. the D707 (I think that's the designation) part next to it is a damper diode that sits inline with the collector and emitter I think is how it's run. so it'll never test correctly in circuit. remove from circuit, red probe to base pin, then black probe to collector and then emitter. if you have anything but junction drops to either, it's bad. it can be good one way and fail the other, still bad.

yeah i hear you there out of circuit it tests fine, with it removed, there is what seems like a dead short across the emitter and collector pads, is this normal? in looking at the schematic it just seemed to me that there should not be a connection there. or zero resistance. so i guess my question is if this is ok.

 

[M K L]

ok, so you should see a low resistence between emitter and collector when the hot is IN and OUT of circuit? this is normal also?

On the pads where it is/was soldered to, yes.

i get the relationship between the base and ground. i just dont get the 0.002 reading in diode mode accross the emitter and collector both IN and OUT of circuit. your saying this is normal?

Of course. That's the reading you get when you put the leads on the same piece of metal and the part of circuit highlighted in red in my picture is one and the same piece of metal.