HOT Mosfet issues

[Robfry2]

Hi guys any advice, the HOT on my monitor chassis is showing continuity in circuit but out of circuit measures fine. Here is the schematic anyone got any advice to what I should check many thanks.

 

[Hexen]

The pins that show continuity in circuit, but not out of circuit... have you measured those points on the PCB when the 'HOT' is out of circuit?

Is it still showing continuity with it off the board?

Is it still showing continuity with the probes reversed?

By 'HOT', you mean Q603, THE 2SC4542/BU50..whatever that says, driving the EHT/FBT?

 

[Hexen]

Would the continuity in circuit be about 23 Ohms between the Base and Emitter?

 

[Hexen]

Check collector to Earth/GND. Any measurement on the board, transistor out of circuit (OOC)?

 

[Robfry2]

Thanks for the help, I took out the transistor reads fine connected it to measurement tool I’ve got it’s not bad. A couple of resistors are out of spec so am going to replace and see if that helps.

 

[eliotcole]

... "HOT Mosfet issues" ... 😏

 

[gunblade]

that is nice design circuit, easy to fault find
this is a tatung svga monitor?
can you explain the actual fault, is it dead?
have you dummy loaded the collector pad of the HOT and checked all the power supply voltages?

 

[Hexen]

Thanks for the help, I took out the transistor reads fine connected it to measurement tool I’ve got it’s not bad. A couple of resistors are out of spec so am going to replace and see if that helps.

Still not sure which transistor... HOT would be output transistor, but isn't that bipolar? The MOSFET is in the PSU? Either way, if it tests OK OOC, then you are probably measuring the resistance of the coil connected to it on the PCB.

A hot transistor means something completely different to me, as in thermal.

 

[Robfry2]

This is the HOT I’m talking about and it is reading continuity between the outside legs. This issue is the fuse keeps blowing. I am replacing the ic102 chip as this was an issue previously and I have checked the mosfet on the power supply side and seems reading fine.

 

[gunblade]

mains fuse blows?
check bridge rectifier, any diodes and ptc in primary section, dummy load the b+ line and isolate flyback pin 2 but a short after the secondary should not cause mains fuse to blow as its a switch mode supply so should go into ocp if any shorts are present

you will read short base/emitter in circuit with a HOT - put meter into diode mode, common lead to collector and then read to emitter and then base, should read like a diode

 

[Hexen]

mains fuse blows?
check bridge rectifier, any diodes and ptc in primary section, dummy load the b+ line and isolate flyback pin 2 but a short after the secondary should not cause mains fuse to blow as its a switch mode supply so should go into ocp if any shorts are present

you will read short base/emitter in circuit with a HOT - put meter into diode mode, common lead to collector and then read to emitter and then base, should read like a diode

This.

 

[Robfry2]

Did a bit of research and it seems normal that base and emitter have continuity, guess the issue is all the the power section.

 

[Robfry2]

mains fuse blows?
check bridge rectifier, any diodes and ptc in primary section, dummy load the b+ line and isolate flyback pin 2 but a short after the secondary should not cause mains fuse to blow as its a switch mode supply so should go into ocp if any shorts are present

you will read short base/emitter in circuit with a HOT - put meter into diode mode, common lead to collector and then read to emitter and then base, should read like a diode

Sorry didn’t read this comment lol.

 

[Robfry2]

mains fuse blows?
check bridge rectifier, any diodes and ptc in primary section, dummy load the b+ line and isolate flyback pin 2 but a short after the secondary should not cause mains fuse to blow as its a switch mode supply so should go into ocp if any shorts are present

you will read short base/emitter in circuit with a HOT - put meter into diode mode, common lead to collector and then read to emitter and then base, should read like a diode

Can you also please tell me what the bridge rectifier and ptc is please thank you.

 

[gunblade]

ptc is the degauss control switch, positive thermal coefficient - marked PH101, remove from chassis for time being, shake it and seeif it rattles
bridge rec is marked D101 I think as schem is not perfectly clear- 4 diodes in that and you should read any dead shorts across any

 

[Robfry2]

Does this help

 

[gunblade]

d101 is the bridge
is the fuse blowing immediatley to black or just parting?

 

[Robfry2]

Power on flash fuse gone. If that helps, I think you helped me before with similar issue. I tested the D101 and seems to read fine I will take out the ph101 and give it a shake. I’m also replacing a couple of resistors that read out of spec while I’ve got it out.

 

[gunblade]

resistor out of spec would highly be unlikely to blow a fuse, a shorted diode or ptc or ntc or mosfet or even shorted out transformer in the supply

 

[Robfry2]

I will test all the diodes replace the mosfet and the controller chip which part is the ntc thank you.