Samsung t240 Totally dead

[pillboxhunter]

Hi, got a Samsung Syncmaster T240 that appears totally dead. When I got it it had a blown fuse F101 3.15A. I replaced it and it immediately blew again and took out the Thermistor TH101 (SCK 056). Further investigation revealed Mosfet Q101 (FQPF 7N65C) was blown. I have replaced the mosfet, all the electolytics and the SCK 056 (with a SCK 057) I also replaced the inverter transformer with a new one from Ebay although the old one reads OK.
Following the above the monitor is still totally dead, not even the power LED lights. Measuring voltages gives no output on any of the pins of CN2 or CN3. I do have 343 volts on C105.
I measured (with a fluke 12 meter) every semiconductor I could find on the board and they all appear to be OK
Plugging the monitor into a computer causes the computer to switch to 1920 by 1200! There is nothing visible on screen even with a powerful torch and the backlights don't come on.
I tested the tubes with an external inverter and they all light up.
I'm stuck now - any suggestions welcome. Pictures of boards attached and schematic of the C version of the board (mine's a D) but it appears identical.

[budm]

Re: Samsung t240 Totally dead

When that MOSFET Q101 shorted out, it probably took out the SMPS IC IC101 (8-PIN SMD) that drive the MOSFET with it, luckily you did not damage the new MOSFET.
Please get the P/N as printed on top of the IC so we can look up the spec sheet.

[pillboxhunter]

Re: Samsung t240 Totally dead

Hi Budm, the number is FAN7602B - I have spare Mosfets if replacement is damaged - fuse doesn't blow so hope it's not. Not sure I'm going to be able to replace that smd chip - don't have a hot air station - although I have a Gas soldering iron with air nozzle so may be able to use that?

[budm]

Re: Samsung t240 Totally dead

Please check the resistance reading between the Ground pin against all other pin, make sure to report if the readings are in OHM, K OHM, or M OHM, monitor unplugged of course.
I have a feeling the IC is bad.
Also the big brown resistor to the right of the MOSFET (picture #1) may have blown open also, it should be <1 Ohm.

[pillboxhunter]

Re: Samsung t240 Totally dead

To pin 1 0.93M, Pin 2 62K, pin 31.095K, pin 5 7.71K, Pin 6 3.5K, Pin 7 Open Circuit, Pin 8 reads 0.6V LO Z.
Big brown resistor reads 0.6 Ohm

[budm]

Re: Samsung t240 Totally dead

Did you measure between Vcc pin and the Ground pin? Do not use chassis ground for the meter because you are working in the primary (hot deadly side) of the circuit.
You may be able to get the IC and have the repair shop or College kid in electronics class solder it in for you.
135V would have blown he startup cap (typically rated at 22~56uF 50VDC)

[pillboxhunter]

Re: Samsung t240 Totally dead

Doh! ground pin to pin 6 with power on reads 1.9v

[budm]

Re: Samsung t240 Totally dead

I would replace that IC.

[pillboxhunter]

Re: Samsung t240 Totally dead

I think that's probably what needs doing - I have a friend who works with smds for his job and he's just replied that he can do it - can't get the chip anywhere except Ebay though and not until 18th Aug at the earliest! How likely is it that there isn't further damage?
Might be time to get a replacement board.

[budm]

Re: Samsung t240 Totally dead

The inverter section where the Inverter transformer and the SMD POWER MOSFET on the bottom side got hot enough to turn the board dark brown, you should check to make sure the MOSFET is also OK.

[pillboxhunter]

Re: Samsung t240 Totally dead

The browning on the board isn't as dark as it looks in the photo and the tranny doesn't read shorted. It's an AP4506GEH on the board (STU312D on the circuit diag).

[pillboxhunter]

Re: Samsung t240 Totally dead

I've ordered another board so I have something to compare measurements against - won't be here for a couple of weeks though.
Thanks for the help so far

[pillboxhunter]

Re: Samsung t240 Totally dead

If converting one of these to LED backlights would removing F301 disable the inverter but leave the rest of the psu board working?

[pillboxhunter]

Re: Samsung t240 Totally dead (NOW FIXED)

Monitor fixed with replacement power board. (arrived much faster than expected from China and looked new - no signs of heating on any components or board.)
Old board IC101, FAN7602B has different readings from new board compared to old so will look to replace that and see if old board can be brought back to life to keep as a spare.