LG 22W2234S troubleshooting and spare parts

[munuainen]

Re: LG 22W2234S troubleshooting and spare parts

Mosfets arrived. Unfortunately the seller had sent AP2761s instead of AP2763s. Upon contacting the seller said the two mosfets should be interchangeable. I'm not quite sure if that's the case. Maybe someone here is competent enough to clarify? I tried installing one and this time nothing blew up but there were no signs of life in the monitor either.

AP2761: http://www.a-power.com.tw/ProductDetails.asp?M_ID=154
AP2763: http://www.a-power.com.tw/ProductDetails.asp?M_ID=157

[jetadm123]

Re: LG 22W2234S troubleshooting and spare parts

Looks like the 2761 is a higher amperage (10A vs 8A)/lower voltage than the 2763 (650V vs 750V).

1) Is the body of the 2761 all plastic like the 2763? In other words, is the tab of the 2761 you screwed onto the heatsink made of metal or plastic?

2) Since you already intalled the 2761, for testing purposes, are you still getting the proper voltage across the main cap?

3) Check the VCC (pin 7) of the LD7522. What is it?

[munuainen]

Re: LG 22W2234S troubleshooting and spare parts

1) Yes, its plastic all over.
2) Around 320 V which I assume is a healthy reading.
3) I'm getting a steady 86 V. I suppose that can't be good as the supply voltage should be 30 volts, right?

[retiredcaps]

Re: LG 22W2234S troubleshooting and spare parts

1
3) I'm getting a steady 86 V. I suppose that can't be good as the supply voltage should be 30 volts, right?

According to datasheet, it should be 30V maximum for VCC. I suggest rechecking your resistor values to make sure you have the right ones.

[munuainen]

Re: LG 22W2234S troubleshooting and spare parts

I also thought the change in voltage must be due to resistors. However, all resistor values in the pcb section related to LD7522 seem fine to me (measured off-circuit). See the attached image for details.

[retiredcaps]

Re: LG 22W2234S troubleshooting and spare parts

However, all resistor values in the pcb section related to LD7522 seem fine to me (measured off-circuit).

See if d803 is shorted?

[munuainen]

Re: LG 22W2234S troubleshooting and spare parts

Nope. All three diodes in the proximity give one-way reading only.

[munuainen]

Re: LG 22W2234S troubleshooting and spare parts

Just thought this might be worth mentioning if it helps with pinpointing the culprit. I plugged in the power to measure voltages on different spots when another bomb hit, killing the mosfet and the two largest resistors. This time it wasn't due to probe shorting.

[retiredcaps]

Re: LG 22W2234S troubleshooting and spare parts

Just thought this might be worth mentioning if it helps with pinpointing the culprit. I plugged in the power to measure voltages on different spots when another bomb hit, killing the mosfet and the two largest resistors. This time it wasn't due to probe shorting.

Well, I suggest starting at the very beginning then. Obviously, something is blowing out the circuit. Follow these instructions by Krankshaft and see what component is possibly shorted.

https://www.badcaps.net/forum/showpo...15&postcount=2

[munuainen]

Re: LG 22W2234S troubleshooting and spare parts

Line fuse fine.

The thermistor RT801 should be 4.5 ohm according to the data sheet . I'm getting almost 11 ohms off-circuit at room temperature.

Line rectifier. Again, unstable readings. Several hunder Kohms anyway.

Line filter cap charging up.

Mosfet I cannot really check, can I? One is dead, the other I don't want to sacrifice.

Output rectifiers are located further up the pcb. Do they really matter at this point?

By the way, when I checked VCC voltage I compared voltage from pins 7 (VCC) and 5 (GND). Pin 5 is directly connected to the minus output pole of the rectifier which obviously is different from the AC connector ground. Did I do right? The reading I got from the pin 5 and AC connector ground before resistors and the mosfet blew up was around 15V.

Gosh I'm such a n00b. I should probably set time aside to study electromagnetics.

[jetadm123]

Re: LG 22W2234S troubleshooting and spare parts

It looks like cap c808 is across the Drain and Source of mosfet q804 (please verify this). If this cap shorts, it provides a direct path to R815 (the 2W resistor). Since q804 is shorted, you will probably have to remove c808 to check it.

[munuainen]

Re: LG 22W2234S troubleshooting and spare parts

C808 definitely not shorted. I'm getting a reading around 1,5Mohm in-circuit, blown mosfet and large resistor having been removed.