ebr733575201 YSUS board voltage

[Plane Nuts]

Have a LG Plasma TV 42PN4500 that had no image when turned on, replaced a few FET's and IGBT's on Ysus Board and Z_sus board, now I get a partial image, as you can see in the attached picture. Voltages are good with the exception of VY, according to the sticker it should be -203v however I get +203v I find it wierd that I get the proper voltage but not proper polarity, I don't undertand how that can be. I also have a few opto couplers (LTP155) that have 1.5 ohms across the diode, instead of the common .5 ohm I am used to seeing on a diode. Unfortunately no oscilloscope as of yet to check waveform.

[R_J]

Re: ebr733575201 YSUS board voltage

How are you measuring VY? What was wrong with the fets/igbt's were they shorted? and if they were shorted Drain/Source to Gate, It is likely either the gate resistor is open or the gate drive ic is also damaged.
What is your VZB voltage?
VY is measured across R201

[Plane Nuts]

Re: ebr733575201 YSUS board voltage

Thank-you R_J for the follow up. To answer your questions.
VY is measured with a DMM on DC volts red on + black on -
Q709 (KF7N60F) all 3 pins shorted together
Q710 (KF7N60F)shorted gate to drain source, open to gate and drain
Q712 (30F124) all three open
VZB voltage is 119.9 V

Don't know which resistor is the gate resistor or the gate drive IC, however I just found out that the big white resistor is shorted!! this looks like a 20 ohm 5 watt resistor (RWS M3 5W 20R0JSMT written on it) does someone have the datasheet for it? or would any 20 Ohm 5W resistor work?

[R_J]

Re: ebr733575201 YSUS board voltage

Resistors almost never short. What resistance does it measure?
The gate resistor is the resistor connected to the fet GATE and is usually driven by a ic. If the ic is blown, there will be no signal getting to the fet to drive it.
If the YV voltage measured across R201 is backwards, I suspect D503 would be backwards and C511 would be exploding.
I don't have a schematic so I don't know which are Q709,710 & 712.

[Plane Nuts]

Re: ebr733575201 YSUS board voltage

I rechecked the resistor and it is good, I had the voltmeter in diode setting and it gave 0.003

I checked the résistors next to the FETs and one seems to be bad giving 4 Mohms, i will try to install a through hole resistor a temporary fix to see if it will work since I have one of the same resistance. there seems to be a diode right after this that I will check while the resistor is out.

as it is late here I will get back with a proper sketch of fet locations(they go from left to right on the copper side Q709, Q710, DXX, Q711, DXXX, and Q712, DXXX) also check D503 and C511

[Plane Nuts]

Re: ebr733575201 YSUS board voltage

To follow up, I replaced the resistor that I found open and Image is back checked the voltages and are at proper values, however VY is still positive, but a this point image looks good so it does not matter anymore. Will let it run for a few hours as a final test. Thank-you R_J

Now my question do I leave this through hole resistor soldered on? or order a SMD resistor and install that? what is everyone's standard practice?

[R_J]

Re: ebr733575201 YSUS board voltage

I would replace it with surface mount if I had one, check scrapped boards if you have them. If thats the only resistor you have, keep the leads short and make sure its not going to short to anything on the board.
If you measure YV across R201 and it is the correct voltage, then it is ok. I guess they could have printed the +&- the other way around to indicate minus voltage.