LG Plasma 50PC3D no VS & 1 second shutdown

[steves7839]

Hello fellow DIY's. I have been searching for an analogue to the following situation:
I've replaced swollen caps on an LG 50PC3D and had a shorted Y-SUS IPM module replaced. The TV clicks on, clicks off after 1 second. No sound or display.

To verify the PS; with the Main and Y/ZSUS disconnected I have used resistor jumpers to force on PS and all DC voltages up to 18V are good, though about 5% low. AC_DET is solid at 5V. As soon as I force on VS_ON, the relay powers off after 1 second. I have loaded VS with a pair of auto bulbs, but DVM shows VS remains at 0V. VA properly reaches 60V before shutdown.

I'll post a pic of the board, jumper setup, and actual measurements this evening. But I wanted to float the symptom for the collective wisdom of the community.

[steves7839]

Re: LG Plasma 50PC3D no VS & 1 second shutdown

Powered up the TV with Y-SUS connected. Green STBY LED, goes red after 1 sec. No display or sound. One anomaly- There is a connector P1001 on the unknown board located behind the lower metal shield, next to an input board. P1001 ohms 0 to GND on all three labelled pins; V5A, V12A, etc. (See photos). I don't want to connect it to the PSB if these pins are in fact shorted, unlikely tho it might be.
Can anyone link me to a block diagram/connection diagram for this TV or identify that board?

[steves7839]

Re: LG Plasma 50PC3D no VS & 1 second shutdown

I measured Q855 out of circuit and found C-B and D-B shorted. Replacement from Coppell TV is installed and I'm ready to test the PSB. However, diode checks in the vicinity show D152 reads a short, in parallel with R152. Neighboring components are Q855 and IC U151. (D152 circled in new pics)
The schematic I have does not include this set's PSB.

Is D152 shorted, or just looks shorted based on other components?

[tom66]

Re: LG Plasma 50PC3D no VS & 1 second shutdown

I would ALWAYS replace the adjacent transistor in any power supply that has blown one transistor. This is because the failure of one transistor can damage the other even if no obvious shorts are present on the other transistor.

[steves7839]

Re: LG Plasma 50PC3D no VS & 1 second shutdown

Q855 fixed it! Enjoyed it next to our 52" Samsung from 2005 for two weeks, then sold it.