LG 42PG1000 Stuck In Standby

[rebel-mc]

Hi, Got one of these in for repair and just can't establish where the fault is.  Power up tv the standby light goes from red to green then after 5 seconds back to red.

So far ive tested all voltages from power supply and everything is there (or at least for 5 seconds).  +5v standby allways there.  Power supply comes on for 5 seconds (relay click) and all voltages there, after 5 seconds all I have is the +5v standby.  On the power supply there are only two power outs (ysus and Mainboard). If I disconnect the Ysus the same thing happens.  If I disconnect the mainboard the same.  If I disconnect both the power supply stays on.  Anyone had any experience with this fault.  I believe the same fault can occur on the 42PG2000 and 42PG3000.  I was thinking the ysus but just can't prove it. 

Again any help would be appriciated.

Thanks