Welcome to JA Robbie,
My Name is John,
You are correct in saying Hp is known for board problems,
You will need to remove the back cover to troubleshoot the plasma you will also need a digital multimeter. I will bet you have a defective Z sustain but you have to be sure you dont want o through your money away by shotguning the set with boards.
First look for missing voltages at power on each connector (not just Vs and Ve look at all the voltages they are marked on label near the top of the panel) should be labled with the proper voltage. If you have one start with the missing voltage and troubleshoot back, it is hard to say what the ultimate failed components would be but you should be able to track it to one board.
Rule 1 is troubleshooting is "start with your sense"
4. Touch (not recommend)
these are the best giveway, if it looks damaged it is if it smells burn it is, if it sounds wrong it is
1. Look - examine each component on the board look for something burnt, swollen or distorted, and look at the fuses.
2. Smell - Smell around the set see if you can smell a burning smell on any of the boards .
3. Listen- Listen for a buzz or an arc out of one of the board, it may be normal but unlikely if you have a defect.
4. Touch- Important because if the parts are getting hot then they are running, but if they stay cold they are not.
As for touch not the best idea on a power supply there is a high risk of shock, as some of the heat sink may not share ground and possible have a potential to earth ground between 5-500 volts.
You will need a DMM to check the diodes and the transistor non should read a short, if any do you will need to remove it and check it out of circuit if the component is not shorted but the short is still in circuit use the schematic to follow the leads to where else the short could be coming from and repeat.
I would have to say unless you are fimilar with repairing with a schematic it can be complex. Your best bet is to do what best buy techs do.... Replace The Board. It is the fastest sure fire way to fix it.
Next remove the connector connecting to Y sustain and the Z sustain to the power supply and attempt to power the set on if the set powers with the connectors disconnected you have more than likely identified the issue as a failed sustain board now to go further.
THIS IS NOT YOUR SET BUT A GOOD EXAMPLE.
Now reconnect the power connect to the y sustain and see if the set powers remember you will not get a picture but the lights on the front should stay light.
If the y main connected causes the set to act up remove the buffer boards and attempt again, if the y still causes the issue you have a failed y board. If disconnecting the buffer resolved the issue you have failed buffers and possible a failed panel, try replacing the buffers.
If the Y board connected the set still light normal disconnect the board and connect the z sustain and test the same as the y, if that causes the issue replace the z.
Hope this helps,
If you have any questions please ask.