Electronics LCD repair


My experiences with an HP pavilion F1723

Hi!

I found a dead Hewlett Packard Pavilion F1723 LCD monitor in a some trash and had a quick look.

The screen did not have any cracks so I thought I'd try it.

When I powered it on the green power light would light up then a few seconds later, the screen would briefly flash "Monitor going to sleep" and the green light would go out.

This repeated every 5- 10 seconds.

 

I googled the part number and discovered that this HP monitor has been the subject of many requests for help, exhibiting  faults including

  • The above mentioned flashing,
  • Dull video
  • Dying after a few minutes use.

I tried the solutions mentioned in the articles I found, including re-soldering everything in sight, examining the board for bad joints and burned resistors and looking for loose components.

I discovered that if I un-plugged the two back light cold-cathode tubes then switching on, the power light would stay on and eventually go yellow.

 

This is what you would expect if the monitor was working but with no signal!

I carefully re-attached the tube connectors (Don't try this at home! the tubes run off very high DC voltage and you could get a nasty shock if , like me, you were stupid enough to touch these connectors! :-)

I then applied a signal to the monitor and up it came!

Great!

Unfortunately, the prospect of fiddling with the tube connectors every time I switched off my monitor did not appeal.

 I thought about the problem and it seemed to me that the sequence of events was as follows.

  1. the monitor would initialize all it's chips.
  2. It would start driving the LCD
  3. It would switch on the backlights.
  4. At this point the power supply appeared to shut down as if an error condition had appeared!

 I thought that this could be caused by a faulty smoothing capacitor on the low-voltage side of the power supply.

If a capacitor fails it may end up with a much lower capacitance. This means it cannot do it's job of supplying peak demands for current from the rest of the circuit.

This is exactly the kind of demand that would be made by the backlights switching on.

 

With this new clue I examined the power supply board and noticed that one of the 16V 1000uF electrolytic capacitors had the tell-tale signs of failure.

  • It was squint.
  • It's top surface was domed out slightly.
  • There was visible residue underneath it.

The reason these signs appear is that when Electrolytic capacitors fail they often boil the liquid components of their construction.

The steam pressure inside the capacitor pushes out the top surface, pushes out the base, causing the squint appearance and eventually vents either through the specially weakened top (That's what the cross shaped score-marks are for!) or through the bottom (As happened here).

I removed the capacitor and saw that it had failed completely.

I cleaned the board location of the residue and replaced the capacitor with a 25 V 100uF electrolytic I had lying about.

The voltage is the maximum working voltage so it was actually further inside it's safe operating limit than the old one.

It was slightly bigger than the 16 V capacitor so I wired it in and insulated it.

I also stuck it down with some of the white thermal glue gunk that I recovered off the old capacitor.

I heated it up with an old bit of copper I held in a pair of pliers insulated (From heat!) with a bit of folded cardboard and heated with my soldering iron.

( don't want to hear of anyone melting the glue directly with their Irons! No one likes to see a dirty soldering tip! :-)

 

I am typing this report using the re-built F1723 and I am happy to report it is working beautifully!

 

I hope this helps someone with repairing this monitor and any other electronic work!

 

Keep Soldering!

Ali Gee