HP monitor power light turns on and off

N

NewbieRepair

New Member
Hi everyone,

This is my first post here and I'm new to electronics repair.

I have an HP LA2006x monitor that's giving me some trouble. The display doesn't come on at all. When I power it up, the power light turns on for a second or two, then goes off, and this cycle keeps repeating.

I couldn't find any obvious signs of failure - no burn marks. Capacitors seem fine (none are bulging). I tried different cables, outlets etc... Issue is still the same.

I'm not sure what to try next. Any advice would be greatly appreciated!
 
Solution
NewbieRepair said:
Alright, I'll check all the capacitors. I've already tested a few and they were fine. Desoldering them and checking their capacitance should be enough, right?
Click to expand...

No, completely pointless, it's the ESR which goes high - you need an ESR meter to test them (as augustinetez mentioned).

It's extremely common that their capacitance is still fine (and within spec.), but the ESR is what matters - I've tested (and replaced) thousands of such failed capacitors. Basically it's down to cheap low quality capacitors, if decent quality ones were fitted, then they DON'T fail - if cheap crap ones were fitted (like Samsung have tended to use) then they fail pretty rapidly.

However, assuming it's the PSU which is at fault, it could be...
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It may be the capacitors in the internal power supply, since it is powered by an AC line cord. Many older monitors have faulty capacitors (due to aging) in the AC to DC power supply, visible by the swelling (or leaking) of the capacitors. Usually replacing just the capacitors will fix the issue (usually 4 to 6 of them, depending on design).
When the caps are swollen, their capacitance changes drastically, causing the power supply to start and it then detects a "bad" voltage or condition, then shuts down. Once the caps are discharged in a second or two, the power supply starts up again, then faults again, over and over...
Even though you looked the caps over, I would check again around the power supply...
 
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NewbieRepair said:
I couldn't find any obvious signs of failure - no burn marks. Capacitors seem fine (none are bulging). I tried different cables, outlets etc... Issue is still the same.
Click to expand...

The caps could still have degraded and dropped in value and/or the ESR increased to unreasonable levels.
 
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NewbieRepair said:
Alright, I'll check all the capacitors. I've already tested a few and they were fine. Desoldering them and checking their capacitance should be enough, right?
Click to expand...

No, completely pointless, it's the ESR which goes high - you need an ESR meter to test them (as augustinetez mentioned).

It's extremely common that their capacitance is still fine (and within spec.), but the ESR is what matters - I've tested (and replaced) thousands of such failed capacitors. Basically it's down to cheap low quality capacitors, if decent quality ones were fitted, then they DON'T fail - if cheap crap ones were fitted (like Samsung have tended to use) then they fail pretty rapidly.

However, assuming it's the PSU which is at fault, it could be capacitors, or it might not be - a common fault is failed secondary side rectifiers, having gone S/C - easily checked by running along the diodes with a multimeter in diode test mode.

If you have a scope, then you can check the ESR of the secondary capacitors (and at the time check the diodes), by applying the scope to each side of the secondary diodes in turn - the input (anode) should have 'high' voltage pulses (depending on the voltage rail). The output (cathode) should have DC voltage, and very little ripple - excessive ripple means high ESR reservoir capacitors, and the same on both sides means the diode is S/C.

Bear in mind, as it's pulsing ON and OFF the input pulses will come and go, and the output DC voltage will rise and fall accordingly.
 
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Solution
I wouldn't be advising the use of an oscilloscope to someone new to electronics repair, you only need to connect the probe ground lead to the wrong place and and all sorts of nasty things can happen, to both the power supply and the scope.

Stick to removing the capacitors and using an ESR meter and doing static (unpowered) multimeter tests on the diodes.
 
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Which was why I specifically said to use it on the SECONDARY side of the transformer only.
 
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ZipZapOuch said:
Don't bother buying an ESR meter - just buy a set of new capacitors. It will be cheaper and you'll know immediately if the thing powered up, it was the bad caps.
Click to expand...
I was thinking about doing that, but I'll stick with the ESR meter. I'll need it in the future, and I can't get new capacitors anywhere in my town. My only option is to desolder them from old electronics, but even then I'm not sure if they're 100% reliable. If the problem turns out to be a capacitor, I'll try to find a good one in some unused electronics.
 
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No!!! - the issue with massive high-ESR capacitor failures is poor quality electrolytics used in switch-mode PSU's - replace them with brand new, high quality ones. I usually order Panasonic or Rubycon capacitors. Electrolytics are something you don't use second hand.

Even if you could buy capacitors in your town (I can't even buy bread in mine ) it's unlikely that decent quality ones will be available - order on-line, I use RS or Farnell, and only order good makes.

If you fit high quality capacitors, then they will most likely outlast the rest of the equipment - this is why capacitors rarely fail in Sony equipment, as they fit higher quality components. The only 'Sony' capacitor failures are in cheap stuff like low cost sound bars, where they don't manufacture them themselves.

Samsung though, commonly fitted electrolytics where the manufacturers rated lifetime was only about 9 months, and had massive warranty failures - although they continued that way for about 5 years.
 
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Thanks for the info, I'll order higher quality ones if issue turns out to be capacitors.
 
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Hey, I'm back! It took me some time to get a meter and return to fixing, but now it's working! The issue was a faulty capacitor with very high ESR. After changing it, the monitor is now working properly.

However, I have another question. When the monitor is on and the display is working (not while it's in sleep mode or off), there's a very quiet buzzing noise. It's not something I hear all the time, but I notice it when I get really close. Is this something to worry about? What could be causing it, and if it fails in the future, where should I look?

Thanks!
 
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Often you get noises from inductors, transformers etc, this is fairly normal.
 
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