HP LP1965 shorted 5v on display board

[tonymich]

I had the green LED flashing when on. Found the 5v line shorted to 1.4 ohms. When I unhook the ribbon cable, I have 5v on it. 1.4 ohms to ground on the display board. I lift the component B5 and I get my 5 volts on the board. have no idea what B5 is. I'm in the process of searching the web for the board M190EG01 V0. Anybody know what B5 is?

[Th3_uN1Qu3]

Re: HP LP1965 shorted 5v on display board

Some clear focused pictures of the board will help.

[tonymich]

Re: HP LP1965 shorted 5v on display board

OK. Here is the portion of the board showing the component B5 that I lifted to remove what appears to be a short on the 5 volt line.

[Th3_uN1Qu3]

Re: HP LP1965 shorted 5v on display board

Judging by the color it appears to be an inductor. It's puzzling though - there's no reason to put an inductor across a DC voltage rail, there's likely something downstream of it that is shorted, the inductor simply passes thru power to that area. I see a fuse there labeled F5. Is that fuse good? Also check the two voltage regulators.

[tonymich]

Re: HP LP1965 shorted 5v on display board

Yeah, the fuse is good. An inductor would be labeled with L and not B wouldn't you think. I measured across B5 and get a short. It appears that there is something on down the line loading the 5 volts. The regulators seem to be OK showing only the one leg tied to the 5 volt line at 1.5 ohms to ground. I hate surface mount stuff with no ID as to what value they are let along what they are. And it's a bitch lifting them out of the circuit.

[tom66]

Re: HP LP1965 shorted 5v on display board

Yeah, the fuse is good. An inductor would be labeled with L and not B wouldn't you think. I measured across B5 and get a short. It appears that there is something on down the line loading the 5 volts. The regulators seem to be OK showing only the one leg tied to the 5 volt line at 1.5 ohms to ground. I hate surface mount stuff with no ID as to what value they are let along what they are. And it's a bitch lifting them out of the circuit.

"B" can mean bead, as in ferrite bead. The colour also matches the common grey seen on a SMD ferrite bead. The ferrite bead essentially attenuates high frequency noise, and cannot short out anything, because it is always in series with a load. I would check to see if the short appears on both sides of B5.

[tonymich]

Re: HP LP1965 shorted 5v on display board

Yes, the short appears on both sides of B5. I've been trying to lift things to see if the short goes away. Very hard to do. Can't seem to find a device that will cause the short.

[tonymich]

Re: HP LP1965 shorted 5v on display board

I guess it's not actually a short since there is 1.5 ohms. I guess it's more of a load that shouldn't be there maybe. I'm wishing for a schematic.

[tom66]

Re: HP LP1965 shorted 5v on display board

I guess it's not actually a short since there is 1.5 ohms. I guess it's more of a load that shouldn't be there maybe. I'm wishing for a schematic.

Many semiconductors don't fail dead short, but do fail "shorted". 1.5 ohms is typical. I've seen a power MOSFET fail at 20 ohms. Doesn't sound like much (only 250mA @ 5V), but on the primary side of a power supply with a 330V bus, that makes a sizeable explosion (Not pretty!!)

1.5 ohms sounds abnormal. It's possible there's a load drawing a lot of current, but I wouldn't expect that at the low voltage used by a multimeter (about 0.3V.)

[alexanna]

Re: HP LP1965 shorted 5v on display board

Yes, the short appears on both sides of B5. I've been trying to lift things to see if the short goes away. Very hard to do. Can't seem to find a device that will cause the short.

If memory serves me correctly, there have been some cases of a shorted SMD cap on the panel board.
Have you checked any of these?
You still may have to lift it off the board if there is a resistor parallel in the circuit.

[tonymich]

Re: HP LP1965 shorted 5v on display board

Yeah, I have had some SMD's short on me in the past. I've had schematics in those cases and was easy to trace stuff out. I'll play around with it for another day lifting a few things but if I don't find something soon, to the recycle bin it goes so I can cut my costs. If I find a shorted cap, I'm not going to know what value it was anyway. Don't really need to waste too much time on a $200 LCD monitor.

[alexanna]

Re: HP LP1965 shorted 5v on display board

Yeah, I have had some SMD's short on me in the past. I've had schematics in those cases and was easy to trace stuff out. I'll play around with it for another day lifting a few things but if I don't find something soon, to the recycle bin it goes so I can cut my costs. If I find a shorted cap, I'm not going to know what value it was anyway. Don't really need to waste too much time on a $200 LCD monitor.

Well I would hope if you decide to recycle the monitor you save some of the parts
CCFLs come in handy for testing, A lot of the mains fuses are of the same spec, Power MOSFETS and Dual diodes are nice to have around.You just never know what you may need.
If you plan to repair another monitor in the future some of these things come in handy

[PlainBill]

Re: HP LP1965 shorted 5v on display board

Yeah, I have had some SMD's short on me in the past. I've had schematics in those cases and was easy to trace stuff out. I'll play around with it for another day lifting a few things but if I don't find something soon, to the recycle bin it goes so I can cut my costs. If I find a shorted cap, I'm not going to know what value it was anyway. Don't really need to waste too much time on a $200 LCD monitor.

There are a several techniques you can use to identify the short. The first MIGHT not work. Work your way along the 5V rail, checking the resistance. You MIGHT be able to see the resistance decrease as you approach the short. The limitation is the accuracy of your DMM at low ranges - typically that aren't very good.

The second is simpler. Set you DMM to the lowest voltage range possible, hold the red probe at the +5V input, and work your way down the 5V rail with the black probe, looking for increasing voltage drops. At the point where the voltage no longer increases, you have located the shorted component.

The third requires extra equipment. Hook a high current 5V supply to the 5V rail. CAREFULLY probe for a hot spot. A 1.5 ohm load on a 5V supply will pass 3A, for a total dissipation of 15 watts!! If it is a shorted SMD cap, you won't have to look to hard, it'll desolder itself!

The high current supply doesn't have to be too expensive - 3 D cells in series will do it. Just observe polarity.

PlainBill

[tonymich]

Re: HP LP1965 shorted 5v on display board

Thanks for all the input and some good troubleshooting ideas. I usually salvage what I can since we have multiple monitors of the same type around here. I'll play around with it a bit and then move on if need be.