HP-L1506 keeps blowing internal fuse

[techfixit]

Hi, I'm looking for some help in trying to diagnose why the fuse keeps blowing in a HP monitor.. 3 monitors to be exact I got them from a local business who were throwing them out because they no longer worked. I've plugged them in to test and all three show no signs of power or lights

I decided to strip them down and investigate and I've discovered the main fuse has blown on all 3. Knowing that fuses generally don't blow for no apparent reason i inspected the boards for other obvious faults and discovered the main filter cap had failed (leaking brown gunge). I replaced the main caps with the exact same spec cap and also fitted a new fuse. Plugged in the display and FLASH.. fuse blows immediately!!!

To simplify things i've focused my attention on just one monitor for now as fixing one should help me solve the problem with the other two. So far I've tested the bridge rectifier, Mosfet and all of the other filter caps which have all tested good.. at least to my limited knowledge and understanding

Could someone please point me in the right direction here as what to test and how to go about doing it please.

[budm]

Re: HP-L1506 keeps blowing internal fuse

What resistance do you read between the two outer pins of the bridge rectifier?
Same question for the two inner pins?
Something is/are badly shorted out to make the fuses turn black like that inside.

"I've tested the bridge rectifier, Mosfet and all of the other filter caps which have all tested good.. at least to my limited knowledge and understanding" What are the resistance readings of those parts?

[techfixit]

Re: HP-L1506 keeps blowing internal fuse

bridge rectifier:

Tested out of circuit, meter set to diode test mode: Pin out 1234 = +~~-

black probe to 1, red to 2 reading: .524
reverse probes 1 to 2 reading: 0L
pins 2 and 3 have no reading either way
pins 3 to 4, black to 3 red to 4 reading: .532
reverse probes, 3 to 4 reading: 0L

MOSFET:

Tested out of circuit. Pinout: 123 = GDS

black probe to 1 red to 2: .17
black probe to 1 red to 3: .17
black probe to 2 red to 3: .000
black probe to 2 red to 1: .17
black probe to 3 red to 1: .17
black probe to 3 red to 2: .000

Reversing probes produces same results

Main Caps:

470uf 35v - .03
470uf 35v - .03
1000uf 25v - .05
1000uf 16v - .02
220uf 10v - .06
1000uf 10v - .04
68uv 450v - 1.2

Tested in circuit using an ESR meter

EDIT: Thought i should note the burnt fuse pictured is a regular 2A/250V glass fuse i put in to test after replacing the main filter cap. The original fuse is a T2AH/250V ceramic fuse. Would this make a difference?

[stj]

Re: HP-L1506 keeps blowing internal fuse

that fet is obviously shorted.

[techfixit]

Re: HP-L1506 keeps blowing internal fuse

that fet is obviously shorted.

What should a good reading look like?

[stj]

Re: HP-L1506 keeps blowing internal fuse

well not 000 that's for sure.

[Agent24]

Re: HP-L1506 keeps blowing internal fuse

Yeah that FET sounds pretty stuffed to me. And it seems like it's shorted through the gate as well so you will probably find damage to the drive circuitry too.

I suggest you use the series lightbulb trick\dim bulb tester when repairing these to stop blowing more fuses and possibly damaging replacement parts if you don't get all the faulty parts replaced first time.


You can use glass fuses instead of ceramic - in terms of operation there is no difference assuming they are the same rating. Ceramic have higher breaking capacity (That is what the H in the T2AH means) and are safer when they blow hard. Glass ones can rupture.

Note that the T in T2AH means Time-Delay or Slow-Blow. You do need this type to stop the fuse blowing on inrush current when the primary capacitor(s) first charge up.

[techfixit]

Re: HP-L1506 keeps blowing internal fuse

Are there any components i can test while i'm waiting on my replacement mosfets to hopefully rule out further damage along the circuit? I'll knock up a dim bulb tester in the mean time and grab some slow-blow fuses. Thanks for everyones input so far, really appreciate the help

[Agent24]

Re: HP-L1506 keeps blowing internal fuse

You could check any snubber components for the FET and make sure they're not blown too. Failure of those could have been what caused the FET to die in the first place.

Also if you can upload a photo of the track side of the board that will help us understand the circuit better. If you upload to the forum itself it is better too. External linked images have a habit of eventually disappearing, which is annoying for anyone referring to the thread in the future.

[budm]

Re: HP-L1506 keeps blowing internal fuse

The IC that drives the MOSFET should also be replaced.

[techfixit]

Re: HP-L1506 keeps blowing internal fuse

The IC that drives the MOSFET should also be replaced.

Is that IC601 your referring to?

[Agent24]

Re: HP-L1506 keeps blowing internal fuse

Going by its proximity to the FET, most likely. What is the part number of IC601?

[techfixit]

Re: HP-L1506 keeps blowing internal fuse

You could check any snubber components for the FET and make sure they're not blown too. Failure of those could have been what caused the FET to die in the first place.

Also if you can upload a photo of the track side of the board that will help us understand the circuit better. If you upload to the forum itself it is better too. External linked images have a habit of eventually disappearing, which is annoying for anyone referring to the thread in the future.

Can you elaborate on snubber components? Would the mosfet failing have caused the main filter cap to dry out or did the mosfet fail because the filter cap did? Sorry about the pics.. I'll upload directly from now on

[techfixit]

Re: HP-L1506 keeps blowing internal fuse

Going by its proximity to the FET, most likely. What is the part number of IC601?

IC601 part number is:

1200AP40
CPDM
0601G

[Agent24]

Re: HP-L1506 keeps blowing internal fuse

Can you elaborate on snubber components? Would the mosfet failing have caused the main filter cap to dry out or did the mosfet fail because the filter cap did? Sorry about the pics.. I'll upload directly from now on

The snubber components will be in parallel with the primary transformer winding the FET switches. Look for a highspeed diode, high voltage capacitor (1kV or more) and maybe some resistor(s).

The attached BenQ schematic is very similar to yours. D603, C603 and R603 make the snubber network. In your case it looks like C603 is actually C614.

Check those components for open circuit.

It's very possible that a failed primary capacitor will blow the FET. The other way around is not so likely.

IC601 part number is:

1200AP40
CPDM
0601G

I found and attached the datasheet. Make sure you get the right version (ends in 40) otherwise it will be the wrong frequency.

[techfixit]

Re: HP-L1506 keeps blowing internal fuse

Some good news to report. I changed the failed Mosfets on all 3 boards and 2 of the monitors are working perfectly now

The third monitor is turning on but then immediately switches off and restarts as soon as the test image appears. I've got 318v at the main filter cap and i've checked all the other filter caps with an ESR meter and all of those tested good.

Has anyone got any suggestions?

[stj]

Re: HP-L1506 keeps blowing internal fuse

try swapping the boards around to narrow it down.
see if all the power boards are good.

[techfixit]

Re: HP-L1506 keeps blowing internal fuse

try swapping the boards around to narrow it down.
see if all the power boards are good.

Sorry should have mentioned i tested the non working board with all 3 lcd's and same result

[stj]

Re: HP-L1506 keeps blowing internal fuse

how about a foto of the board.

[techfixit]

Re: HP-L1506 keeps blowing internal fuse

how about a foto of the board.

It's the same board in the previous pics i posted