Benq FP91G+ few milliseconds to black - beyond my expertise

[pfrcom]

When I got this Benq FP91G+ its power light wouldn't even come on when the power button was pressed

One of the C5707 pair indicated by red arrows tested faulty, so I replaced the pair

A solder joint on the component between those C5707 was cracked, so both its legs have been resoldered

Because a joint was cracked, I've also resoldered legs of the components indicated by yellow arrows - are they usually FU9024 ?

Not in this instance - one is marked F1 F37 FQU 11P06, the other F1 F38 FQU 11P06

I've also resoldered legs of the transformers

The C5707 pair indicated by green arrows is original

All capacitors have been replaced, not necessarily by high quality ones, but sourced from a local specialist supplier (not a chain store)

Now after pressing the button, the power light stays green

There'll be a brief flash from the backlights, usually with the Benq logo visible on screen for a fraction of a second

Help please - how do I progress from here ?

[mariushm]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

This issue can also be caused by CFL lights going bad (too old) or due to the wires having a poor connection to the lamp ends.

The wires going to each lamp are simply soldered to the end of a lamps, as you can see in this page here: http://www.ccflwarehouse.com/lasotu.html

See the pictures in the link above

Often the wire breaks under that silicon jacket, or the solder no longer makes good contact and the connection is interrupted.

The inverter has a sort of "protection" mechanism in which if a lamp doesn't start in a second or so, it turns off all lamps.

So you'd have to carefully open the back of the panel to reach those silicon jackets, pull them gently to get access to the solder and visually check if the wires are properly connected and to be sure the insulation is not damaged by electric arks. Be sure to do this with the monitor turned off, the lamps have 700v going through them when turned on.

Ideally, you'd just use another cfl tube with the wires already connected and known to work (maybe removed from a monitor that failed for other reasons) so you save yourself the trouble of disassembling the thing.. this way you just unplug one lamp at a time and replace it with your test lamp and if the monitor stays on you know that lamp you disconnected was bad.

[budm]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

Fuse PF801 read less tahn 1 Ohm?

[pfrcom]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

Fuse PF801 read less tahn 1 Ohm?

Reads ~ 556 Ohm in either direction

[pfrcom]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

3) Open fuse

Some boards will have a fuse (F designation) or picofuse (PF designation) and they may be open because a transistor was shorted or some other component went bad.

If you have a manual range multimeter, set it to 200 ohms (two hundred). You can test for open fuses "in circuit" by (with power off and unplugged)

a) put black probe on one pin and red probe on other pin - read/record ohm

A reading of less than 1.0 ohms indicates a good fuse. Anything higher than 1.0 ohm is a sign of an open fuse. Do NOT replace fuses and turn on the power back on without checking for what caused the fuse to go open otherwise you will just be replacing the fuse again.

Damn - looks like I wasn't thorough enough

I'd already desoldered, and put back, diodes which, in circuit, showed ohms in both directions - but seemed OK out of circuit

How bad would it be, to swap in a resetable fuse salvaged from a motherboard ?

Or for that matter, a one-time fuse salvaged from a motherboard ?

[retiredcaps]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

Resolder all the inverter transformer pins regardless how they look. If you don't the picofuse is likely to blow again. Don't ask me how I know!

[retiredcaps]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

Reads ~ 556 Ohm in either direction

Here is a case where the fuse isn't quite 100% open. On really cheap multimeters the threshold for continuity is any reading less than 1800 ohms which could mislead the user into thinking the fuse is good because the multimeter beeps continuous.

[pfrcom]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

Thanks budm - looks like you've saved another one for me

Replaced PF801 as shown in first attachment, with fuse salvaged from a computer motherboard - probably a keyboard fuse in its original incarnation

Leads were barely long enough to be soldered, shown by second attachment, apologies for the blurred pic - measure ~ 0.02 ohm solder joint to solder joint, so should suffice

I hope people who know what they're doing aren't horrified by a 5A fuse replacing the original 3A - only alternative on hand was unmarked, so I thought better the devil you know ...

Now the monitor lights up and complains about no connection, so time to reassemble and test properly

@retiredcaps - the inverter transformers are the black components with yellow tape on them, aren't they ? - I think I've already done them all, sorry to be vague but this one's been hanging around a while

[retiredcaps]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

@retiredcaps - the inverter transformers are the black components with yellow tape on them, aren't they ?

Yes.

[smason]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

Now that's an interesting case.
My (flawed) logic would have been to think "ok the backlights come on briefly, if the fuse was blown, they wouldn't come on at all" and I probably wouldn't have gotten around to testing the fuse.

[pfrcom]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

My (flawed) logic would have been to think "ok the backlights come on briefly, if the fuse was blown, they wouldn't come on at all" and I probably wouldn't have gotten around to testing the fuse.

Mine too - and the reason this one's been hanging around for a while

Talk about a DOH moment when budm asked about PF801, then I measured it

[retiredcaps]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

Since I have worked on 4 or 5 of these Benq variations, I meaure the fuse regardless of the symptoms. It only takes 5 seconds and then I move onto checking all the usual suspects.

553 ohm fuse is not the same as an open fuse (infinite resistance).

[budm]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

This Belfuse, when it goes bad (over current), instead of going completely open circuit, it has high resistance instead, so it is like feeding B+ through 500 Ohms resistor to run the inverter circuits.

[pfrcom]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

UPDATE

Pleasure of this repair lasted only a few months before being replaced by pain from the monitor refusing to switch on one morning

So it got put aside until I needed a working monitor

Expecting my recycled picofuse to be the culprit, I was surprised to find that it tested OK

Not necessarily the case for F601, the fuse beside the AC input socket

F601 has no audible beep when tested with the Diode test / Continuity setting of my Fluke 77

Although it does measure ~6.45 M if I use the meter's Ohm setting - presumably this number is MilliOhms, indicating a good fuse ?

Am I barking up the wrong tree, or have I found the likely culprit ?

I'd already tested the four C5707 as good, before checking the fuses

HELP PLEASE

[stj]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

suspect a problem with the caps - eiher the 4 blue ceramics or the 2 gray poyesters.
most likely the polyesters.

[retiredcaps]

Re: Benq FP91G+ few milliseconds to black - beyond my expertise

Although it does measure ~6.45 M if I use the meter's Ohm setting - presumably this number is MilliOhms, indicating a good fuse ?

No, that is MEGA ohms on the Fluke 77.

The Fluke 77 can only show 0.1 ohm resolution. Straight from the manual.