Benq FP737S - Q7T3

[pedro]

I started a fresh thread on this because too often an existing thread get confused by multiple addons with different symptoms, and I for one get a bit confused trying to relate a post on page 3 to whichever post earlier in the thread ...

This unit has been previously repaired, with the 2SC5707's, the FU9024's and picofuse PF751 replaced. Symptom: power LED responds to video, backlight dead, inverter fuse PF751 open.

PSU switcher and voltages check out OK. All the above transistors, as well as Q741,742,752 and 753 test OK in circuit. Pulled the two FETs (I do this as it breaks the circuit at a convenient point and allows testing them off-board) and they test OK out. All solder joints pass close visual. All four inverter transformer secondaries agree within 1 ohm. All caps test OK on ESR. Based on the symptoms, I don't see capacitance deviation involved.

With FETs out and only the PSU and backlights connected (i.e. no video board/panel) I jumpered PF751. Current after small inrush was < 1mA. (Note that with the video board disconnected, the TL1451 doesn't get a Vcc feed).

Fed the +15V (through meter) direct to each diode at the FET output position. Each inverter section lights its backlights, currents were 475mA and 495mA.

Routine recap and resoldering joints is obviously planned, but first I'm trying to sort out the cause of the fuse failure. Possibilities I see are:

1. The fuse failure was random.
2. One of the bipolars (741/742/752/753) in the FET drive control is wacky.
3. The TL1451 is shite.

Comments?

[PlainBill]

Re: Benq FP737S - Q7T3

Good piece of failure analysis. I agree, assuming the fuse failure was a random event is pushing the bounds of probability. I WOULD add item 4 to your list of possible causes - problems with one of the film caps in the Royer oscillator. If one of those goes open it would cause a current spike.

PlainBill

[Rtech]

Re: Benq FP737S - Q7T3

The PF fuse often goes,seemingly for no apparent reason,and I see this quite regularly on Dell monitors which use the same cct.Have tried testing components,as you have,but in the end,simply replacing it,has seemed to be the answer.

[shovenose]

Re: Benq FP737S - Q7T3

I would try to replace the fuse and see what happens

[alexanna]

Re: Benq FP737S - Q7T3

Some of the benq borads I have seen fail, do so after they have been woken up from the sleep mode [amber light] several times, possibly over night. And if I remember correctly I don't think I have lost a fuse with the transistor failure ,It has been with a failure with the power fet.But any thing is possible.
If you could rig a way to test [with extension leads ]cover the p/s up with it's back plastic case and monitor the peak current flow as it comes out of sleep mode.
Al.

[pedro]

Re: Benq FP737S - Q7T3

Some of the benq boards I have seen fail, do so after they have been woken up from the sleep mode [amber light] several times, possibly over night. And if I remember correctly I don't think I have lost a fuse with the transistor failure ,It has been with a failure with the power fet.But any thing is possible.
If you could rig a way to test [with extension leads ]cover the p/s up with it's back plastic case and monitor the peak current flow as it comes out of sleep mode.

Did the lazy version of that - no cover, and set the meter on the 15V at the fuse location to peak detect. Powered up and then cycled the video source. The running current was 1.08A and the best inrush I could capture was 1.11A. Repeated this dozens of times, same figures. Let it sleep for a fairwhile and repeat, same result. Left it on smoke test and current sat at 1.08A. Did a temp check (ambient 26C) on 5707's, 9024's, and all LV heatsinks. Hottest was 41C, this is with the board horizontal and no cover.

Good piece of failure analysis. I agree, assuming the fuse failure was a random event is pushing the bounds of probability. I WOULD add item 4 to your list of possible causes - problems with one of the film caps in the Royer oscillator. If one of those goes open it would cause a current spike.

But that would be a one-off event, surely, unless it went intermittent? I wondered if such a failure would raise the inverter freq noticeably, and might explain the inverter running current differences I observed (475 vs 495 mA).

The PF fuse often goes,seemingly for no apparent reason,and I see this quite regularly on Dell monitors which use the same cct.Have tried testing components,as you have,but in the end,simply replacing it,has seemed to be the answer.

and

I would try to replace the fuse and see what happens

That's what I have effectively done today after putting the 9024's back, bridging the dead fuse with the meter. If no more magic smoke appears, it'll be a fuse/recap/resolder/reassemble.

Thanks guys for your input.

[aheliason]

Re: Benq FP737S - Q7T3

Benq Q7T3. I've replaced the blown 3A SB fuse. Unit ran for about two days then blew the fuse again. Now I've pulled the 470 and 1000 uf electrolytics. Trouble is, I mixed them up by mistake. Anyone have a parts placement or schematic? This new member needs some help! Thanks.

[retiredcaps]

Re: Benq FP737S - Q7T3

Unit ran for about two days then blew the fuse again. Now I've pulled the 470 and 1000 uf electrolytics.

You must resolder all the inverter transformer pins or the picofuse will blow again. There is a 14 page thread of this monitor at

https://www.badcaps.net/forum/showth...light=fp737%2A

There should be the info you need to figure which cap goes where.

[aheliason]

Re: Benq FP737S - Q7T3

Thank you for the link to the discussion. I found the schematic referred to on page 2 as "Benq Inverter Schematic" to be very similar to my Q7T3 PCB. The component designators match up but all the 500 series are unpopulated on my board. To start with I'm replacing all the 470 and 1000 ufd electrolytics with 35 volt 105 degree ones. Also will resolder the xfmrs, as you suggested. Will let you know how this works out.