PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

[Newbie2]

Hello guys,

I got another BenQ FP731 (Q7T3) monitor on the blink. Its original 2SC5707 transistors were replaced with D44H11 transistors since the originals failed, and this monitor doesn't like the D44H11 transistors. I have another identical BenQ FP731 (Q7T3) monitor that works perfectly with the D44H11 replacements.

This monitor did get its four 2SC5707 transistors (Q739, Q740, Q759, Q760) replaced with D44H11 transistors, its "bel" 3A fuse (PF751) replaced with the exact same fuse and the power section was recapped with Panasonic FC capacitors with identical values and sizes of the original "Elite" capacitors.

My buddy who's an electrical enginner suggested "transistor DC bias" and said that the D44H11 transistors I put it must have low hFE values. I tested a couple that I had and found a few good ones (between 200-213 hFE) while the others were between 182-195 hFE. I installed all the higher hFE D44H11 transistors and still the problem remains. My buddy said that this monitor needs transistors with high gain, and I was thinking NPN Darlington transistors due to their superior gain to regular NPN BJTs.

I did a search and I think that the 2N6388 (PDF spec sheet: https://cdn.badcaps-static.com/pdfs/...4ecf5b2869.pdf) and 2SD1277 (PDF spec sheet: ) Darlington transistors could do the job. My buddy isn't available to check the specs, so I want you guys that can help me to see if they're suitable.

I did a short video on how this monitor behaves, you can download it here: http://www.mediafire.com/?tcd1aezgc9g

Also I got the official BenQ schematic for the Q7T3 power board of this monitor, it can be downloaded here: http://www.mediafire.com/?qd9u13ecnw9

Please help me and make good suggestions on this!

[Newbie2]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

Also (sorry mods and Topcat if you see this for double posting as I couldn't edit my post above this one) here is a picture I took of the front of the entire power board (download link here): http://www.mediafire.com/?f7g9h1dnp1x

[Krankshaft]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

I wouldn't be so quick to dismiss these transistors yet. If they work fine on an identical monitor then your problem lies in the monitor itself and not the transistors.

Is there some reason why PF501 doesn't have either a fuse or jumper wire across it?

All of the other pico fuses that were axed by the beancounters have jumper wires across them.

If this is how it is on the identical monitor the works dismiss that thought.

Be sure to check for any marginal solder joints especially on and around the HV transformers.

Did you try swapping panels between the identical monitors?

A bad backlight can cause the inverter to shut down in this manner and this is the easiest way to rule this out.

[Newbie2]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

I forgot to say that this monitor sometimes does blink and sometimes works properly, and when it works properly its brightness is the same as the other FP731 monitor so I don't believe the backlights on this monitor are going belly up on me yet.

When it's working and it wants to go into its little power blink routine first if it's on the display will go a little dim for one second (my buddy thinks a backlight shut off at that moment) and then the screen fully shuts off with the green power light still on. Then I'll have to turn it on and off until the screen turns on for more than 4 seconds.

I'm pretty sure both of these monitors have the same power board layout. One reason is they were purchased at the same time and the stickers on the back of them show that they were manufactured in the same month of the same year (2005). The picture of the power board was taken from the good perfectly working FP731 monitor.

I haven't really checked the solder joints on this monitor, I only checked the solder joints that I made when I replaced the transistors and capacitors. I'll check them when I open this monitor up again for the 5th time.

Sorry about my inexperience since I have little electrical experience and I am a computer technician.

My guess is that the little green Lelon 100uf 25V cap at C795 (visible in the schematic and photo of the power board) is going bad even though its vent has not bulged. I said that to my buddy and he firmly thinks the transistors are the problem, not any caps or other components.

[Wizard]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

Must change capacitors.

Cheers, Wizard

[Newbie2]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

Must change capacitors.

Cheers, Wizard

I did replace all the original "Elite" 470uf 35V and 1000uf 16V caps with Panasonic FC replacements of the same size and values, so those caps I don't think need to be changed.

-------------
I'm getting a little suspect on that tiny Lelon 100uf 25V capacitor, it doesn't look bad but I believe (I may not be right) it is on the SCP circuit as shown on the schematic of this power board so when I get some time I'll open up the monitor and check that cap. I got a slightly bigger but not much taller 8mm Panasonic FC 100uf 63V cap to replace it if needed.

So far here's what I think are possibilities besides the transistors:
1. Dry/bad solder joints in the inverter area of the power board only occasionally working, causing the power blinks when they are not working.

2. The small caps in the inverter.

Someone experienced who studied the BenQ Q7T3 power board and similar BenQ power boards (davmax and Krankshaft for example) can really help me here if possible.

[Newbie2]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

I got new evidence.

The problematic FP731 monitor I have just did its dreaded power blinks, and I found something interesting about its behaviour.

It was on and the screen went dim for a second and the screen went totally blank. To test the solder joint theory, I banged the monitor a few times on its side. I turned it on, and it turned on for 10 seconds instead of 4 seconds. It never did that in a power blink situation. I turned it off, and turned it on. The dim BenQ logo appeared, and I hit the side of the monitor at that point. The BenQ logo suddenly brightened right after I hit the monitor's side, and turned on normally. Currently it is still on and displaying an image.

Now when I open this monitor up I'll have something to check in the inverter area of the power board, the solder joints. My solder joints are good, I just need to check the other joints and reflow the solder and add more if the joint checked out bad.

[Newbie2]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

I disassembled the monitor yesterday and inspected its power board. I saw just what I was suspecting, there were numerous dry joints around the inverter area.

Fixed those solder joints, and the monitor works perfectly! (for now, I'll see if it continues its reliability into the long-term)

[Krankshaft]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

Thats why I always do a once over on repaired boards before powering them up.

Solder joints can cause wierd things to happen.

All of the heat generated from these boards doesn't help matters either.

[verbs]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

I am a Newbie here,

But let me say that this forum is bang-on, for the help on my Benq F731.
I had two blown 2sc5707 and the Bel 3AMP fuse. Also to top it off I have blanking out problems with the bad solder joint on the HV's transformers on the inverter side. WOW! You guys are great!

Just to add my 2-cents worth, my local store was on back order for the 2sc5707, but did reference the D44H11 with 2SD1267 transistors. They work too, but my shielding cover allowed for the larger chip package.

thanks
verbs

[Wizard]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

that's cool. Good job then!

When I redo the capacitors, I check and resolder other items.

Cheers, Wizard

[EGuevarae]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

I am a Newbie here,

But let me say that this forum is bang-on, for the help on my Benq F731.
I had two blown 2sc5707 and the Bel 3AMP fuse. Also to top it off I have blanking out problems with the bad solder joint on the HV's transformers on the inverter side. WOW! You guys are great!

Just to add my 2-cents worth, my local store was on back order for the 2sc5707, but did reference the D44H11 with 2SD1267 transistors. They work too, but my shielding cover allowed for the larger chip package.

thanks
verbs

So, if you can't get a set of 2sc5707 you can substitute them with some 2sd1267 ? Someone here can confirm that based on specs/datasheets?

[Gianni]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

For what I can say, 2SD1267 has lower HFE (min 70 max 250) compared to 2SC5707 (min 200 max 560).
There are some differences also for current and voltage capability (60V vs 80V and (4/8A vs 8/11A).

The HFE should not be a problem since the D44H11 used by Newbie2 has HFE min 80 (can't find max value) but the voltage and current are important; D4H11 is closer to 2SC5707 specs.
Unfortunately I found the datasheets coming form 3 different manufacturer and as usual they give parameters based on different test condition.

Ciao
Gianni

[futrose]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

I am having the same issue with my BenQ FP731 monitor, wouldn't turn on at all so I replaced a bad bulb, (was black on the end) and now the monitor comes on for about 2 seconds then goes off. If I am going to resolder the HV transformers do I touch up just the solder on the bottom of the of the board or all the individual connections on the sides of the transformers too? And am I correct that the fuse (PF751) should have an ohms value if it is good and not 0?
Thanks.

[PlainBill]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

I am having the same issue with my BenQ FP731 monitor, wouldn't turn on at all so I replaced a bad bulb, (was black on the end) and now the monitor comes on for about 2 seconds then goes off. If I am going to resolder the HV transformers do I touch up just the solder on the bottom of the of the board or all the individual connections on the sides of the transformers too? And am I correct that the fuse (PF751) should have an ohms value if it is good and not 0?
Thanks.

Just resolder the connections to the pc board unless the ones on the side of the transformer are obviously bad.

A good fuse should have a resistance below 1 ohm.

The problem you are describing appears to be the 'two seconds to black' problem.

PlainBill

[futrose]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

Yeah, is there a thread that explains how to trouble shoot the "2 seconds to black" issue? I changed some of the caps (35v) and when I tested the voltage on these (voltage meter on both leads) I was only getting 16V. Is that correct?

[PlainBill]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

There isn't any detailed procedure because each model of monitor is different. The background goes like this: The inverter gets the BLON signal, and the inverter controller turns on all CCFLs at full brightness. After a couple of seconds it starts monitoring the voltage across each CCFL and the current through each CCFL. If voltage or current is too high or too low, the controller shuts down all CCFLs.

In general the first step is to determine of the problem is the inverter or a CCFL. For this I suggest using a case mod light kit. The inverter from the kit will check the CCFLs in the monitor, the CCFLs from the kit will check the inverter in the monitor.

With a schematic it would be fairly easy to determine where the problem lies. Without one, it gets very tedious, especially if you do not have an oscilloscope available.

One place to start is to locate the inverter controller and download the data sheet and application notes for it.

PlainBill

[CodyJ]

Re: PLEASE HELP! Repaired BenQ FP731 (Q7T3) on the blink!

Hello to all, I am new! But not to electronics. I have no experience with the tubes CCFL light bulbs in these monitors... but here is how it happened. The monitor and PC were both on, monitor was in standby (working great just a couple minutes ago). I came back and moved the mouse, the screen light up green, could see my desktop but then it got brighter and then went DARK, can still see it so i'm going towards an inverter failure or CCFL bulb failure. How would I pinpoint this as from what I am reading the inverter can fail but the symptoms of HOW it failed I would like some personal advice as to if anyone has had the same experience and what was it .. inverter or CCFL bulbs. Monitor works fine just awfully hard to see.

Thank to al and hope I am not bumping the topic bad as I picked it up on google.