Benq FP767 PS caps

[bullwinkle]

Re: Benq FP767 PS caps

C5707 are known to fail in BenQ boards as I observed in Dell LCD monitors. Dont forget to check their individual hfe values as a precautionary measure.

[Bobdee]

Re: Benq FP767 PS caps

Great posting Bill thanks

[PlainBill]

Re: Benq FP767 PS caps

Great walkthrough, PlainBill.

I've always wondered which is the easiest way to test the bridge rectifier.
From reading these forums, I've gathered that it's a good idea to desolder one terminal of a diode in order to test the bridge.
If we number the diodes on the next figure from 1 to 4, from top to bottom, which terminal should I desolder?

Thanks

In the case of the bridge in this type of power supply it isn't necessary. There is no input transformer to confuse the issue. Just do it with the cord unplugged. Measure from each terminal to each other in turn. If you see a short between any two terminals there is a problem. Then it becomes a matter of determining if the problem is the bridge or the power FET. Since it's less messy to remove the bridge, I usually do that IF NECESSARY. Even with a fuse in the circuit, a shorted diode or a shorted FET will often have visual damage.

PlainBill

[maverick530]

Re: Benq FP767 PS caps

I re-tested the board and I miss-spoke: I DO get a GREEN LIGHT on the front panel and a small weezing noise when the monitor is "turned off" from the front panel.

What does this change?

I will test the voltages across the mother cap and the cables that go to the logic board as soon as I can. Thanks,

[maverick530]

Re: Benq FP767 PS caps

A first step is to use a DVM and Carefully! check the voltage across C605 with the power cord plugged in and the power switch on.

PlainBill

What do you mean by "Carefully"? Careful I don't touch the high-voltage and shock myself? Or careful because there is a wrong way to probe and I might break something (either my meter or my board)? If so, what settings do I set my DMM for and what probe do I place where? Ex. do I touch the black wire on the negative pin of cap?

[Krankshaft]

Re: Benq FP767 PS caps

Wouldn't contact with the output of the inverter circuit give you an RF burn due to the high frequency output?

[Bobdee]

Re: Benq FP767 PS caps

I re-tested the board and I miss-spoke: I DO get a GREEN LIGHT on the front panel and a small weezing noise when the monitor is "turned off" from the front panel.

What does this change?

I will test the voltages across the mother cap and the cables that go to the logic board as soon as I can. Thanks,

Hello Maverick
Dont forget to check the Fuse PF701 that Plainbill asked you to as well, he will need this info, more so now that you have green LED

[PlainBill]

Re: Benq FP767 PS caps

Wouldn't contact with the output of the inverter circuit give you an RF burn due to the high frequency output?

I don't know. I've never tried it, and won't suggest it to anyone else.

HOWEVER, note that I was suggesting he measure the voltage across the main input cap because he indicated he didn't even get a green LED.

PlainBill

[PlainBill]

Re: Benq FP767 PS caps

I re-tested the board and I miss-spoke: I DO get a GREEN LIGHT on the front panel and a small weezing noise when the monitor is "turned off" from the front panel.

What does this change?

I will test the voltages across the mother cap and the cables that go to the logic board as soon as I can. Thanks,

No need to check the cap voltage. Voltages to the main (logic) board would be interesting, but not vital. The voltages on each side of PF701 are more significant at this point.

PlainBill

[PlainBill]

Re: Benq FP767 PS caps

Great walkthrough, PlainBill.

I've always wondered which is the easiest way to test the bridge rectifier.
From reading these forums, I've gathered that it's a good idea to desolder one terminal of a diode in order to test the bridge.
If we number the diodes on the next figure from 1 to 4, from top to bottom, which terminal should I desolder?

Thanks

To correct my earlier answer, don't bother unsoldering any leads for the first test. Measure across the center pair of leads. These are the AC input to the bridge. If they are shorted it's either a shorted bridge or another problem on the AC side. Then measure across the outer pair. This is the DC output. You may see an increasing resistance as the filter capacitor charges.

PlainBill

[Krankshaft]

Re: Benq FP767 PS caps

I re-tested the board and I miss-spoke: I DO get a GREEN LIGHT on the front panel and a small weezing noise when the monitor is "turned off" from the front panel.

It seems like the main power supply is fine he mis posted he does get a green light. That green LED is probably powered off the logic board which means that it's getting power from the main supply.

In that case test all 4 C5707s and the inverters pico fuse.

With a DMM switch it to diode check measure from base to collector then flip the position of the probes. Voltage drop in one direction is fine do that same from base to emitter same testing rule applies.

My HP 1955 Benq build board had the base, collector, and emitter locations silkscreened on the board.

[maverick530]

Re: Benq FP767 PS caps

In that case test all 4 C5707s and the inverters pico fuse.

Voltage drop in one direction is fine do that same from base to emitter same testing rule applies.

Diode testing the pico fuse IN CIRCUIT AND POWERED OFF, the pico fuse (PF701) measures 1 (infinity) in one direction, and .216 ohms in the other direction.

I did also test the pico fuse POWERED ON (but I don't think it was under load, meaning I didn't press the monitor's power button and have the green light on) and it measured a .7V in one direction.


Diode testing the C5707s IN CIRCUIT AND POWERED OFF, 2 of the 4 measured open (0 resistance) and the other 2 measured around .600 ohms but NEVER voltage dropped in any direction. IS THIS OKAY that they didn't voltage drop in one direction?


In an LCD, if only 2 of the 4 C5707s are bad, wouldn't at least one of the florescent tubes light up on the monitor, or do both have to function to see any picture?

[PlainBill]

Re: Benq FP767 PS caps

Diode testing the pico fuse IN CIRCUIT AND POWERED OFF, the pico fuse (PF701) measures 1 (infinity) in one direction, and .216 ohms in the other direction.

I did also test the pico fuse POWERED ON (but I don't think it was under load, meaning I didn't press the monitor's power button and have the green light on) and it measured a .7V in one direction.


Diode testing the C5707s IN CIRCUIT AND POWERED OFF, 2 of the 4 measured open (0 resistance) and the other 2 measured around .600 ohms but NEVER voltage dropped in any direction. IS THIS OKAY that they didn't voltage drop in one direction?


In an LCD, if only 2 of the 4 C5707s are bad, wouldn't at least one of the florescent tubes light up on the monitor, or do both have to function to see any picture?

For the pico fuse, measure the resistance acroos the fuse. Interpreting what you reported, it appears the fuse is open.

I don't have a good grasp of the circuit around the C5707's so it would be best to wait for Krankshaft's suggestions. However, I will point out that if only one transistor of the 4 is shorted, that will still be enough to blow the fuse. And with the fuse open, no power goes to the inverter.

PlainBill

[maverick530]

Re: Benq FP767 PS caps

No need to check the cap voltage. Voltages to the main (logic) board would be interesting, but not vital. The voltages on each side of PF701 are more significant at this point.

PlainBill

I tested the voltages to the logic board with the power board turned on but disconnected to the logic board, and couldn't get any solid readings when touching each wire. My DMM fluctuated between +-.1v a bit, but never stabilized. I'm thinking the connector must be connected to the logic board to make a complete circuit, or to start the power draw in some way? Or maybe I'm just not probing it right. I grounded the black wire of my DMM to ground and touched the red (positive) wire to the pins.

[maverick530]

Re: Benq FP767 PS caps

I also forgot to mention that the two C5707 transistors that are suspected bad are the top two, meaning they are right next to each other. This looks like a common occurrence that both will go out on the same circuit segment--can anyone confirm this? I just want to make sure they're not testing bad BECAUSE they're on the same segment and only one of them is REALLY bad (or worse, it's that square cap thingie between them (C824)). This is for learning purposes; I'll still replace any and all I suspect to be bad for completeness.

[maverick530]

Re: Benq FP767 PS caps

I think it's pretty well certain that I will have to replace some (if not all) C5707 transistors.

I found some at Bluestar International, but what's the difference between:

2SC5707

and

C5707

? Which ones should I get? Any other suggestions as to where to purchase them? Thanks in advance,

[maverick530]

Re: Benq FP767 PS caps

The pico fuse (PF701) is a 5A fuse, it says "Lf 5A" on it. Or "bel m6 5A" (on another board of mine, I actually have 2 of these boards).

But I have no idea the rating. Will this
5 AMP 125V FAST PICO AXIAL work as a replacement?

Do pico fuses have polarity? In other words, does it matter which way I test them or which way I put them in? I don't see any marks on the fuse or the board itself to indicate any polarity.

[Bobdee]

Re: Benq FP767 PS caps

I also forgot to mention that the two C5707 transistors that are suspected bad are the top two, meaning they are right next to each other. This looks like a common occurrence that both will go out on the same circuit segment--can anyone confirm this? I just want to make sure they're not testing bad BECAUSE they're on the same segment and only one of them is REALLY bad (or worse, it's that square cap thingie between them (C824)). This is for learning purposes; I'll still replace any and all I suspect to be bad for completeness.

Pairs of transistors always measure the same when in circuit,they either measure good or bad obviously, but when you remove the bad pair usually only one is bad when tested out of circuit,
It is recomended that you replace in pairs thou, some replace all four, I never do
dont worry about that cap it usually ok in most cases
No diff between c5707 and 2sc5707 but the latter is usually genuine sanyo replacements to my knowledge

[Bobdee]

Re: Benq FP767 PS caps

I think it's pretty well certain that I will have to replace some (if not all) C5707 transistors.

I found some at Bluestar International, but what's the difference between:

2SC5707

and

C5707

? Which ones should I get? Any other suggestions as to where to purchase them? Thanks in advance,

I get mine from Ebay just type in 2sc5707
you might find some repair kits on there sometimes with fuse and fets

[maverick530]

Re: Benq FP767 PS caps

...so should I go ahead and replace the C5707 transistors and the pico fuse and see what happens? Or is there anything else I need to test before making my initial attempt at fixing this board?