Backlight problem on viewSonic VP171b

[PlainBill]

Re: Backlight problem on viewSonic VP171b

There are several causes of the transistors blowing. In addition to the transformers that retiredcaps mentioned, check the timing caps on the inverter (the large blue film cap next to each transformer). Also check for cracked solder joints on the inductor.

PlainBill

[retiredcaps]

Re: Backlight problem on viewSonic VP171b

Do you know what is causing these transistors to blow?

In post #11, I'm fairly sure PlainBill is referring to the Benq boards where the C5707s short because of dodgy/poor soldering on the inverter transformers.

Maybe resolder/reflow yours and see.

[edfromut]

Re: Backlight problem on viewSonic VP171b

My monitor worked for about 2 weeks then it went blank again. Q5 and Q6 blew again. I replaced them and the monitor worked for about 10 seconds and then they blew again. Do you know what is causing these transistors to blow?

[edfromut]

Re: Backlight problem on viewSonic VP171b

I replaced Q5 and Q6 and that did the trick. For $5 total it's like brad new. Thanks PlainBill !!

[PlainBill]

Re: Backlight problem on viewSonic VP171b

I have the same problem as previous posts, one second delay and then the screen goes black. It looks like I have a shorted transistor (Q5), but I can't identify it. The transistors Q3, Q4, Q5, and Q6 are all the same. They are very small and the only markings on them is "DK" on one line and "QN" underneath it.

Can anyone identify these transistors?

They are 2SC4672, the details are here.

PlainBill

[edfromut]

Re: Backlight problem on viewSonic VP171b

I have the same problem as previous posts, one second delay and then the screen goes black. It looks like I have a shorted transistor (Q5), but I can't identify it. The transistors Q3, Q4, Q5, and Q6 are all the same. They are very small and the only markings on them is "DK" on one line and "QN" underneath it.

Can anyone identify these transistors?

[PlainBill]

Re: Backlight problem on viewSonic VP171b

sorry, don't know if this is the right place for this question, can anyone tell me what is the procedure to remove the cover of this VP171b monitor to get to the boards?

Viewsonics have some of the more difficult covers to remove. In general, remove all the screws attaching the mount. There may be four more at the corners, remove them too. Usually there is a seam that runs around the monitor between the front and the back.

Since you saw fit to double post, at this point you can search for videos on Youtube.

PlainBill

[joehill3]

Re: Backlight problem on viewSonic VP171b

sorry, don't know if this is the right place for this question, can anyone tell me what is the procedure to remove the cover of this VP171b monitor to get to the boards?

[matteo_g]

Re: Backlight problem on viewSonic VP171b

You are correct, Jowel80. In my haste and since the MOSFET was in an
SO-8 package, thinking it was an IC I called it "U8" instead of "Q8" as it
really is.

PlainBill is also correct in stating that there could be multiple causes of the
2-seconds-then-blackout symptom. U1 (BA9741), the switcher chip, will
shutdown in the same amount time if either of its two power supply outputs
have zero voltage (which it interprets as a short circuit, thus invoking its
short-circuit protection shutdown). There is an LC circuit in line with the
MOSFET, and other devices that ultimately feedback the output voltage to
U1. I did not totally reverse engineer the entire circuit, so I am not sure if
the feedback is actually coming from primary (low voltage) or the secondary
(high voltage) side of the transformer. It would be best to check if there is
output voltage on the primary side at the output of the MOSFET (for each
power supply circuit) and go from there. It will be there and be measurable
for the 2 seconds before U1 shutsdown. If there is voltage all the way to
the primary of the transformers, then something may be wrong with the
transformers themselves and/or their soldering, or with the feedback circuits
thereafter (if that is where they are located). Since you can't effectively
measure the secondary voltage without an attenuating/high-voltage probe,
you'll have to check for problems there using continuity and ohm readings
when the power is off. If voltage does not exist at one or both of the
primaries, you need to probe on the gate side of the MOSFETs and
trace back from there. There's not much more than a resistor or two before
you reach U1, so if the gate signal is dead, it could be due to a faulty
switcher chip.

The good news is that since there are two identical circuits, you can go back
and forth in order to determine where the difference lies.

[jowel80]

Re: Backlight problem on viewSonic VP171b

Sorry, i was waiting to order bad parts. And it was a typo error.
Sorry again.

thanks!

[EGuevarae]

Re: Backlight problem on viewSonic VP171b

Hi Matteo_G, i have the same problem on my LCD screen. The part that i have to replace is not the same. My U8 is a EEPROM and not a main P-channel MOSFET. Can you help me with that ASAP to clarify things?

Thanks!

ASAP? This is kind of a free/spare time forum ....

[PlainBill]

Re: Backlight problem on viewSonic VP171b

I checked on the board and the part that you are talking about seem to be Q8 and not U8. U8 is a EEPROM and Q8 is the part number that you are talking about. So I'll go with Q8

Hold it!!!! There are a number of reasons for the 'two seconds to black' problem, a bad mosfet would be one of them. Before replacing anything check Q7 and Q8. Each of them is as likely to fail. Also, it could be a bad transformer, bad solder joint, frayed wire, or bad CCFL.

PlainBill

[jowel80]

Re: Backlight problem on viewSonic VP171b

I checked on the board and the part that you are talking about seem to be Q8 and not U8. U8 is a EEPROM and Q8 is the part number that you are talking about. So I'll go with Q8

[PlainBill]

Re: Backlight problem on viewSonic VP171b

Hi Matteo_G, i have the same problem on my LCD screen. The part that i have to replace is not the same. My U8 is a EEPROM and not a main P-channel MOSFET. Can you help me with that ASAP to clarify things?

Thanks!

Pictures and a part number will help clarifying the issue.

PlainBill

[jowel80]

Re: Backlight problem on viewSonic VP171b

Hi Matteo_G, i have the same problem on my LCD screen. The part that i have to replace is not the same. My U8 is a EEPROM and not a main P-channel MOSFET. Can you help me with that ASAP to clarify things?

Thanks!

[PlainBill]

Re: Backlight problem on viewSonic VP171b

That is a good question since replacing the bad MOSFET might result in
another failure. I checked the resistance across the 4 backlight lamps and
also their resistance to ground. All seemed normal there, or at least
consistent (infinite resistance everywhere).

Update and declaration of success: I received the replacement MOSFETs on
time via FedEx, soldered them in, and reassembled the monitor. It is up and
running and functioning properly.

My conclusion is that perhaps the design of the circuit is a bit marginal. The
MOSFETs only have a maximum VDS of -30Vdc and the body/flyback diode
can only handle 1.7A vs. the 5A that the FET can conduct. It's possible that
over time the diode will fail because of this under rating, and then leave the
FET unprotected resulting in its ultimate destruction. Perhaps adding an
external flyback diode might help. I have 5 more new MOSFETS and about a
dozen more Planar monitors of identical model type throughout my factory.
I'm ready if and when the next one does the same thing, which won't surprise
me.

I can't speak for this particular model, but one common cause is bad solder joints on the inverter transformers.

PlainBill

[matteo_g]

Re: Backlight problem on viewSonic VP171b

That is a good question since replacing the bad MOSFET might result in
another failure. I checked the resistance across the 4 backlight lamps and
also their resistance to ground. All seemed normal there, or at least
consistent (infinite resistance everywhere).

Update and declaration of success: I received the replacement MOSFETs on
time via FedEx, soldered them in, and reassembled the monitor. It is up and
running and functioning properly.

My conclusion is that perhaps the design of the circuit is a bit marginal. The
MOSFETs only have a maximum VDS of -30Vdc and the body/flyback diode
can only handle 1.7A vs. the 5A that the FET can conduct. It's possible that
over time the diode will fail because of this under rating, and then leave the
FET unprotected resulting in its ultimate destruction. Perhaps adding an
external flyback diode might help. I have 5 more new MOSFETS and about a
dozen more Planar monitors of identical model type throughout my factory.
I'm ready if and when the next one does the same thing, which won't surprise
me.

[PCBONEZ]

Re: Backlight problem on viewSonic VP171b

Did you figure out what made the MOSFET blow?

[matteo_g]

Re: Backlight problem on viewSonic VP171b

I just joined this forum so I could post this reply. I figured it might help out
others who end up in a similar predicament.

With a factory assembly line partially down because of a failed Planar
touchscreen backlight, and no other staff or spare touchscreens available
since everyone was off for the holidays, I was forced to dig into
troubleshooting the EMAX LCD backlight driver board. The board on this bad
monitor is identical to the one cited in this thread.

Thanks to some initial leads from this thread, I was able to eliminate the caps
and also get confirmation on the key symptom of this board's particular failure
mode: The monitor does light up for about 1 second after power up, and
then it will go dark. This turned out to be key to verifying the root cause.
From other searches around the web, it would seem this is a common failure
mode for these Emax LCD backlight driver boards that have two or more
outputs. Here's why.

As it turns out, U1 (BA9741) is a dual switch-mode power supply chip that
drives the primary sides of two high-voltage step-up transformers. The
datasheet is readily accessible online. It has two outputs, each that drive
the two separate high voltage supplies on opposite sides of the board. In
short, the reason why the monitor is lighting up for 1 second is because one
stage is working, but the other stage is not. Because the dead stage has
zero volts at its ultimate output, the short circuit detection feature of the
BA9741 shuts down the entire chip (both stages) in about 1 second. You
may notice when the monitor backlight comes on, it's only at about half
brightness anyway.

I dug deeper and found that the root cause had nothing to do with the
capacitors. Having dual, identical circuits to compare against each obviously
made what could have been a difficult job much easier. The problem was due
to a failed U8 (SDM9435A), the main P-channel MOSFET in an SO-8 package
that is used to switch 12V to the primary of one of the high voltage
transformers. I ordered up some replacements from Mouser for $0.80 each
and will receive them tomorrow. Should be a piece of cake from here, but I
will post another reply confirming that the fix worked.

[Starlite528]

Re: Backlight problem on viewSonic VP171b

You may also have bad tubes.
Try the inverter with different tubes, or the tubes with a different inverter.

A little buzzing is normal.
The windings in the Torrids and transformers can vibrate a little.
It's louder because you have it out of the case.

.

I've not taken it apart that far. I would think that it would only knock out half the lamps if they were bad, then again I have no idea how the inverter is programmed to react.

I don't know what kind of lamps or where to get new ones.