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Rajiv Mehra
 
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Default LCD monitor's Inverter bad ?

Hi everyone,
I have a LCD monitor, a QVision 15 inch touchscreen. The model is J5PX.
It's backlight has some problem. I would appreciate if anyone could please
help me out here.

Symptoms:
On power on the screen flashes on for half a second and then it goes blank.
Also if I give it video signal it also flashes once.

I opened it up and used a flashlight to shine through the back, the display
is OK. Even during the 1/2 second turn on, the display appears just fine.

The monitor has two backlights, one at the top and one at the bottom. Both
the backlights' reflectors have two CCFLs. the bottom one is the one which
flashes on. The top one does not light up, but it's one end does glow for a
second. It appears that the bottom CCFL is the one that used to be lit up
all the time (while the monitor was working properly). I can see that the
bottom CCFLs have their ends blackened a little bit. The top CCFLs do not
have black ends.

Of course, I have tried swapping the top and the bottom ones (the connectors
are the same), but the same things happens. The bottom CCFL just flashes on
for half a second and then ...nothing.

From all this it appears that either the inverter is bad or the driving
circuitry has gone bad.

About the inverter:
The inverter has 4 wires going to it from the "motherboard" of the LCD
monitor. Two are just GND. One is +5v DC while the monitor has a video
signal supplied to it. The 4th seems to be 'floating'.

I did some search on UseNet and found some tips. People are saying that the
tiny fuses burn out. In my case there is only one fusible resistor and that
is not open.

Questions:

Has anyone encountered and solved the same problem?
What is the possibility of having the inverter faulty? How can I test it out
?
The inverter is being given some signal which controls the brightness. I
suspect that circuit might be faulty. How to test that ?

Inverters specs:
Inverter part numbers: CDA-004A
Manufacturer:- CHI-SAM

There are some numbers on the track side too
F27003004A
HK100002
CHI 0110
E

A Google search brought me to this page. The second inverter from the top
(the 15" model) looks very similar to mine.

http://www.chisam.com.tw/p6-1.htm

Any tips / suggestions in solving this problem would be highly appreciated.
Thanks
Rajiv


[If you wish to reply to my e-mail ID, please delete the junk from my e-mail
address.]


  #2   Report Post  
Rajiv Mehra
 
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One more thing:
I just read that the inverter needs 12 volts input. none of those three
wires have a 12 volt input on them.
The +5v is just a "tuen on display" signal.

BUT then again ....if there is no 12 v input then how does the light flash
on for a second??
Any input ???


---------------------------------------------------
"Rajiv Mehra" wrote in message
...
Hi everyone,
I have a LCD monitor, a QVision 15 inch touchscreen. The model is J5PX.
It's backlight has some problem. I would appreciate if anyone could please
help me out here.

Symptoms:
On power on the screen flashes on for half a second and then it goes
blank. Also if I give it video signal it also flashes once.

I opened it up and used a flashlight to shine through the back, the
display is OK. Even during the 1/2 second turn on, the display appears
just fine.

The monitor has two backlights, one at the top and one at the bottom. Both
the backlights' reflectors have two CCFLs. the bottom one is the one
which flashes on. The top one does not light up, but it's one end does
glow for a second. It appears that the bottom CCFL is the one that used to
be lit up all the time (while the monitor was working properly). I can see
that the bottom CCFLs have their ends blackened a little bit. The top
CCFLs do not have black ends.

Of course, I have tried swapping the top and the bottom ones (the
connectors are the same), but the same things happens. The bottom CCFL
just flashes on for half a second and then ...nothing.

From all this it appears that either the inverter is bad or the driving
circuitry has gone bad.

About the inverter:
The inverter has 4 wires going to it from the "motherboard" of the LCD
monitor. Two are just GND. One is +5v DC while the monitor has a video
signal supplied to it. The 4th seems to be 'floating'.

I did some search on UseNet and found some tips. People are saying that
the tiny fuses burn out. In my case there is only one fusible resistor and
that is not open.

Questions:

Has anyone encountered and solved the same problem?
What is the possibility of having the inverter faulty? How can I test it
out ?
The inverter is being given some signal which controls the brightness. I
suspect that circuit might be faulty. How to test that ?

Inverters specs:
Inverter part numbers: CDA-004A
Manufacturer:- CHI-SAM

There are some numbers on the track side too
F27003004A
HK100002
CHI 0110
E

A Google search brought me to this page. The second inverter from the top
(the 15" model) looks very similar to mine.

http://www.chisam.com.tw/p6-1.htm

Any tips / suggestions in solving this problem would be highly
appreciated.
Thanks
Rajiv


[If you wish to reply to my e-mail ID, please delete the junk from my
e-mail address.]



  #3   Report Post  
James Sweet
 
Posts: n/a
Default


"Rajiv Mehra" wrote in message
...
Hi everyone,
I have a LCD monitor, a QVision 15 inch touchscreen. The model is J5PX.
It's backlight has some problem. I would appreciate if anyone could please
help me out here.

Symptoms:
On power on the screen flashes on for half a second and then it goes

blank.
Also if I give it video signal it also flashes once.

I opened it up and used a flashlight to shine through the back, the

display
is OK. Even during the 1/2 second turn on, the display appears just fine.

The monitor has two backlights, one at the top and one at the bottom. Both
the backlights' reflectors have two CCFLs. the bottom one is the one

which
flashes on. The top one does not light up, but it's one end does glow for

a
second. It appears that the bottom CCFL is the one that used to be lit up
all the time (while the monitor was working properly). I can see that the
bottom CCFLs have their ends blackened a little bit. The top CCFLs do not
have black ends.

Of course, I have tried swapping the top and the bottom ones (the

connectors
are the same), but the same things happens. The bottom CCFL just flashes

on
for half a second and then ...nothing.

From all this it appears that either the inverter is bad or the driving
circuitry has gone bad.

About the inverter:
The inverter has 4 wires going to it from the "motherboard" of the LCD
monitor. Two are just GND. One is +5v DC while the monitor has a video
signal supplied to it. The 4th seems to be 'floating'.

I did some search on UseNet and found some tips. People are saying that

the
tiny fuses burn out. In my case there is only one fusible resistor and

that
is not open.

Questions:

Has anyone encountered and solved the same problem?
What is the possibility of having the inverter faulty? How can I test it

out
?
The inverter is being given some signal which controls the brightness. I
suspect that circuit might be faulty. How to test that ?

Inverters specs:
Inverter part numbers: CDA-004A
Manufacturer:- CHI-SAM

There are some numbers on the track side too
F27003004A
HK100002
CHI 0110
E



If the inverter pinout is that simple you may be able to just replace them
with cheap surplus inverters. All Electronics had some last I checked, could
be the tubes are worn out as well and will no longer reliably run from the
output of the inverter.


  #4   Report Post  
dkuhajda
 
Posts: n/a
Default

Check for open surface mount fuses on the inverter.
If ALL the inverters are not running, top and bottom lamp, ALL the
inverters go into shutdown. Same will happen if one of the lamps is simply
worn out/bad.

Replace with same size slow blow fuses and run test unit for days off and
on.

David


  #5   Report Post  
Rajiv Mehra
 
Posts: n/a
Default

David,
I cannot find any SMT fuses. What do they look like??
I saw a lot of SMT resistors, capacitors and transistors (with values
on them). I can see one fuse (a green colored standard resistor type
thing with F4A printed on it). I guess it is the only fuse on the PCB.
I checked it, that is not open.

Furthermore I do not think the LCD monitor was running with both top
AND the bottom CCFls running at all time. I say that because ONLY the
bottom CCFLs have their ends blackened. The top ones are fine. So I
guess when the brighness is turned down, only the bottom CCFLs light
up. But when you turn up the brighness, the top CCFLs ALSO light up.
Of course I am just guessing, but maybe someone out there (with a lot
of experience with Inverters) can confirm my speculation???

I also noted that the ccfls with black ends still flash on for half a
second.

Anyway, I am still perplexed by why there is no 12V dc input coming to
the inverter board.

Any tips ?
Thanks!


dkuhajda wrote in message ...
Check for open surface mount fuses on the inverter.
If ALL the inverters are not running, top and bottom lamp, ALL the
inverters go into shutdown. Same will happen if one of the lamps is simply
worn out/bad.

Replace with same size slow blow fuses and run test unit for days off and
on.

David



  #6   Report Post  
John Gill
 
Posts: n/a
Default

Rajiv:
I just repaired an inverter for a 15" Viewsonic LCD. It was a dual
lamp one like yours. Two transistors that drive the transformer for
one of the lamps were bad. I used the diode position on my DMM to
check the SMD transistors. Replacing the transistors with regular
larger TO-92 case style ones fixed the problem.
I found that if you hold a small NE-2 bulb near the transformer, the
gas in the bulb will glow if there is high-voltage AC close to it.
If the bulb glows, the inverter for that section is working OK.

Most inverters will shut down if there are problems on the board.
The bulb would glow for just a second or two, then go out. It would
not glow near the transformer with the bad driver transistors.
The 4 wires going to the inverter on mine were
12 Volts, ground, enable, and dimming (some will change the voltage
on the lamps to control brightness).
It helps having a dual-bulb inverter in that you can check back and
forth between the parts on the two ends of the inverter board.
I have seen places on the Internet that sell the inverters for
between $70 and $130 (plus shipping).
Hope this helps...
John
  #7   Report Post  
Bob Shuman
 
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Default

You can find them much cheaper than the prices you quoted. Some places even
offer repaired units.

Use Google and enter the manufacturer and model number of the inverter to
find sources.

Bob

"John Gill" wrote in message
om...
I have seen places on the Internet that sell the inverters for
between $70 and $130 (plus shipping).



  #8   Report Post  
Rajiv Mehra
 
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John,
Thanks for your response.
I checked the transistors using my DMM but they seem all right to me.
One thing is interesting, though. My inverter has two transistors for
each lamp pair. It appears that the transistors are connected in
parallel (their EBC legs are shorted with the paired transistor) But I
guess they have it that way because they just want more current
handling capability.

I have a few questions for you:-

1. About your Viewsonic monitor,
a. Do you have a pair of lamps at the top and bottom (a total of 4
lamps)?
b. Do the lamps at the top and bottom light up simultaneously?

2. If the lamps connected to one side of the inverter do not light up
(or are faulty) does the other side's lamps also turn off ?

3. Did you actually measure a +12 on one of the 4 wires going to the
inverter? Is that DC or AC ? Is that voltage stable at 12 v or does it
varies with the brightness? (although I understand that there is a
separate dimming control voltage)

4. When the inverter shuts down (due to a fault or something) does the
12V stop coming in? In other words the fault detection circuit…..it is
off the inverter board (maybe on the motherboard)?

The thing is… none of the 4 wires going to my inverter has 12 volts on
it. I have GND on two of them and one has +5v (which is display on
signal) and the 4th is kinda floating !

I used my DMM to measure DC on the 4th wire, unless it has 12 v AC my
measurements are valid.

Still I do not understand how come one of the CCFL flashes on when
switching on when the inverter is not getting 12 volts. My inverter
sure does need 12 volts, as evident from this page
http://www.chisam.com.tw/p3.htm
The one at the top (model # CDA-004A) is mine.

Unless it gets 12 volts for half a second !

Any input is highly appreciated!
-Rajiv



(John Gill) wrote in message . com...
Rajiv:
I just repaired an inverter for a 15" Viewsonic LCD. It was a dual
lamp one like yours. Two transistors that drive the transformer for
one of the lamps were bad. I used the diode position on my DMM to
check the SMD transistors. Replacing the transistors with regular
larger TO-92 case style ones fixed the problem.
I found that if you hold a small NE-2 bulb near the transformer, the
gas in the bulb will glow if there is high-voltage AC close to it.
If the bulb glows, the inverter for that section is working OK.

  #9   Report Post  
MacMan85
 
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AFAIK All inverters use 12V as the supply. All TFT screens I've seen
have all the lamps switched on all the time - the brightness signal
alters the intensity of all lamps, not switches some of them off. Of
the four connection wires, one should be 12V, one Ground, one switches
the tubes on if there's a valid input signal, and the other controls
the brightness.

In your case the inverter is faulty - if one of the outputs goes bad,
a fault signal switches off all the inverter outputs. It is completely
normal for the display to flash on when you switch it on or apply a
video signal, it takes a second or so for the inverter fault circuit
to detect the fault and switch off the outputs.

The fault could be an output transistor or a faulty transformer. I've
found the only way to repair these is to fit a new inverter. I have
had some success using the cheap inverters sold with CCFLs for
computer case modding - you might find these at Radio Shack or
similar. You will lose the brightness control though.

Hope this helps,
Richard



  #10   Report Post  
Rajiv Mehra
 
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Hi Richard,

Thanks for the response.

You seem to be pretty knowledgeable so can I ask you some questions?.



First

What if my CCFLs are faulty? Don't you think in that case too the
inverter will shutdown?



I just want to be sure before I put some money in a new inverter (some
$60 but even then the exact same part is not available anywhere. They
do have similar inverters which can make 2 CCFLs light up, instead of
4 like mine http://www.lcdpart.com/Products/cda039f.html)



I did see some $7 inverters on All electronics. They might be my only
solution.



Second

Why does only the bottom pair of CCFLs in my monitor have black ends?

I am guessing that only those ccfls were switched on when the monitor
was all right, am I wrong ?



Last:

Do I really have to light up all 4 CCFLs? What if I get 2 of those
cheap inverters and light up two CCFls , one at top and other at
bottom. I do understand that the display's brightness might suffer but
having 4 CCFLs sounds like overkill to me. Maybe that is why the CCFLs
at the top never switched on.



Thanks for your time!



Regards,

Rajiv Mehra



PS: BTW there are inverters out there which take 5 volts (as the
supply) instead of 12 vdc. I saw them on some websites. One can be
seen here http://www.chisam.com.tw/p3.htm


MacMan85 wrote in message . ..
AFAIK All inverters use 12V as the supply. All TFT screens I've seen
have all the lamps switched on all the time - the brightness signal
alters the intensity of all lamps, not switches some of them off. Of
the four connection wires, one should be 12V, one Ground, one switches
the tubes on if ..........



  #11   Report Post  
James Sweet
 
Posts: n/a
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"Rajiv Mehra" wrote in message
...
Hi Richard,

Thanks for the response.

You seem to be pretty knowledgeable so can I ask you some questions?.



First

What if my CCFLs are faulty? Don't you think in that case too the
inverter will shutdown?



I just want to be sure before I put some money in a new inverter (some
$60 but even then the exact same part is not available anywhere. They
do have similar inverters which can make 2 CCFLs light up, instead of
4 like mine http://www.lcdpart.com/Products/cda039f.html)



I did see some $7 inverters on All electronics. They might be my only
solution.




Try the $7 inverter first, chances are it'll work, at the very least it'll
show you if the tube is good and if you don't end up using it you can always
use it for some other project.


  #12   Report Post  
MacMan85
 
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On 18 Nov 2004 08:14:16 -0800, (Rajiv Mehra)
wrote:

Hi Richard,


What if my CCFLs are faulty? Don't you think in that case too the
inverter will shutdown?

I just want to be sure before I put some money in a new inverter (some

$60 but even then the exact same part is not available anywhere. They
do have similar inverters which can make 2 CCFLs light up, instead of
4 like mine
http://www.lcdpart.com/Products/cda039f.html)

If the tubes light up even briefly they are not faulty - there isn't
really anything to go wrong with a CCFL - it's just a tube with an
electrode at each end filled with gas.

I did see some $7 inverters on All electronics. They might be my only
solution.


Buy one and use it to test the tubes if you are not sure!

Why does only the bottom pair of CCFLs in my monitor have black ends?

I am guessing that only those ccfls were switched on when the monitor
was all right, am I wrong ?


I have no idea - I have a set of tubes I took out of an old 18" screen
- some have black ends, some don't, and they were all switched on when
the screen was in use.

Do I really have to light up all 4 CCFLs? What if I get 2 of those
cheap inverters and light up two CCFls , one at top and other at
bottom. I do understand that the display's brightness might suffer but
having 4 CCFLs sounds like overkill to me. Maybe that is why the CCFLs
at the top never switched on.


Different screens have different requirements for backlighting - Some
use a tube at the top and bottom, mostly laptops I think, some have
tubes equi-spaced behind the panel. You could try using just two tubes
and see if the result is acceptable.

HTH
Richard
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James Sweet
 
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"MacMan85" wrote in message
...
On 18 Nov 2004 08:14:16 -0800, (Rajiv Mehra)
wrote:

Hi Richard,


What if my CCFLs are faulty? Don't you think in that case too the
inverter will shutdown?

I just want to be sure before I put some money in a new inverter (some

$60 but even then the exact same part is not available anywhere. They
do have similar inverters which can make 2 CCFLs light up, instead of
4 like mine
http://www.lcdpart.com/Products/cda039f.html)

If the tubes light up even briefly they are not faulty - there isn't
really anything to go wrong with a CCFL - it's just a tube with an
electrode at each end filled with gas.



That's not entirely true, as they age, the emissive coatings on the
electrodes wear out and the voltage required to sustain an arc rises. It's
possible for it to reach a point where the inverter has enough kick to
strike an arc but can't sustain a high enough voltage to keep it going. I
would certainly suspect the inverter first though.


  #14   Report Post  
Junior Member
 
Posts: 1
Default

First, sorry for bringing such an old thread back to life but I wonder if anyone has any other ideas about this. I have the same problem with a KDS PixelTouch monitor. I have found an interesting twist to this problem though. If I leave the inverter plugged in but remove the screws holding it and position it outside the monitor, it works perfectly.

Also, if I place the inverter board underneath the LCD and assemble it while it is working, it will continue to work until power is lost. Normally I would just replace the inverter but PixelTouch no longer has any of these. In addition, the original company that made these monitors (gvision, who sold them to KDS for rebranding) has the inverters for $25 but will not sell them. They state they are for internal use only and I'd have to send it in for a $50 repair plus the $25 part.

I initially thought that it was the positioning of the cables but I've tried to reposition the connecting cables to no avail. As far as I can tell, the inverter looks perfect and performs perfect until it is installed properly.

Any ideas?
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Posted to sci.electronics.repair
external usenet poster
 
Posts: 259
Default LCD monitor's Inverter bad ?

Maybe there is a defect in the circuit board, or in some part, that when it
is mounted, something is going opened. Another thing, is that maybe there is
a component that has become temperature sensitive, and it is failing when
there is a little more heat from being mounted.

Try using a heat gun on the inverter to see if it fails. If it stops
working, you can then use some freeze spray on one component at a time until
you find the failed one. Most of the time, the electrolytic capacitors go
bad on these boards.

--

JANA
_____


"stevezemlicka" wrote in message
...

First, sorry for bringing such an old thread back to life but I wonder
if anyone has any other ideas about this. I have the same problem with
a KDS PixelTouch monitor. I have found an interesting twist to this
problem though. If I leave the inverter plugged in but remove the
screws holding it and position it outside the monitor, it works
perfectly.

Also, if I place the inverter board underneath the LCD and assemble it
while it is working, it will continue to work until power is lost.
Normally I would just replace the inverter but PixelTouch no longer has
any of these. In addition, the original company that made these
monitors (gvision, who sold them to KDS for rebranding) has the
inverters for $25 but will not sell them. They state they are for
internal use only and I'd have to send it in for a $50 repair plus the
$25 part.

I initially thought that it was the positioning of the cables but I've
tried to reposition the connecting cables to no avail. As far as I can
tell, the inverter looks perfect and performs perfect until it is
installed properly.

Any ideas?




--
stevezemlicka


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