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'[EE] LCD monitor backlight repair'
2003\08\11@042403 by t

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Hi all,

Some time ago I had a damaged LCD 15" computer monitor
which had a backlight that would only work for a few
seconds after being powered up, and then the backlight
would turn off. After some experimentation, and
probably more by accident, I found that I could make
the backlight stay on all the time by changing a
resistor value on the backlight drive circuit. I never
found out if this 'fix' would have lasted because the
monitor was eventually binned due to the LCD itself
having been damaged too.

I have just been offered another LCD monitor which has
a 'good' LCD panel, and the monitor otherwise works
fine, but the backlight goes off a few seconds after
the monitor is turned on.

So can someone suggest to me why it is that these
backlights fail in this way? That is, they power up,
stay on for a few seconds, and then die away? I found
that in the old monitor I had, I could fiddle with the
backlight drive board to keep it oscillating. So on one
hand, this could be a common fault with the backlight
drive PCB. Or, maybe it is actually due to a fault or
premature ageing effect in the tube itself, resulting
in a change in some characteristic that the drive
circuit depends upon greatly to keep oscillating?

Cheers

Trevor.

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2003\08\11@044515 by Vasile Surducan

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The backlight have a simetrical structure.
Two tubes, two high voltage drivers. I had the same problems with
one having TL494 as drivers and three transistors near every tube.
If any of that two circuits have problems (like greater currents than
usualy) the shut off circuit is working. You have to discover the shut of
scheme and force the driver always on. Then see which driver becomes hot
and workaround. Now you may disconect one tube and keep on just the
another. Then do the same with the other. Check the capacitors in series
with the tubes and high voltage transformers. Usualy these are damaged
or you have a died power MOS transistor.

top 10 wishes,
Vasile
http://surducan.netfirms.com


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2003\08\11@045552 by t

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Thanks Vasile, this is extremely useful. So, am I right
in thinking that if the tube does light for a second,
then the fault is almost certainly a fault with the
driver circuit itself, and not a fault in the tube
causing the shutdown? Happily, it seems that you are
suggesting that the driver circuit is very likely to be
the blame rather than the tube, which is good, since I
am much more likely to be able to replace / repair it.


Cheers

Trevor.


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2003\08\11@140014 by John Ferrell

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If you can identify the original use for the screen, there are lots of
aftermarket parts suppliers that can be located with Google.
{Original Message removed}

2003\08\11@192805 by Duane Hofstetter

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{Quote hidden}

I work on LCD panels quite often and usually the inverter blows a fuse.
Instead of modifying the inverter you would be better off changing the CCFT.
Digikey carries some bulbs as does BGmicro and All Electronics.
don't get too caught up in the voltage and wattage specs. Generally you'll
be lucky to just find
one with the correct dimensions.
CCFTs like most fluorescent bulbs get a dark band at one or both ends before
going out all together.
I believe that they start drawing a little extra current before they die,
which causes the blown fuse or the blinking out that you are experiencing.

Duane

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2003\08\11@223245 by Alexandre Souza

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> I work on LCD panels quite often and usually the inverter blows a fuse.
> Instead of modifying the inverter you would be better off changing the
CCFT.
> Digikey carries some bulbs as does BGmicro and All Electronics.
> don't get too caught up in the voltage and wattage specs. Generally you'll
> be lucky to just find
> one with the correct dimensions.
> CCFTs like most fluorescent bulbs get a dark band at one or both ends
before
> going out all together.
> I believe that they start drawing a little extra current before they die,
> which causes the blown fuse or the blinking out that you are experiencing.

   Duane, would you mind in developing more about this subject? How about a
good "example" of CCFT inverter, and some explanation about the workings?

   These things seems to be simple, but I always found hard to make these
beasties work. Why that capcacitor on the end of the circuit, on the voltage
output for the lamp??? :o)


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2003\08\12@012725 by gtyler

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I have just repaired one two, used a row of ulta brite leds instead. The cap
resonates with the transformer inductance and limits the tube current.
george

{Original Message removed}

2003\08\12@144405 by Peter L. Peres

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Your problem is most likely due to tube degassing (increased internal
pressure while operating). It will need a higher voltage to run then. If
you can make the converter give this higher voltage then the tube will
likely work a little longer. The real fix is to replace the tube.

Peter

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2003\08\12@153705 by Peter L. Peres

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> Duane, would you mind in developing more about this subject? How about a
> good "example" of CCFT inverter, and some explanation about the
> workings?

I'm not Duane. Most cheap inverters use something called the Royer circuit
which is a symmetrical oscillator using two bipolar transistors and a
transformer. There are also chips that can drive this cricuit, also with
FETs, but this seems to be used rarely (it costs more money). It has
relatively low efficiency, since it drives the transformer into
saturation.

Most not cheap inverters (as in laptop, palmtop, efficient, dimmable) use
an advanced version of the same circuit that uses a specialised chip to
drive the transformer at resonance but without saturating it. You have to
read chip datasheets to understand what they do. These can be 95+%
efficient and work extremely well, often outlasting the lamp. They use
almost exclusively MOSFETs for switching.

> These things seems to be simple, but I always found hard to make these
> beasties work. Why that capcacitor on the end of the circuit, on the
> voltage output for the lamp??? :o)

They are not simple, they *look* simple. There are lots of critical items
in the circuit, like the transformer core and the lamp, and small
capacitors that need to sustain several amperes of current and hundreds
of volts, and the unloaded Q of the transformer which must be high enough
to produce the lamp ignition voltage but low enough to prevent blowing the
transistors to kingdom come in case the lamp is cold and does not light
immediately and more like this.

The capacitor in series with the lamp makes sure that the lamp does not
have dc through it. The lamp is in fact a mercury vapor rectifier that
just so happens to emit light. It tends to amplify any asymmetry in the
electrodes by generating dc. The capacitor prevents that. This is
especially true with dimmed FL lamps which have relatively cold
electrodes in operation. The problem is common to all mercury vapor lamps
(including common fluorescent tubes).

Peter

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2003\08\12@155331 by Alexandre Souza

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> The capacitor in series with the lamp makes sure that the lamp does not
> have dc through it. The lamp is in fact a mercury vapor rectifier that
> just so happens to emit light. It tends to amplify any asymmetry in the
> electrodes by generating dc. The capacitor prevents that. This is
> especially true with dimmed FL lamps which have relatively cold
> electrodes in operation. The problem is common to all mercury vapor lamps
> (including common fluorescent tubes).

   Veeeeeeeeery interesting, Peter! I'll look for some datasheets and learn
more. Thanks a lot! ;o)


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2003\08\13@024828 by Vasile Surducan

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On Tue, 12 Aug 2003, Peter L. Peres wrote:

{Quote hidden}

 Peter, I'm not very sure you have right here. The circuit does not work
at resonance this is sure. A series resonance  (R-plasma, L-transformer
coil, C-series capacitor) which is current resonance will not help at all.
If would be a parallel resonance then yes, I would be agree. But is not.
The capacitors values are usualy 2x33pF/2KV. There are also usually two
bipolars as you say and one NMOS. I have doubts the DC created by tube
rectifying will bother the circuit. It's posible just to saturate the
coil as reversed supply ...? Changing the 33pF with 100pF does not
change too much the current through load. I think this is just a XC
limiter and nothing more. More than that, I've exeperiment with short
circuiting one of the capacitors (one stage, for one tube) and there was
no difference in current load comparing with the standard one. Also a
saturated coil becomes very quickly hot, but it wasn't hot there.

best regards,
Vasile

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2003\08\13@154235 by R Prosser
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The problem is, DC damages / shortens the life of the tubes.
RP





 Peter, I'm not very sure you have right here. The circuit does not work
at resonance this is sure. A series resonance  (R-plasma, L-transformer
coil, C-series capacitor) which is current resonance will not help at all.
If would be a parallel resonance then yes, I would be agree. But is not.
The capacitors values are usualy 2x33pF/2KV. There are also usually two
bipolars as you say and one NMOS. I have doubts the DC created by tube
rectifying will bother the circuit. It's posible just to saturate the
coil as reversed supply ...? Changing the 33pF with 100pF does not
change too much the current through load. I think this is just a XC
limiter and nothing more. More than that, I've exeperiment with short
circuiting one of the capacitors (one stage, for one tube) and there was
no difference in current load comparing with the standard one. Also a
saturated coil becomes very quickly hot, but it wasn't hot there.

best regards,
Vasile

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2003\08\13@154614 by Peter L. Peres

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> electrodes in operation. The problem is common to all mercury vapor
lamps
> (including common fluorescent tubes).

> Veeeeeeeeery interesting, Peter! I'll look for some datasheets and learn
> more. Thanks a lot! ;o)

Is it that veeeeeeery interesting ? I forgot to mention that the dc from
the lamp also affects the oscillator and can put the transformer into
saturation and 'cook' it.

Peter

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2003\08\14@041754 by Vasile Surducan

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On Wed, 13 Aug 2003, Peter L. Peres wrote:

> > electrodes in operation. The problem is common to all mercury vapor
> lamps
> > (including common fluorescent tubes).
>
> > Veeeeeeeeery interesting, Peter! I'll look for some datasheets and learn
> > more. Thanks a lot! ;o)
>
> Is it that veeeeeeery interesting ? I forgot to mention that the dc from
> the lamp also affects the oscillator and can put the transformer into
> saturation and 'cook' it.
>
 Maybe I will dissapoint you, but is not. :)
 Use a scope with a 100:1 probe and see the amount of DC voltage compared
 with the ac output voltage.

 Vasile

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2003\08\14@175447 by Peter L. Peres

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>  Peter, I'm not very sure you have right here. The circuit does not work
> at resonance this is sure. A series resonance (R-plasma, L-transformer
> coil, C-series capacitor) which is current resonance will not help at
> all. If would be a parallel resonance then yes, I would be agree. But is
> not. The capacitors values are usualy 2x33pF/2KV. There are also usually
> two bipolars as you say and one NMOS. I have doubts the DC created by
> tube rectifying will bother the circuit.

I did not write anything about resonance, or about current limiting. I
only said they prevent dc through the *lamp* mostly, but also through the
transformer. The transformer will only be affected in a self-oscillating
circuit, and only when dimming the lamp (the lamp will flicker and the osc
can change modes or 'sing'). In a driven converter there is current
feedback and the chip will not allow the transformer to be saturated.

If you want to see what the lamp does, rectifier wise, put a 10mA
d'Arsonval (!!!) dc miliammeter in parallel with the capacitor, turn on
the circuit and then dim the lamp and see what happens. The effect is much
stronger with old lamps. Note that the behavior depends a lot on the kind
of lamp used. A FL lamp run with dc can fail after 100 hours or less.

Peter

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2003\08\15@114014 by Peter L. Peres

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> Maybe I will dissapoint you, but is not. :) Use a scope with a 100:1
> probe and see the amount of DC voltage compared with the ac output
> voltage.

On which oscillator ? You cannot disappoint me here as the low cost
oscillators have already completely disappointed me. The design is so
tight that the difference in efficiency from say 60% to 55% caused by
saturation can trip a thermal fuse. This does not happen in steered
oscillators.

Peter

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2003\08\16@051847 by Trevor Page

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> -----Original Message-----
> From: pic microcontroller discussion list
> [KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU] On Behalf Of gtyler
> Sent: 12 August 2003 06:27
> To: RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU
> Subject: Re: [EE] LCD monitor backlight repair
>
>
> I have just repaired one two, used a row of ulta brite leds
> instead. The cap
> resonates with the transformer inductance and limits the tube current.
> george

This is interesting. Did you find that fitting a row of bright white
LEDs offered a reasonably good replacement for the tube? Did it work
just as well, and were there any gotcha's with this scheme? It sounds
relatively straightforward, to me, to just somehow fix the row of LEDs
inside where the tube went and run them off the point where the supply
enters the unit.

If the results aren't fantastic then it may still be the best option for
me, purely because of low cost. And it's DIY. I'll be just happy to make
a dead TFT screen useable. Spending money on replacement parts to get
the original backlight system working again could possibly not be very
economical considering the cost of a new unit (as with just about
everything these days).

Would you say there is any limit to the size of the display this repair
method could be used with? The dead display I have on it's way to me is
an 18". The fault, as with the one I had before, is that the backlight
turns off several seconds after powering up. Damn, I really like the
idea of fixing it with white LEDs. Go on, someone tell me why it just
ain't that simple!

These white LEDs can be stunningly bright so if I have enough of them
closely spaced, surely they would give enough light.

Cheers

Trevor.

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2003\08\16@114359 by gtyler

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   The display was about 6" diagonal and I used 9 leds. The problem was
price! cost about $20 for leds and  I only did it because I could not find
another tube. The inverter is still Ok but now is not used anymore. I used 3
strings of 3 leds with a 100R resistor on 12v.
   The device was a 386 sx computer MB in a custom box used for some kind
of beuty treatment, the computer actually did nothing as the output was only
connected to a piece of veroboard with an opamp oscillator producing +-12v
at a few khz. The lady who owns it is convinced that it works again after I
fixed the backlight!

George

----- Original Message -----
From: "Trevor Page" <spamBeGonetspamBeGonespamINTROSPECTIV.ECLIPSE.CO.UK>
To: <TakeThisOuTPICLISTEraseMEspamspam_OUTMITVMA.MIT.EDU>
Sent: Saturday, August 16, 2003 11:17 AM
Subject: Re: [EE] LCD monitor backlight repair


> > {Original Message removed}

2003\08\16@120101 by Trevor Page

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Maybe a row of white LEDs to replace a faulty tube in an 18" TFT is a
mad idea after all, firstly due to the cost of the white LEDs. And 18"
is going to take a lot of LEDs. Secondly, I did some searching on
groups.google about this subject and read some comments about how using
white LEDs isn't going to give anything like the correct colours on the
screen. However, it is something to consider as a last resort.

I just spoke to the guy who's sending me this TFT display about the
fault. Apparently it usually stays on for just a few seconds after
turning on, but 'sometimes stays on for longer'.

Ah well, wait and see I suppose.

Trev

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