Post by thread:[EE] LCD monitor backlight repair

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. -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

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 On Mon, 11 Aug 2003 spam_OUTtTakeThisOuTINTROSPECTIV.ECLIPSE.CO.UK wrote: {Quote hidden}> 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. > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

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. {Quote hidden}> 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 > > > On Mon, 11 Aug 2003 .....tKILLspam@spam@INTROSPECTIV.ECLIPSE.CO.UK > wrote: > > > 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. > > > > -- > > http://www.piclist.com hint: The list server > can filter out subtopics > > (like ads or off topics) for you. See > http://www.piclist.com/#topics > > > > -- > http://www.piclist.com hint: The list server can > filter out subtopics > (like ads or off topics) for you. See > http://www.piclist.com/#topics > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

{Quote hidden}> > 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. 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 > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> 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) --- Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.507 / Virus Database: 304 - Release Date: 04/08/03 -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> 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 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> 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) --- Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.507 / Virus Database: 304 - Release Date: 04/08/03 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On Tue, 12 Aug 2003, Peter L. Peres wrote: {Quote hidden}> > 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, 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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

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 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body

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 -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestspam_OUTmitvma.mit.edu

> 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 -- http://www.piclist.com hint: To leave the PICList @spam@piclist-unsubscribe-requestKILLspammitvma.mit.edu

> -----Original Message----- > From: pic microcontroller discussion list > [KILLspamPICLISTKILLspamMITVMA.MIT.EDU] On Behalf Of gtyler > Sent: 12 August 2003 06:27 > To: RemoveMEPICLISTTakeThisOuTMITVMA.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. -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

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" <spamBeGonetspamBeGoneINTROSPECTIV.ECLIPSE.CO.UK> To: <TakeThisOuTPICLISTEraseMEspam_OUTMITVMA.MIT.EDU> Sent: Saturday, August 16, 2003 11:17 AM Subject: Re: [EE] LCD monitor backlight repair > > {Original Message removed}

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 -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics