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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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What's next after bad caps LG W2252TQ-TF schematic?
I picked up a free LG W2252TQ-TF.
lights up for a second then quits. Figgered bad caps, but I pulled and tested 'em all. All good. No bulges. Checked all the diodes. Other semiconductors aren't shorted. Two fets in the backlight supply probably have the highest stress level, but I thought I'd ask for ideas before trying to get them out. It's gonna be difficult to power it up disassembled. Ideas on other possible high failure rate parts? Schematic? Thanks, mike |
#2
Posted to sci.electronics.repair
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What's next after bad caps LG W2252TQ-TF schematic?
"mike" wrote in message ... I picked up a free LG W2252TQ-TF. lights up for a second then quits. Figgered bad caps, but I pulled and tested 'em all. All good. No bulges. Checked all the diodes. Other semiconductors aren't shorted. Two fets in the backlight supply probably have the highest stress level, but I thought I'd ask for ideas before trying to get them out. It's gonna be difficult to power it up disassembled. Ideas on other possible high failure rate parts? Schematic? Thanks, mike Backlight tube itself ? Most of the lamp inverters that I've seen, have current monitoring. If a 'wrong' current caused by a defective tube is detected, the inverter just shuts down. It makes them hard to troubleshoot without an appropriate load. You can check that the supply voltage stays up, and that the 'on' signal to the inverter stays asserted. I've had a couple of the high voltage tube coupling caps fail over the years, and also transformers. You can make sure that the backlight itself has quit, and it's not just that the screen has gone black, by shining a powerful light at the front of the screen, at an angle. Usually, you will be able to see at lest some vestige of what should be being displayed, if it *is* that the backlight has gone out. Arfa |
#3
Posted to sci.electronics.repair
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What's next after bad caps LG W2252TQ-TF schematic?
On 5/18/2013 3:03 AM, Arfa Daily wrote:
"mike" wrote in message ... I picked up a free LG W2252TQ-TF. lights up for a second then quits. Figgered bad caps, but I pulled and tested 'em all. All good. No bulges. Checked all the diodes. Other semiconductors aren't shorted. Two fets in the backlight supply probably have the highest stress level, but I thought I'd ask for ideas before trying to get them out. It's gonna be difficult to power it up disassembled. Ideas on other possible high failure rate parts? Schematic? Thanks, mike Backlight tube itself ? Most of the lamp inverters that I've seen, have current monitoring. If a 'wrong' current caused by a defective tube is detected, the inverter just shuts down. It makes them hard to troubleshoot without an appropriate load. You can check that the supply voltage stays up, and that the 'on' signal to the inverter stays asserted. I've had a couple of the high voltage tube coupling caps fail over the years, and also transformers. You can make sure that the backlight itself has quit, and it's not just that the screen has gone black, by shining a powerful light at the front of the screen, at an angle. Usually, you will be able to see at lest some vestige of what should be being displayed, if it *is* that the backlight has gone out. Arfa Good call. One of the backlight secondaries is open. Thought I'd take a shot at trying to fix it. Hypothesizing that the connection to the pin might be open, I measured some capacitances to attempt to determine which end. The good secondary measures 2.68pF from primary to either end of the secondary. The bad secondary measures 2.68pF from primary to one end of the secondary and 10pF to the other end. Now, I'm confused. I expected to have the sum of the two measurements on the open secondary to add up to about 2.68pF. Another thing I'd never seen before was that each end of the winding has two turns of what looks like 5 or so wires twisted together headed for the connector pin. The wire to the pin is potted, so can't see the actual pin connection. Can't get my head around how or why they did that??? The winding is in the clear, looks like a single layer, so if I can find the break, I can probably fix it. |
#4
Posted to sci.electronics.repair
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What's next after bad caps LG W2252TQ-TF schematic?
"mike" wrote in message ... On 5/18/2013 3:03 AM, Arfa Daily wrote: "mike" wrote in message ... I picked up a free LG W2252TQ-TF. lights up for a second then quits. Figgered bad caps, but I pulled and tested 'em all. All good. No bulges. Checked all the diodes. Other semiconductors aren't shorted. Two fets in the backlight supply probably have the highest stress level, but I thought I'd ask for ideas before trying to get them out. It's gonna be difficult to power it up disassembled. Ideas on other possible high failure rate parts? Schematic? Thanks, mike Backlight tube itself ? Most of the lamp inverters that I've seen, have current monitoring. If a 'wrong' current caused by a defective tube is detected, the inverter just shuts down. It makes them hard to troubleshoot without an appropriate load. You can check that the supply voltage stays up, and that the 'on' signal to the inverter stays asserted. I've had a couple of the high voltage tube coupling caps fail over the years, and also transformers. You can make sure that the backlight itself has quit, and it's not just that the screen has gone black, by shining a powerful light at the front of the screen, at an angle. Usually, you will be able to see at lest some vestige of what should be being displayed, if it *is* that the backlight has gone out. Arfa Good call. One of the backlight secondaries is open. Thought I'd take a shot at trying to fix it. Hypothesizing that the connection to the pin might be open, I measured some capacitances to attempt to determine which end. The good secondary measures 2.68pF from primary to either end of the secondary. The bad secondary measures 2.68pF from primary to one end of the secondary and 10pF to the other end. Now, I'm confused. I expected to have the sum of the two measurements on the open secondary to add up to about 2.68pF. Another thing I'd never seen before was that each end of the winding has two turns of what looks like 5 or so wires twisted together headed for the connector pin. The wire to the pin is potted, so can't see the actual pin connection. Can't get my head around how or why they did that??? The winding is in the clear, looks like a single layer, so if I can find the break, I can probably fix it. Sod's law of course, says that it will be at the inside of the winding ... :-\ Failing that, you might be able to cull a suitable transformer from a scrap board. Quite a few use the same or very similar transformers. It might even be worth Googling for a replacement, as some of these are available. Likewise, complete inverter boards for some models, are available at quite reasonable prices. i.e. http://www.shopjimmy.com/lg-eay38280...t-inverter.htm and http://www.ebay.com/itm/New-replacem...-/400219921779 or http://www.aliexpress.com/store/prod...735816735.html Arfa |
#5
Posted to sci.electronics.repair
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What's next after bad caps LG W2252TQ-TF schematic?
On Sat, 18 May 2013 00:56:52 -0700, mike wrote:
I picked up a free LG W2252TQ-TF. lights up for a second then quits. Figgered bad caps, but I pulled and tested 'em all. All good. No bulges. Checked all the diodes. Other semiconductors aren't shorted. Two fets in the backlight supply probably have the highest stress level, but I thought I'd ask for ideas before trying to get them out. It's gonna be difficult to power it up disassembled. Ideas on other possible high failure rate parts? Schematic? Thanks, mike Much good advice may be available at www.badcaps.net/forum Be prepared to provide good quality pictures. Try www.elektrotanya.com for a service manual. PlainBill |
#6
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What's next after bad caps LG W2252TQ-TF schematic?
On 5/18/2013 10:39 AM, Arfa Daily wrote:
"mike" wrote in message ... On 5/18/2013 3:03 AM, Arfa Daily wrote: "mike" wrote in message ... I picked up a free LG W2252TQ-TF. lights up for a second then quits. Figgered bad caps, but I pulled and tested 'em all. All good. No bulges. Checked all the diodes. Other semiconductors aren't shorted. Two fets in the backlight supply probably have the highest stress level, but I thought I'd ask for ideas before trying to get them out. It's gonna be difficult to power it up disassembled. Ideas on other possible high failure rate parts? Schematic? Thanks, mike Backlight tube itself ? Most of the lamp inverters that I've seen, have current monitoring. If a 'wrong' current caused by a defective tube is detected, the inverter just shuts down. It makes them hard to troubleshoot without an appropriate load. You can check that the supply voltage stays up, and that the 'on' signal to the inverter stays asserted. I've had a couple of the high voltage tube coupling caps fail over the years, and also transformers. You can make sure that the backlight itself has quit, and it's not just that the screen has gone black, by shining a powerful light at the front of the screen, at an angle. Usually, you will be able to see at lest some vestige of what should be being displayed, if it *is* that the backlight has gone out. Arfa Good call. One of the backlight secondaries is open. Thought I'd take a shot at trying to fix it. Hypothesizing that the connection to the pin might be open, I measured some capacitances to attempt to determine which end. The good secondary measures 2.68pF from primary to either end of the secondary. The bad secondary measures 2.68pF from primary to one end of the secondary and 10pF to the other end. Now, I'm confused. I expected to have the sum of the two measurements on the open secondary to add up to about 2.68pF. Another thing I'd never seen before was that each end of the winding has two turns of what looks like 5 or so wires twisted together headed for the connector pin. The wire to the pin is potted, so can't see the actual pin connection. Can't get my head around how or why they did that??? The winding is in the clear, looks like a single layer, so if I can find the break, I can probably fix it. Sod's law of course, says that it will be at the inside of the winding ... :-\ Failing that, you might be able to cull a suitable transformer from a scrap board. Quite a few use the same or very similar transformers. It might even be worth Googling for a replacement, as some of these are available. Likewise, complete inverter boards for some models, are available at quite reasonable prices. i.e. http://www.shopjimmy.com/lg-eay38280...t-inverter.htm and http://www.ebay.com/itm/New-replacem...-/400219921779 or http://www.aliexpress.com/store/prod...735816735.html Arfa Thanks, guys. Found the service manual. Looks like I can get a transformer for $12 or so. I've got lots of time, so I think I'll take a whack at fixing the transformer. Be just my luck to buy a transformer and find other issues. Never can tell what you're gettin' at a garage sale. Thanks, mike |
#7
Posted to sci.electronics.repair
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What's next after bad caps LG W2252TQ-TF schematic?
On 5/18/2013 1:26 PM, mike wrote:
On 5/18/2013 10:39 AM, Arfa Daily wrote: "mike" wrote in message ... On 5/18/2013 3:03 AM, Arfa Daily wrote: "mike" wrote in message ... I picked up a free LG W2252TQ-TF. lights up for a second then quits. Figgered bad caps, but I pulled and tested 'em all. All good. No bulges. Checked all the diodes. Other semiconductors aren't shorted. Two fets in the backlight supply probably have the highest stress level, but I thought I'd ask for ideas before trying to get them out. It's gonna be difficult to power it up disassembled. Ideas on other possible high failure rate parts? Schematic? Thanks, mike Backlight tube itself ? Most of the lamp inverters that I've seen, have current monitoring. If a 'wrong' current caused by a defective tube is detected, the inverter just shuts down. It makes them hard to troubleshoot without an appropriate load. You can check that the supply voltage stays up, and that the 'on' signal to the inverter stays asserted. I've had a couple of the high voltage tube coupling caps fail over the years, and also transformers. You can make sure that the backlight itself has quit, and it's not just that the screen has gone black, by shining a powerful light at the front of the screen, at an angle. Usually, you will be able to see at lest some vestige of what should be being displayed, if it *is* that the backlight has gone out. Arfa Good call. One of the backlight secondaries is open. Thought I'd take a shot at trying to fix it. Hypothesizing that the connection to the pin might be open, I measured some capacitances to attempt to determine which end. The good secondary measures 2.68pF from primary to either end of the secondary. The bad secondary measures 2.68pF from primary to one end of the secondary and 10pF to the other end. Now, I'm confused. I expected to have the sum of the two measurements on the open secondary to add up to about 2.68pF. Another thing I'd never seen before was that each end of the winding has two turns of what looks like 5 or so wires twisted together headed for the connector pin. The wire to the pin is potted, so can't see the actual pin connection. Can't get my head around how or why they did that??? The winding is in the clear, looks like a single layer, so if I can find the break, I can probably fix it. Sod's law of course, says that it will be at the inside of the winding ... :-\ Failing that, you might be able to cull a suitable transformer from a scrap board. Quite a few use the same or very similar transformers. It might even be worth Googling for a replacement, as some of these are available. Likewise, complete inverter boards for some models, are available at quite reasonable prices. i.e. http://www.shopjimmy.com/lg-eay38280...t-inverter.htm and http://www.ebay.com/itm/New-replacem...-/400219921779 or http://www.aliexpress.com/store/prod...735816735.html Arfa Thanks, guys. Found the service manual. Looks like I can get a transformer for $12 or so. I've got lots of time, so I think I'll take a whack at fixing the transformer. Be just my luck to buy a transformer and find other issues. Never can tell what you're gettin' at a garage sale. Thanks, mike The plot thickens... LG W2252TQ-TF Powers up but no backlight. has four backlight lamps with two transformer secondaries. TMS93137CT is the transformer. One of the secondaries is open. The other one measures about 1.5K. I zapped it with a stun gun. The open secondary now measures 1.5K also. But the "fix" didn't survive the reinstallation of the transformer. It's open again. So, I ordered a new transformer. Got it today, and the SAME secondary is open. Looks new and has the right part number on it. The secondary winding is definitely there, just open. Same side as the original. Maybe some manufacturing defect. When I opened up the monitor, both my original secondaries measure ~1K, but after powering it up, the bad one is open again and no backlight at all. By now, you're asking, "is there a question in there?"... nope, just venting. If anybody has a suggestion, I'm all ears. Meanwhile contacting the vendor.... Thanks, mike |
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