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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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Alba LCD19advdw, LCD TV and DVD, 2009
Failed backlighting. Firstly 2 off 8pin SOIC with 4 pins commoned for
heatsinking, to drive backlight Tx, on the mains SMPS LV side. Markings look like PRAAN FOS 8862C under 30x microscope, google comes up with nothing, anyone suggest likely mistrancription alphanumerics? DVD plays and sound emerges, not tried a torch on LCD as all a lash-up at the moment. That Tx powers 2 separate tubes with only 2 3KV caps the other components, 1 of 3.9pF and other 4.7pF , different size tubes likely?. Both separate secondaries of that Tx measure 830 ohm each 500V Insulation testing showed 200M for each of those 3KV caps. Not knowmingly have any puffy 3KV caps around and tried 2 x 27pF 12KV ones instead. The whole unit went into a protection mode it seems, no DVD or sound. Replaced the original 3KV caps and back to the previous situation. Does this suggest the 2 drivers are working, they cold test DVM-D pin to pin combinations matching across each ? Unplugging each lamp in turn makes no difference. Only the second time I've peered into backlight operation , so somewhat unknown territory, previous time was leaky HV caps. |
#2
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Alba LCD19advdw, LCD TV and DVD, 2009
I've some tubular ceramic caps, 1p8 unknown voltage, scraped back the
covering and 2.4mm electrode separation , so perhaps 3KV or so. Tried 2 of those on each secondary and no change 2 extenders underneath, so I tried swapping tubes over , no change. Wired in the tube of a CFL onto the plain unmonitored side of the Tx, no glow. Removed it and fitted it on the monitored side and 6 seconds after standby to on, 1 second maintained burst of lamp light. I could repeat this procedure but only ever 1 second , presumably before the monitoring decides there is an internal tube short or something. These CFL tubes have about 10R heaters? either end, but nothing 30 Meg, over the 4 ends of the original tubes. So assuming the burried tail connections are ok and the tubes are not physically broken , what can go wrong with these TV backlight flourescent tubes, and both of them? What if any ohmage across the pairs of wires at each end would I expect? |
#3
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Alba LCD19advdw, LCD TV and DVD, 2009
So it is 2 wires per tube and 4 tubes, not 2 , it seems.
Tried 2 CFL tubes on the monitored side and both light for 1 second. So I can leave one CFL in there and use the other supply to test each TV tube plugged-in in turn, surely 4 could not have failed at once, for 1 second. |
#4
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Alba LCD19advdw, LCD TV and DVD, 2009
Tested each tube and number 4 will not work.
Perhaps dodgey connection of one of the other pair, but now the original pair of 2 tubes at the top work for 1 second. Cleaning pcb around HV areas and resoldering HV and monitoring solder points , still 1 second only. What sort of compact load to represent the duff tube? a few neon bulbs in series perhaps. I don't understand the monitoring as connects to midway point of 2 tubes and no meggy resistors around, just 10s of K. One SMD diode marked A2 is used elsewhere in the LV circuitry, as is transtor marked 3FW. 2 perfectly normal pitch spacing SO23 Q marked 7.AJ or 7AJ are not repeated elsewhere but not obviously HV |
#5
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Alba LCD19advdw, LCD TV and DVD, 2009
7AJ are 3 pin diodes, D on the overlay
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#6
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Alba LCD19advdw, LCD TV and DVD, 2009
On 27/04/2016 12:43, N_Cook wrote:
7AJ are 3 pin diodes, D on the overlay not 7AJ but A7J I "replaced" the duff tube with 3 neons in series on the ps, and the display and backlighting and DVD play works fine. So looks like the unmonitored side needed a load also to bring the monitoring into valid envelope. I have to admit its not obvious which area of the screen is lacking light, needs wrapping up properly and a normal viewing position perhaps. Would there be 2 long tubes top and bottom and 2 short along the sides or 2 short each along top and bottom, I've not ventured fully inside. The neons run at 24 deg C over ambient, so perhaps 5 or 6 would be better if more than a temporary fudge. Going the other way would a CFL lamp make a TV/laptop backlight exerciser? 10 ohms between pins 1 and 2 and another 10R between pins 3 and 4 and then the tube across 1&4 or 2&3. |
#7
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Alba LCD19advdw, LCD TV and DVD, 2009
Have a small collection of thin tube flourescents from
fax/scanner/laptops of unknown states. I know at least one is green but I did not label as such, so made an exerciser for 2 wire tubes , from the CFL board. Tubes between pins 2 and 3. 2x 10R,.5w soon blew , 2x 0.33R,2W in positions 1 to 2 and 3 to 4 , probably too far the other way, but allowed to run each of them for a few seconds to check for colour and non-evenness or strobing. |
#8
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Alba LCD19advdw, LCD TV and DVD, 2009
N_Cook wrote:
I "replaced" the duff tube with 3 neons in series on the ps, and the display and backlighting and DVD play works fine. So looks like the unmonitored side needed a load also to bring the monitoring into valid envelope. I have to admit its not obvious which area of the screen is lacking light, needs wrapping up properly and a normal viewing position perhaps. Would there be 2 long tubes top and bottom and 2 short along the sides or 2 short each along top and bottom, I've not ventured fully inside. The neons run at 24 deg C over ambient, so perhaps 5 or 6 would be better if more than a temporary fudge. I have been looking for a practical/cheap solution for the same problem on a DELL 1707FPt computer display. It has four backlight tubes two at the top and two at the bottom. One of them was cracked and very darkened inside, the rest were fine. Problem is the inverter is not happy without a load on that output, runs for one second and shuts down. I tried three 3W resistors in series adding 120K and it works fine, yet the resistors get quite hot. The missing tube is barely noticeable, it just causes the top 1/3 of the srceen to be slightly darker but goes mostly unnoticed. I would be happy to make it work with three tubes, installing the resistors permanently is one solution but I wondered if the inverter could be modified somehow to disable or ignore that output. The inverter uses a transformer for each pair of tubes so not sure if there is anything that can be done. |
#9
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Alba LCD19advdw, LCD TV and DVD, 2009
On 07/05/2016 13:21, Jeroni Paul wrote:
N_Cook wrote: I "replaced" the duff tube with 3 neons in series on the ps, and the display and backlighting and DVD play works fine. So looks like the unmonitored side needed a load also to bring the monitoring into valid envelope. I have to admit its not obvious which area of the screen is lacking light, needs wrapping up properly and a normal viewing position perhaps. Would there be 2 long tubes top and bottom and 2 short along the sides or 2 short each along top and bottom, I've not ventured fully inside. The neons run at 24 deg C over ambient, so perhaps 5 or 6 would be better if more than a temporary fudge. I have been looking for a practical/cheap solution for the same problem on a DELL 1707FPt computer display. It has four backlight tubes two at the top and two at the bottom. One of them was cracked and very darkened inside, the rest were fine. Problem is the inverter is not happy without a load on that output, runs for one second and shuts down. I tried three 3W resistors in series adding 120K and it works fine, yet the resistors get quite hot. The missing tube is barely noticeable, it just causes the top 1/3 of the srceen to be slightly darker but goes mostly unnoticed. I would be happy to make it work with three tubes, installing the resistors permanently is one solution but I wondered if the inverter could be modified somehow to disable or ignore that output. The inverter uses a transformer for each pair of tubes so not sure if there is anything that can be done. This TV was running perfectly happily with 6 neons in series without any droppers, replacing the failed tube for loading purposes, for 3 hours as a try out of whether it was possible |
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