Hi everybody, and thank you for your precious help.
I'm repairing 3 light scanners (club lighting) and they have the same problem, the power doesn't start up or in some cases after 2-3 seconds. The third one was working but after I tested it on bench without load (it has 36ohm internal resistor on +5V and about 100 on 12V) it starting have the same problem. If I give just a +12V pulse from an external battery after D14 (see figure) they all start and keep working perfectly connected to load (18A on 5V, 8A on 12V). They are up from 4years already 24/365 but in one I changed both power-transistors, both condensers C5-C6, the C9 (in my circuit is 2,2mF), even the controller TL494, tested all diodes and transistors junctions, and I made all test ever bypassing the components near TL494 dedicated for protection (short, overvoltage, overcurrent and so on).
So brief as I could be, I suppose the responsability is on the power stage, cause it produces only 0,87 to 1,4V (on D14) when connected to 220V on all the three exemplars I tested: that also means the switching is working.. at low condition. At this low power conditions in my opinion the controller doesn't work, not activate the couple T3-T4 and leave the power switching side to operate as minimum.
I need to overcome this problem but it's not easy, I already tried to short the NTC to improve the current at the start without success, I would increase the capacity of C9 to let more current flow.. and try to rise potential on D14 to let the controller work, but I'm afraid it is calculated to have resonance with the transformer or to rise up too much the current. Maybe I can have a try for some seconds adding 1mF to C9 and seeing if something changes. There is also C10-R11 which looks like a "cos-fi" reduction, instead of a trick to produce at boot a sort of peak to startup the controller, I don't think they may help the process.
Does someone here has faced the dame issue before? I'm quite disoriented. Thank you a lot!
PS: the circuit is really similar but not the same, differences are on the circuit fault checks areas
I'm repairing 3 light scanners (club lighting) and they have the same problem, the power doesn't start up or in some cases after 2-3 seconds. The third one was working but after I tested it on bench without load (it has 36ohm internal resistor on +5V and about 100 on 12V) it starting have the same problem. If I give just a +12V pulse from an external battery after D14 (see figure) they all start and keep working perfectly connected to load (18A on 5V, 8A on 12V). They are up from 4years already 24/365 but in one I changed both power-transistors, both condensers C5-C6, the C9 (in my circuit is 2,2mF), even the controller TL494, tested all diodes and transistors junctions, and I made all test ever bypassing the components near TL494 dedicated for protection (short, overvoltage, overcurrent and so on).
So brief as I could be, I suppose the responsability is on the power stage, cause it produces only 0,87 to 1,4V (on D14) when connected to 220V on all the three exemplars I tested: that also means the switching is working.. at low condition. At this low power conditions in my opinion the controller doesn't work, not activate the couple T3-T4 and leave the power switching side to operate as minimum.
I need to overcome this problem but it's not easy, I already tried to short the NTC to improve the current at the start without success, I would increase the capacity of C9 to let more current flow.. and try to rise potential on D14 to let the controller work, but I'm afraid it is calculated to have resonance with the transformer or to rise up too much the current. Maybe I can have a try for some seconds adding 1mF to C9 and seeing if something changes. There is also C10-R11 which looks like a "cos-fi" reduction, instead of a trick to produce at boot a sort of peak to startup the controller, I don't think they may help the process.
Does someone here has faced the dame issue before? I'm quite disoriented. Thank you a lot!
PS: the circuit is really similar but not the same, differences are on the circuit fault checks areas
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