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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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Unexpected inverter board shutdown
Hi everybody,
I've got a strange problem that really puzzles me, concerning an HV inverter board for the CFL lamps in a noname flat panel monitor with Hyundai boards and components. Problem: Inverter board shuts down after generating HV for about half a second. Details: Two outputs on the board for altogether four lamps, two transformers, two XA something switchmode controller ICs, one unidentified small IC, four power transistors, a couple of discrete transistors and diodes, all SMD. What I did to locate the fault: 1. Tested outside the monitor with known good voltage supply. Same. 2. Tested without lamps. Same. 3. Hooked lamps to one output at a time only. Same. 4. Checked all discrete semiconductors. All seem ok. 5. Replaced the only electrolytic cap. Same. 6. Checked transformer operation using an oscilloscope probe on top. Pure sine on both transformers. Independent of whether lamps are connected. No distortions. Frequency about 42 kHz. I googled a lot but found nothing so far. Now I'm at my wit's end. Maybe anyone knows something about the shutdown logic used in such boards? Thanks for your help! Regards, Leo |
#2
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Unexpected inverter board shutdown
Over Current/Voltage protection. Without the lamps connected prob O/V, with
lamps prob O/C. Would need the service manuals fo the specific circuit, including voltage and current measurements. Nominal is to just replace the invertors and lamps at one time. "Leo Meyer" wrote in message ... Hi everybody, I've got a strange problem that really puzzles me, concerning an HV inverter board for the CFL lamps in a noname flat panel monitor with Hyundai boards and components. Problem: Inverter board shuts down after generating HV for about half a second. Details: Two outputs on the board for altogether four lamps, two transformers, two XA something switchmode controller ICs, one unidentified small IC, four power transistors, a couple of discrete transistors and diodes, all SMD. What I did to locate the fault: 1. Tested outside the monitor with known good voltage supply. Same. 2. Tested without lamps. Same. 3. Hooked lamps to one output at a time only. Same. 4. Checked all discrete semiconductors. All seem ok. 5. Replaced the only electrolytic cap. Same. 6. Checked transformer operation using an oscilloscope probe on top. Pure sine on both transformers. Independent of whether lamps are connected. No distortions. Frequency about 42 kHz. I googled a lot but found nothing so far. Now I'm at my wit's end. Maybe anyone knows something about the shutdown logic used in such boards? Thanks for your help! Regards, Leo |
#3
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Unexpected inverter board shutdown
Leo Meyer wrote:
Hi everybody, I've got a strange problem that really puzzles me, concerning an HV inverter board for the CFL lamps in a noname flat panel monitor with Hyundai boards and components. Problem: Inverter board shuts down after generating HV for about half a second. Details: Two outputs on the board for altogether four lamps, two transformers, two XA something switchmode controller ICs, one unidentified small IC, four power transistors, a couple of discrete transistors and diodes, all SMD. What I did to locate the fault: 1. Tested outside the monitor with known good voltage supply. Same. 2. Tested without lamps. Same. 3. Hooked lamps to one output at a time only. Same. 4. Checked all discrete semiconductors. All seem ok. 5. Replaced the only electrolytic cap. Same. 6. Checked transformer operation using an oscilloscope probe on top. Pure sine on both transformers. Independent of whether lamps are connected. No distortions. Frequency about 42 kHz. I googled a lot but found nothing so far. Now I'm at my wit's end. Maybe anyone knows something about the shutdown logic used in such boards? Thanks for your help! Regards, Leo It's probably detecting bad or no lamps, there's a resistor on many of those that can drift up in value making that detection too sensitive. |
#4
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Unexpected inverter board shutdown
Leo:
Some inverter boards use a ceramic capacitor across the primary of the high-voltage transformer. I have had a few inverters where this capacitor was bad causing the inverter to shut down very quickly. I think the value of the capacitor was about 0.1Uf at 100 volts. Hope this helps... John |
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