Re: Samsung SyncMaster 940T blows inverter fuse
The power supply is always-on when the power cable is plugged in and the power switch is flipped on, but the panel and inverter don't activate until I push the front-panel power button. I'm using that to stop things also, and I try to have someone near the power outlet if I'm doing something that seems particularly risky.
I'll have to get someone to help me with this. I'm more interested in not destroying anything than I am in figuring out if I saw a little bit of light from somewhere. If this is causing an arc in a transformer, there should be a safer way to test for it (like measuring the switching frequency or duty cycle). I'm only theorizing that this could be happening based on the current increase of 400mA - if there is something switching power to the transformer on and off, this behavior is likely regulating the amount of current going to the transformer (via duty cycle or switching frequency, I think) as it can't change the voltage. That signal would be coming from the inverter control IC (U201), which would cause the FETs (U202 and U203, labeled 4502C) to switch power from the power supply to the transformers on and off. I would guess, based on this, that either the inverter IC's switching behavior is changing in a bad way, or one/both of the FETs is damaged. Since the thing works at all, I don't think one of the FETs is shorted but I can check that tonight. I think I know what part that is, so I might be able to hook my oscilloscope to whatever pin attaches to the inverter IC and watch the wave. Doing the same to the output pins might be useful also.
The ring tester indicates if any turns in a coil are shorted together. I'm not sure how well these work on the inverter transformers, as the wire gauge is tiny and there are spacers every so often that may impact the ringing behavior (I have no idea). Other inverter transformers I've tested that work fine light 3 LEDs on the high voltage side and none on the low voltage side, which is about what I'm seeing now. As I understand things, a few shorted turns would reduce the resistance of the transformer and would also reduce the output voltage a bit. I already checked the resistance, and I'd need to have some idea of what to look for in the voltage divider on the return from the CCFLs as I don't have any tools that can safely measure 650v (high voltage rating).
Originally posted by alexanna
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I'll have to get someone to help me with this. I'm more interested in not destroying anything than I am in figuring out if I saw a little bit of light from somewhere. If this is causing an arc in a transformer, there should be a safer way to test for it (like measuring the switching frequency or duty cycle). I'm only theorizing that this could be happening based on the current increase of 400mA - if there is something switching power to the transformer on and off, this behavior is likely regulating the amount of current going to the transformer (via duty cycle or switching frequency, I think) as it can't change the voltage. That signal would be coming from the inverter control IC (U201), which would cause the FETs (U202 and U203, labeled 4502C) to switch power from the power supply to the transformers on and off. I would guess, based on this, that either the inverter IC's switching behavior is changing in a bad way, or one/both of the FETs is damaged. Since the thing works at all, I don't think one of the FETs is shorted but I can check that tonight. I think I know what part that is, so I might be able to hook my oscilloscope to whatever pin attaches to the inverter IC and watch the wave. Doing the same to the output pins might be useful also.
Originally posted by alexanna
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