I have a Samsung LN32B360C5D set that has a mind of its own. Its like the little girl who when she was good she was very, very good but when she was bad she was horrible. If it hasn't been plugged in for a while -- a few hours to a few days -- it acts like it should. ...for a while... The standby light comes on and it responds correctly to all the side panel buttons as well as the remote control and has a great picture. However, either after turning it off or after it has been on 10-15 minutes, when you turn it back on it is in a 28-29 second perpetual loop -- power-on chime, 1.5-2 sec backlights on, 28-29 seconds backlight goes off, 1.5-2 seconds later you hear the power-on chimes and it just repeats. Once it starts looping none of the controls, button or IR, have any effect -- it just loops on its own and will start looping as soon as the power cord is plugged in.
All the voltages look good. In fact, even when it is not responding to the buttons and IR they are still sending correct signals to the main board. The only difference I have found that correllates to when it works and when it doesn't is the pin coming from the chip under the finned heat sink on the main board -- I haven't been able to find a schematic of the main board -- that switches on the pull-down transistor Q302 for pin 1 (power on/off) works correctly when the set is working correctly but just sits at 0V when it isn't. 0V leaves the pull-down off letting pin one go high which is the on signal.
I have an oscilloscope but don't really have the correct probes for looking at ripple noise on a switching PSU so I can't say that the power rails are completely clean. Just in case I re-capped the whole thing with capacitors of the correct voltage, capacitance and low ESR but it made zero difference in the symptoms. I do see the high voltage frequency/duty cycle change when the power-on chimes and then when the backlight comes on and again when the backlight goes off. No way for me to know which comes first but since the PSU uses PWM it appears that it is working correctly and these are just the changes that result from feedback from the various loads but probably primarily the inverter load.
Since the voltages and inputs look fine, just the one output line that doesn't always seem to be working, I'm thinking of removing the glued-on heat sink and reflowing the solder on the chip that contols that on/off signal but before I do I thought I would run this by folks to see if there might be some better approach. I would love to be able to find a schematic of the main board but either my google skills are insufficient or there just simply isn't one online anywhere.
Thanks in advance for whatever help and advice you might offer,
Howard
All the voltages look good. In fact, even when it is not responding to the buttons and IR they are still sending correct signals to the main board. The only difference I have found that correllates to when it works and when it doesn't is the pin coming from the chip under the finned heat sink on the main board -- I haven't been able to find a schematic of the main board -- that switches on the pull-down transistor Q302 for pin 1 (power on/off) works correctly when the set is working correctly but just sits at 0V when it isn't. 0V leaves the pull-down off letting pin one go high which is the on signal.
I have an oscilloscope but don't really have the correct probes for looking at ripple noise on a switching PSU so I can't say that the power rails are completely clean. Just in case I re-capped the whole thing with capacitors of the correct voltage, capacitance and low ESR but it made zero difference in the symptoms. I do see the high voltage frequency/duty cycle change when the power-on chimes and then when the backlight comes on and again when the backlight goes off. No way for me to know which comes first but since the PSU uses PWM it appears that it is working correctly and these are just the changes that result from feedback from the various loads but probably primarily the inverter load.
Since the voltages and inputs look fine, just the one output line that doesn't always seem to be working, I'm thinking of removing the glued-on heat sink and reflowing the solder on the chip that contols that on/off signal but before I do I thought I would run this by folks to see if there might be some better approach. I would love to be able to find a schematic of the main board but either my google skills are insufficient or there just simply isn't one online anywhere.
Thanks in advance for whatever help and advice you might offer,
Howard
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