Hello all,
My TV has failed, and I'm hoping that you good folks could offer some advice.
Here's the skinny:
The TV is a Polaroid FLM-3732 that I bought new 6 years ago or so. It never had any problem at all prior to this failure.
The symptom: One evening after a couple hours of constant operation, it turned off. No standby LED at all. Disconnecting from AC power for 10 seconds or so, then reconnecting would get the standby LED back on, but pressing the power button would only result in a momentary change of the LED from red to blue, then it would go dark again.
I opened it up on the bench, so that I could check power supply rail voltages, etc. The +5VSB (standby, I assume) is coming up on power up, and looks stable and clean. When the power button is pressed, the +24, +12, and +5.1 rails come up, and appear to be stable - but maybe slightly low. A moment later there is a faint tick, and all rails drop to 0, including +5VSB. This is repeatable by removing AC power for a few seconds.
I also found three capacitors bulging. They were the two 1000uF/10v which are in parallel on the +5VSB circuit, and the 1000uF/35v which is on the 24v rail. I replaced them, and in the interest of eliminating any additional capacitor trouble, I also replaced all other electrolytics on the board with the exception of the two 100uF/450v bulk caps, which I tested with a capacitance meter, and both showed dead-on. The caps that I used for replacements are ones that we had in stock at work, so some are a bit higher voltage than the originals, but identical capacitance. I didn't think that could possibly hurt anything. The brands are United Chemicon LXY and KMG, and Illinois Capacitor CKHM.
After recapping, I reinstalled the power supply, only to see the same symptom as before.
Having read another thread here about a similar problem with an identical power supply, I tested all diodes with a good Fluke multimeter's diode check function. No problems here as far as I can tell. I also checked the junctions of all FETs. They look good, too. I can't see any markings on the smaller transistors on the back of the board, and thus I'm not sure which pin is which for testing.
After checking all of this, mostly out of frustration I put the supply back into the TV again, and tried a few times to get it to power on. Surprisingly, after a few tries, it did! I was able to turn it on and off with the power button a few times until I let it set in the "off" state for a few minutes, then back to the original symptom. I also noticed that after this bit of foolery, the heat sink to which the main two switching FETs are attached became very hot to the touch, and a small amount of smoke was coming from the toroid inductor that is mounted under a different heat sink (the one that the bridge is attached to).
I brought the power supply to work with me, where I have access to much better tools. I powered up the board with no load attached, and activated the other rails by applying a 3.2k Ohm resistor from +5VSB to STB. All rails came up, and looked nice and clean on the oscilloscope, but I didn't leave it on for very long, as I didn't want to overheat anything any more than necessary.
This is where I stand now. If you've read this whole novel, I'd like to thank you. I know it's long, but there's no point in asking for help if I'm not going to give you all of the pertinent information, right? Please don't hesitate to let me know if I haven't been clear about something, of if I've left out important info.
Any suggestions are most welcome!
Pictures will follow shortly.
Thanks,
Troy
My TV has failed, and I'm hoping that you good folks could offer some advice.
Here's the skinny:
The TV is a Polaroid FLM-3732 that I bought new 6 years ago or so. It never had any problem at all prior to this failure.
The symptom: One evening after a couple hours of constant operation, it turned off. No standby LED at all. Disconnecting from AC power for 10 seconds or so, then reconnecting would get the standby LED back on, but pressing the power button would only result in a momentary change of the LED from red to blue, then it would go dark again.
I opened it up on the bench, so that I could check power supply rail voltages, etc. The +5VSB (standby, I assume) is coming up on power up, and looks stable and clean. When the power button is pressed, the +24, +12, and +5.1 rails come up, and appear to be stable - but maybe slightly low. A moment later there is a faint tick, and all rails drop to 0, including +5VSB. This is repeatable by removing AC power for a few seconds.
I also found three capacitors bulging. They were the two 1000uF/10v which are in parallel on the +5VSB circuit, and the 1000uF/35v which is on the 24v rail. I replaced them, and in the interest of eliminating any additional capacitor trouble, I also replaced all other electrolytics on the board with the exception of the two 100uF/450v bulk caps, which I tested with a capacitance meter, and both showed dead-on. The caps that I used for replacements are ones that we had in stock at work, so some are a bit higher voltage than the originals, but identical capacitance. I didn't think that could possibly hurt anything. The brands are United Chemicon LXY and KMG, and Illinois Capacitor CKHM.
After recapping, I reinstalled the power supply, only to see the same symptom as before.
Having read another thread here about a similar problem with an identical power supply, I tested all diodes with a good Fluke multimeter's diode check function. No problems here as far as I can tell. I also checked the junctions of all FETs. They look good, too. I can't see any markings on the smaller transistors on the back of the board, and thus I'm not sure which pin is which for testing.
After checking all of this, mostly out of frustration I put the supply back into the TV again, and tried a few times to get it to power on. Surprisingly, after a few tries, it did! I was able to turn it on and off with the power button a few times until I let it set in the "off" state for a few minutes, then back to the original symptom. I also noticed that after this bit of foolery, the heat sink to which the main two switching FETs are attached became very hot to the touch, and a small amount of smoke was coming from the toroid inductor that is mounted under a different heat sink (the one that the bridge is attached to).
I brought the power supply to work with me, where I have access to much better tools. I powered up the board with no load attached, and activated the other rails by applying a 3.2k Ohm resistor from +5VSB to STB. All rails came up, and looked nice and clean on the oscilloscope, but I didn't leave it on for very long, as I didn't want to overheat anything any more than necessary.
This is where I stand now. If you've read this whole novel, I'd like to thank you. I know it's long, but there's no point in asking for help if I'm not going to give you all of the pertinent information, right? Please don't hesitate to let me know if I haven't been clear about something, of if I've left out important info.
Any suggestions are most welcome!
Pictures will follow shortly.
Thanks,
Troy
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