This is a 'social' freebie. One of my friends had given it to me. His description of the problem was "You can hear the heartbeat, but there are no signs of life". When I plug it in, I can hear what he was talking about - a slow (every 2-3 seconds) pulse from the speakers, but no response from the buttons on the front.
This uses a power brick (12V, 4.5A), which lit both filaments of an 1157 automotive lamp brightly. Scoping the output showed a clean voltage. I then downloaded the service manual and disassembled the monitor. The design was simple - input DC went to 5V and 3.3V bucking regulators and to the inverter. Disconnecting the inverter made no difference, disconnecting the tuner portion of the TV also made no difference. I did notice the same 2-3 second pulse on the 3.3V supply. A long look at the schematic showed the output of the 3.3 volt supply went to dozens of ICs. I was preparing myself for a difficult troubleshooting project when I noticed the same 2-3 second pulse appeared on the 2.5V supply, which was derived from the 5V supply. And that led me back to finding the same 2-3 second pulse on the 12 volt supply.
Since I didn't have an equivalent supply handy, things sat there for about a month when I had the opportunity to buy a variable bench supply (0-30V, 0-5A). Hooking that up to the power connector resulted in elimination of the pulse, but the backlights would not come on completely. Checking the output of the power brick, I discovered that while it was labeled as providing 12V at 4.5A, it actually put out 14V. Boosting the output of the bench supply to 14 volts gave me full backlights. With that information I hooked up a Kensington 'Universal' power adapter and discovered I had a working TV/monitor.
Next I turned my attention to the 'power brick'. A quick investigation showed the 1157 lamp draws 2.7A if both filaments are on, well below the specs of the power supply. Opening the power brick was easy, removing the supply from it's case was anything but. Eventually I got it out and stared checking. Of course, it was well coated with adhesive, the same one that held it in the case. Checking the ESR of the caps (SamYoung) showed they were roughly at spec. Removing them from the board confirmed this. After some thought, I ordered replacements for the three output caps and the mains filter cap. The startup cap was buried under more adhesive, besides I had those on hand. Installing the caps and testing it gave me the same pulsing sound. Prying the adhesive off, I was able to extract a 47uF, 50V cap of uncertain ancestry. Replacing it, I hooked the power supply up, and was rewarded with the same pulse - but only a single beat!!! Turning to the TV / monitor, I discovered it was working.
Conclusions - Check power supplies at full load. And just because a power supply produces an output at no load, don't assume the startup cap is good.
PlainBill
This uses a power brick (12V, 4.5A), which lit both filaments of an 1157 automotive lamp brightly. Scoping the output showed a clean voltage. I then downloaded the service manual and disassembled the monitor. The design was simple - input DC went to 5V and 3.3V bucking regulators and to the inverter. Disconnecting the inverter made no difference, disconnecting the tuner portion of the TV also made no difference. I did notice the same 2-3 second pulse on the 3.3V supply. A long look at the schematic showed the output of the 3.3 volt supply went to dozens of ICs. I was preparing myself for a difficult troubleshooting project when I noticed the same 2-3 second pulse appeared on the 2.5V supply, which was derived from the 5V supply. And that led me back to finding the same 2-3 second pulse on the 12 volt supply.
Since I didn't have an equivalent supply handy, things sat there for about a month when I had the opportunity to buy a variable bench supply (0-30V, 0-5A). Hooking that up to the power connector resulted in elimination of the pulse, but the backlights would not come on completely. Checking the output of the power brick, I discovered that while it was labeled as providing 12V at 4.5A, it actually put out 14V. Boosting the output of the bench supply to 14 volts gave me full backlights. With that information I hooked up a Kensington 'Universal' power adapter and discovered I had a working TV/monitor.
Next I turned my attention to the 'power brick'. A quick investigation showed the 1157 lamp draws 2.7A if both filaments are on, well below the specs of the power supply. Opening the power brick was easy, removing the supply from it's case was anything but. Eventually I got it out and stared checking. Of course, it was well coated with adhesive, the same one that held it in the case. Checking the ESR of the caps (SamYoung) showed they were roughly at spec. Removing them from the board confirmed this. After some thought, I ordered replacements for the three output caps and the mains filter cap. The startup cap was buried under more adhesive, besides I had those on hand. Installing the caps and testing it gave me the same pulsing sound. Prying the adhesive off, I was able to extract a 47uF, 50V cap of uncertain ancestry. Replacing it, I hooked the power supply up, and was rewarded with the same pulse - but only a single beat!!! Turning to the TV / monitor, I discovered it was working.
Conclusions - Check power supplies at full load. And just because a power supply produces an output at no load, don't assume the startup cap is good.
PlainBill
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