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Cause determination is important in this product due to the complex system control circuits and its relationship to the power supply & shutdown.
Cause determination is important in this product due to the complex system control circuits and its relationship to the power supply & shutdown.
- The standby section of the main power supply must operate before anything else can occur. The power supply has its own microcomputer, as described earlier, for the purpose of shutting the unit down, and powering up in the proper sequence. There must be two two power supplies present in the standby, on for the power supply microprocessor, and one for the main chassis microprocessor. The main chassis microprocessor is located on the scaler module, issues On & Off commands to the power supply microprocessor in order to begin either process. This microcomputer also have the ability to shut the unit down. Shutdowns involved with the microprocessor involve an error code.
- Latching shutdowns are involved with D819 located at the Low B-Audio PCB. These shutdowns can be cause by voltage source problems relating to over voltage on the Low B-Audio PCB. Fan shut down conditions are a function of Main Microprocessor, but are generated by monitoring circuits located on the Low B-audio board.
- No attempt to start, no sound or picture and no 5V source available for either microprocessor. If either of these source is missing, the Main Power Supply module should be replaced. If the Standby sources are present, check for an On | Off signal to be generated by the main chassis microprocessor. If 3 Volt High signal does not appear when the power switch is pressed, suspect a problem in the Main chassis microprocessor | scaler PCB.
- A latching shut down problem can be verified by measuring the voltage appears at the cathode of D819 after the power supply returns to OFF state. 0.6Vdc appearing at the cathode of this diode indicates a latching shut down.
If a latching shutdown voltage is indicated by 0.6Vdc appearing at the cathode of D819, next be determined if the shutdown is due a problem with the Main Supply or an over-voltage problem on the Low B-Audio PCB. D609 is a steering diode connecting the shut down line of the power supply micro with the protect line attached to the Gate of D819. When the unit shut down, the anode of this diode rise momentarily if the problem is located on the Main Power Supply. If this is the case, the Main Power Supply should be replaced.
It is best to check the DC level of the anode of this diode said above, using a DC coupled scope. Most DVMs won't react fast enough to measure a momentary rise in the DC level.
If this line does not rise when the unit shuts down, the problem is likely associated with secondary supplies developed by the Low B - Audio PCB
This product has two cooling fans. They are mounted above the power supply and the left of center above the Y section of Plasma Display. If either of these cease to operate a singnal in the form of a DC pulse will be input to the main supply microprocessor and the unit will shut down. This shut down is not a latching shut down, and the set may be turned back ON. The "Or" Gate monitoring the rotation of the two fans is located at Low B-Audio board and is output to the TV micro at PJ81, Pin-23.
The circuit of the 'Or' Gate consists around D897 & D898. If either fan should stop or fail to start, a High generated by fan will be steered through one of these diodes to the Fan Stop line and the TV microprocessor will shut down the unit.
This type of shut down is characterized by the fact that it requires about 30 seconds to occur, the shut down is no latching type, the unit can be turned On again, and a 0.25 second flashes and then a 2 second pause; then repeat; is generated.
