TOSHIBA - 42HP83 PLASMA - POWER SUPPLY SHUT DOWN PROBLEM
The main power board produces a verity of supply voltages used by various boards in the set. If a power supply panel has been replaced, or at the event of panel change, certain adjustments should be done. These adjustments are [Vs, Va, Vsc, Vset & Ve]; determines the voltage supplied to plasma screen and are controlled by a variable resistor located on the plasma screen supply panel. As incorrect voltage levels may cause 'shut-down' or screen damage, it is important to measure these voltages at each test points. The voltage requirements will be clearly marked on the label of the panel.
Shut-downs in the 43HP83 plasma receiver can be divided into [2] categories. The unit can shut down due to a problem, such as an in-operational fan, or due to an over voltage condition. Both these shut downs are noticed by the main microprocessor, but can be identified by error symptoms.'
# Shut downs due to fan trouble simply cause the unit to turned off, and exhibit an error code by blinking the Power LED. When this occur, the unit will be allowed to re-start by turning it Off and back On again. In this situation technician normally will have enough time to trouble shoot the unit and in some cases read the error codes in the 'Service Menu'.
## Over voltage shut down is a function of the power supply distribution system that latches the unit OFF when it occurs. It will not attempt to start again until the set is unplugged from AC power socket. When re-apply AC again, the same thing will repeat. In addition to latching of unit, the shut down usually occurs immediately generate a Dead set, with no indication other than blinking power LED. If this is the condition, make sure that it is with power supply regulator board itself.
### There is an over voltage protection circuit integrated with power main power board to detect any over voltage condition at its output voltages. When the protect line in this circuit rises, the Gate of SCR [Silicon Controlled Rectifier] also will rise. This will cause [D819] to conduct and apply DC voltage to the base of [Q819]. Q819 will conduct and the Power On line will be pulled low. This causes the power relays in the unit to open. As well as opening the relays and shutting the whole unit down. When Q819 pulls the power line low, signal is send to main micro-controller realizing about it, and the unit starts blinking LED, indicating shut down.
Since trouble shooting is done by board level, all that needs to be done is to determine, which among the two boards is causing shut down. Actually this is the main part of service, where a technician should have give more concentration. It is a must have its schematic diagram to follow the circuit. Schematic diagrams can be downloaded from internet. With only a schematic diagram, user can't do any thing with it, as it will be a drawing which not resemble to anything with in the TV.
If a voltage of about 1.8VDC appears across R872 as the unit is turned ON, it means that the shut down is being initiated by the 3.3VS1 source developed by the low B2 power supply. If this is the case, Low B2 board should be replaced.
Since this is the only source monitored on this board. If no voltage is detected across R872, the shut down problem might be related to one of the low B1 sources and that board should be trouble shooed. In most cases, it is advice to replace the board as card basis. It will be best especially in case of Plasma TVs.
Use the same method to check the sources developed by Low B1, and that board. If confirmed faulty, replace the board. Never try to repair it.
