TOSHIBA PDP - 42HP83 _ POWER SUPPLY SHUT DOWN

Contn:


     Shutdowns in this Plasma receiver can be divided into tow categories.  The unit can shut down due to a problem, such as an inoperable fan, or it can shut down due to an over-voltage condition.  Both of these shutdowns are noted by the microprocessor, but can be identified by two different error symptoms.

• Shut downs such as the lack of a fan rotation simply cause the microprocessor to turn of the unit off and exhibit an error code by blinking the Power LED.  When these occur, the unit will be allowed to restart by turning it off and back again on.  In this situation normally have enough time to troubleshoot the unit and in some cases read the error codes in the service menu.
• Over-voltage shut down is a function of a distribution system and latches the unit off when it occurs.  It will not attempt to start again, until it is unplugged and AC is applied again.  In addition to latching the unit off, the shut down usually occurs immediately, generating a dead set with no indication, other than blinking power LED indicating a shut down condition.  This indicates a Power Supply Related problem.

CAUSES OF SHUTDOWN [BOARD LEVEL]

• The system monitors most of the DC sources being developed by the regulators on the Low B1 & B2 power supply boards.  When an over voltage condition is detected by this shut down circuit, the protect line will rise, causing the unit to shut down and signalling the the microprocessor that this has occurred. 

Shut-Down Circuitry

• When the protect line in this circuit rises, the gate of SCR D819 will also rise, and cause the SCR to conduct and to apply a DC voltage to the base of Q819, which in turn will conduct, and the Power TV or Power On Line will be pulled low.  This cause the Power Relays in the unit to open.  As well as opening the relays and shutting the unit down, when Q819 pulls the Power Line, Low signalling the microprocessor that the unit has shut down and blinking LED  display indicating shut down will begin.  The protect line that operates this shut down monitors five different voltage sources.
• On the Low B1 Power Supply Board, the 6.5VD, the 3.3VD, the 5.2V2 and the 9 volt sources are monitored. 
• On the Low B2 Power supply board, the 3.3VS1 source is monitored.   These two sources are monitored by Zener diodes in series with steering or blocking  diodes and a reisistor connected between the product line and the source.  
• For an example: Monitoring system as it applies to 5V2 source on the Loq B1 board.

     Since troubleshooting is done to Board Level, all that need to be done is to determine which of the two boards is causing shut down.  The only one source is monitored on the Low B2 board.  So need  to make only one measurement to determine the cause to board level.  That measurement involves  the [3.3VS1] source located  on the Low B2 supply board

To measure and make a determination

     Using a DC oscilloscope , measure the voltage developed across resistor [R872] at turn On.  Connect properly the leads of scope first to the measuring points, remove the AC to reset the unit and reapply again. 

     If the Low B2 board is defective, scope will measure a momentary voltage across R872 when the unit is turned ON.  This measurement can also be done with a digital multimeter, set to voltage hold mode.

     What happens here is, the break over point of the Zener diode [D872] is 4.3Vdc.  If it conducts due to an over voltage on the [3.3VS1] supply line, this voltage will appear at its Anode.  After it passes through [D873], it will be reduced approximately (0.6V) due to the diode junction.  [R872] & [R819] form a voltage divider network.  Both of them are 1000Ohms, meaning, the voltage appear across each resistor will be ~ 1/2 of the voltage appearing at the Cathode of D873, [4.3 - 0.6 = 3.7].  Half of this 3.7V is ~1.8Vdc.

     If a voltage [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 that 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 is probably related to one of the Low B1 Sources, and that board should then be suspected.  Can use the same method to check the sources developed by the Low B1 board, but since the repair is limited to board level, it is not necessary.  The Low B1 board should simply be changed.