TOSHIBA - 42HP84 42HPX84 POWER SUPPLY TROUBLESHOOTING
The Power Supply to these models Plasma TVs is divided into
two modules. The Main Power Supply and the Low B - Audio Output Module.
The main supply in this unit provides seven supply sources. It
is considered as a module and can only be serviced to modular level. If
it is determined to be defective, it must be changed. No repair to
these boards are advised.
The Low -B Audio Out module provides four low voltage sources.
It can be serviced to component level or can be exchanged as a modular
assembly. The audio out module contains the Audio Output stage for the
unit.
The Main Power supply shown below is considered a modular
replacement item. Understanding its design allows to make sound
technical judgments when troubleshooting. There is a jumper mounted
just to the left of two relays on the Main Supply board. If this board
should need replacement, be sure to transfer this jumper from the
original power supply board to the replacement board. This jumper is
necessary for operation of this board.
Main Power supply board
Standby supply board.
The 5V Standby source is used to power the Main Microprocessor circuit, while the unit is in Standby mode. The Microprocessor is located on the Scaler Module of the receiver and is used to control all operational aspects of the unit. The system control unit communicates with the power supply microprocessor to activate the main supply, display error codes, pr trigger unit to shut down if necessary.
Power Supply Micro board
The "PS - Micro" 5 Volt source can be found at pin 28 of the PKG 4 (PKG 4 has 30 pins). It is directly soldered to the main power supply board.
The "5-V Standby or 5Vm" source can be found at pin 9 of P807 on the main power supply.
In addition to the Standby and Main power supplies, there is a Low B supply too. The Low B audio Out module shown below generates another four sources.
The Audio Output IC [Q610] is mounted on a heat sink on the upper left side of the PCB assembly. The Low B power supply areas consisting of {Q820, Q829, Q835 & Q870} occupy the reminder of the PCB.
The 5V Standby source from the Power Supply Board is used to enable the TV microprocessor and other sections of the signal board, when the unit is OFF. Next is a junction of two protection inputs. Pin-13 of P807 is directly connected to Pin-7 of P808 on the Power Supply. This protect input is from the Power Supply microprocessor and the other is from circuits monitoring the voltages generated on the Low B board.
The Power Supply microprocessor monitors the voltage generated by the Main Power Supply and if they are abnormal the protect line originating at the Power Supply Microprocessor will go High, causing shut down and an error code. {Power :LED Flashes in (0.5) seconds intervals.}
If a voltage generated on the Low B audio board should become excessive, the protect line originating on the board itself will rise, causing shut down and an error code. {Power LED Flashes in 0.5 seconds intervals}.
To effect shut down when one of these lines rise, they are both coupled to the Gate of an SCR [D819] as a protect line. A High on this line causes the SCR to couduct, Pulling the Power On line from the TV microprocessor Low, and placing the unit to latching shut down state.
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.
