Remember if a voltage is missing check for proper resistance
before proceedings.
Understanding the Power On Sequence when Troubleshooting a
possible Power Supply Failure will simplify the process of isolating which
circuit board failed to operate properly. In this Section we will investigate
the Power on Sequence and examine ways to locate quickly where the failure
occurred.
Check the Power On LED for Operation. A Red LED indicates a
presence of 5V STB and AC-ON/DETECT. Failure
of the Power ON LED to light is an indication of loss of 5V STB or AC ON/
Detect remembers the 5V STB and AC-ON/DETECT are developed on the SMPS and sent
to the Main Board.
Listen for Relay Click; the click of the Relay is an
indication of RL-ON going high. RL-ON is sent from the Main Board to the SMPS
and when present the U701 controls the Relay Operation. RL-ON going high and no
Relay is a failure of the SMPS, RL-ON staying low is a failure of the Main
Board.
Relay Operation means that the SMPS if working properly will
output the 16V supply to the Main Board.
This voltage will allow the Tuner, Audio and Video Circuits on the Main
Board to function, and if connected to an Antenna Input, Audio would be
present. If the Relays closed and these supplies failed suspect a problem with
the SMPS.
The next step of operation calls for the M5V_ON line from
the Main Board to the SMPS to go high on P803 pin 21. A high on the M5V_ON Line
activates the 5V VCC line. Loss of 5V VCC results in no “Raster”, no Display
Panel Reset, and no Y, Z, Control or X Board operation. Any of these boards or
failure of the SMPS could cause loss of5V VCC and M5V_ON going high. M5V_ON
staying low indicates a problem on the Main Board.
VS-ON is the last step of the Power Sequence and is
responsible for bringing the VS and VA Voltages up. The VS-ON signal pin 20 P803 is sent from the Main Board to the
SMPS as a high, VS and VA and full operation of the Display Panel are now
enabled. Loss of VS-ON results in loss of VA and VS and no Raster, no Panel
Display Reset but Audio would be present. If VS-ON went high and VS and VA
where missing the problem could be caused by a failure on the SMPS or a circuit
using these voltages. A Resistance check should narrow the possible failures
quickly.
STATIC TEST
This test can confirm the proper operation of
the SMPS without the need to exchange the board. This Power Supply can operate in a No Load State. This means that
by applying AC power to CN101 and all other plugs disconnected, this power
supply will function. Simply removing P803 (Lower Right-Hand Side of the
Board), will cause the “AUTO” Pin 22 to go high from its normal low state
allowing the Power Supply to go to full power on mode when AC Power is
Supplied. Be careful after this test and make sure the VA and VS lines have discharged
before reconnecting the supply cables.
If either Y-SUS or Z-SUS is causing the power supply to
shutdown, unplug the Z-SUS. This will
allow the Y-SUS to function. If you unplug the Y-SUS from the SMPS, and jump
the 5V VCC line to the Y- SUS for Control Board Power the Z-SUS will function.
If the Y-SUS and Z-SUS Boards are working normal, when the
SMPS comes up to full power on, “Display Panel Reset” will be visible. Shorting
the Auto Pattern Gen. test points at this time should result with test patterns
on the screen (if not check for 16V and VA to the X Boards).
For a “Stand-Alone” static test for the Power Supply, apply
the usual 2-100Watt light Bulbs test on the Vs output line for a simulated
load. If the Power Supply operates in this condition, it is assured it can
maintain its output power under load.
Ground the Auto Ground (Pin 22) on P803
AC Power Applied AC Det (Pin 18) and 5V STB (Pins 9 ~ 12)
are 5V.
100Ω¼ watt resistor added from 5V STB (Pins9 ~ 12) to RL ON
(Pin 19) closes relay RL101 turning on the 16V Supply.
100Ω¼ watt resistor added from 5V STB (Pins 9 ~ 12) toM5V_
ON (Pin 21) brings the 5V VCC line high.
100Ω¼ watt resistor added from 5V STB (Pins 9 ~ 12) toVS _ON
(Pin 20) bringsthe VA and VS Lines high.
USING BULBS AS LOAD
Unplug all connectors.
Using two 100W light bulbs [filament type only], attach one end to Vs
P801 pin 1 & 2 and the other end to chassis ground. Apply AC to CN101. If the light bulbs turn on, allow the SMPS to run for several
minutes to be sure it will operate under load.
If this test is successful and all the other voltages are generated, can
be assured the power supply is OK.
Note: The light bulb
test is not necessary for the SMPS to turn on and stay on. This SMPS will run without a load. But, it is necessary to test the SMPS under
load.
CLICK ON THE PICTURES TO MAGNIFY
