Troubleshooting - Philips DVDR985 Technical Training Manual

The Power Supply
This Power Supply operates whenever AC power
is applied. The primary winding of the trans-
former operates using Hot Ground. Be sure to
use an isolation transformer on the AC input sup-
ply. When working with Switch Mode Power
Supplies, there are three general symptoms to
be considered: No operation or dead, improper
regulation, and one or more supply lines miss-
ing.
No Operation
This symptom can be caused by many compo-
nents. Of course, check the Standby supplies
coming from the SMPS. If you can identify a
missing supply, then move on to the next section
SPMS problems.
When AC is applied to the unit, The System
Control Microcomputer Resets and turns On
some of the switched supplies, via the IStndby
ON control voltage. The 5VSW and 8VSW sup-
plies are the first group of supplies to be
switched On. The Analog Microcomputer
Initializes its I 2 C bus and communicates with the
Tuner, the Audio Demodulator, and the Front
Panel. If those circuits operate properly, the ION
control voltage goes low to turn on the second
set of switched voltages. If the ION switching
voltage does not appear, the problem is on the
Analog Board or the Front Panel. The Analog
Board Microcomputer only allows 10 Seconds for
this process. and it will only try once.
If the ION switching voltage appears the Analog
Board and the Front Panel are operating well
enough to turn on power. The 3.3Vdc is the main
supply to most of the microcomputers on many
of the PCBs. The Analog Board's Microcomputer
also sends out the IReset signal to 7902 on the
Digital Board. This IC produces a delayed
Resetn signal line for the Host Decoder, the
VSM, the VIP, and the Line Doubler. The Host
Decoder activates the I 2 C Bus on the Digital
Board. The Host Decoder also produces four

TroubleShooting

reset signals for: the DIVIO Board, two of them
go to the EMPRESS and one to the BE. The
Host Decoder expects a response on the I 2 C
Bus from: the EMPRESS, the VIP, and both
Progressive Scan ICs. The BE performs its own
reset and self analysis and communicates to the
VSM via two signals that it is ready to operate.
The SUR line goes Low on Pin 24 of the VSM
and the PSEN signal must appear. The PSEN
signal is 5.3 MHz. If either the SUR line or the
PSEN signal do not appear the, Host Decoder
will communicate to the Analog Board's
Microcomputer, and the Power will be shut
down.
If the BE has responded properly and the rest of
the Digital Board operates properly, the
Host decoder communicates to the VSM which
communicates to System Control Microcomputer
on the Analog Board to tell the Front Panel to
turn On the Display and wait for customer
Keyboard input.
The challenge is to figure out where in this
process the machine stops. The Analog
Microcomputer allows 10 seconds for the VSM
to give the all is well signal. If it does not occur,
the ION switching voltage is removed and the
unit goes into shutdown.
SMPS Problems
Separate the power Supply from the rest of the
unit. Connect it to an isolation transformer.
Check for 160Vdc on the drain of 7125. If it is
not present, there is an open in the AC input cir-
cuit. This includes the bridge rectifier and the pri-
mary winding of the transformer.
Next, check the gate of 7125. It should have
about 3 - 4 volts on it. If it is near 0Vdc, check
two things. Check the FET for a short. And,
check the pull up resistors supplying the gate,
3125 and 3126, and 3141. If the voltage is 4Vdc
or more, the FET should be conducting.
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