Philips DVDR985 Technical Training Manual page 107

Check the voltage on the source. It should be a
few tenths at most. If it is higher, check the
source resistors and the FET. It is a good idea
anytime the FET is replaced to replace the
source resistors. They can read proper and be
out of tolerance, causing a repeat failure in a
short time.
Is the unit ticking? Place a scope on the drain of
the FET. Is there a signal there? If the signal is
about 2 Hz, the Overload Circuit may be activat-
ing. Remove AC power and check each of the
output lines for a short, and the overload transis-
tors, 7141 and 7143. If no problem can be identi-
fied, supply AC and monitor the base of 7143. If
it reaches .6V, there is an improper load in the
circuit or the FET is shorted.
Improper Regulation
This type fault will show itself in one of three
ways: The outputs are too high, the outputs are
too low, or alternate high and low outputs. The
precise regulation loop monitors the 5Vdc and
3.9Vdc supplies. Measure the voltage drop on
the diode portion of the optic coupler, 7200.
If the voltages on the output lines are too high,
the voltage drop on the diode portion of the optic
coupler should be 1-1.2Vdc. The transistor por-
tion should be conducting hard with little or no
voltage drop. If there is a voltage drop on the
transistor portion, the optic coupler is bad.
If the transistor portion of the optic coupler has
little or no voltage drop, the secondary side of
the power supply is asking for less output. The
problem involves 7140 and associated compo-
nents. If 7140 itself checks out, the negative sup-
ply to the transistor from 6142 may not be sup-
plying enough control voltage.
If the optic diode has a volt or less, the shunt
regulator (7251) and/or the voltage divider resis-
tors can be the problem. Check the diode portion
of the optic coupler.
If the voltages on the outputs are too low, the
voltage drop on the diode in the optic coupler
should be less than a volt. The transistor side
should have a 5-6V drop. If these voltages are
proper, the secondary portion of the power sup-
ply is asking for more output, and the problem is
on the primary side.
If the diode portion has a volt or more on it, the
Shunt Regulator is improperly driving the optic
coupler. The problem is with the Shunt Regulator
or the voltage dividers.
If the diode portion does not have a voltage drop
of .9V or more and the transistor side has a
small voltage drop 3V or less, the transistor por-
tion of the optic coupler is faulty.
For a condition of having alternate high and low
output voltages, there is a rectification and filter-
ing problem, or there is an excessive load on
one of the output lines. If there is an excessive
load on the 5 Vdc line, the 5V could read normal,
while the other outputs rise to abnormal levels.
The Supply is working harder to maintain the 5V
on the monitored line, but the other outputs
receive proportionally more energy. This effect
makes their output voltage elevated.
For a condition of having just one output miss-
ing, there is an open in the Rectification/Filtering
circuit. This could include: a fuseable resistor,
the winding of the transformer, or problems with
the foil pattern on the board. Once the open is
identified, measure the line for a short before
providing AC to the supply.
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