Troubleshooting; Transistors And Diodes; Integrated Circuits; Capacitors - Philips PM3055 Service Manual

14-9
14.5
TROUBLE SHOOT ING
14.5.1
Introduction
The following information
is
provided
to
facilitate trouble shooting.
Information contained
in
other sections
of the
manual should
also be
used
to locate the defect. An understanding of the circuit
is
helpful
in
locating troubles, particularly where integrated circuits
are used.
Refer
to the circuit description
for this information.
14.5.2 Trouble-shooting techniques
If
a
fault appears, the
following test sequence can be used
to
find
the
defective
part:
-
Check
if the settings of
the controls of
the oscilloscope
are
correct. Consult the Operating Instructions.
Check
the
equipment
to
which
the
oscilloscope
is
connected
and the
interconnection
cables.
-
Check
if the oscilloscope
is
well-calibrated.
If
not,
refer
to
section
13.
"Checking and Adjusting".
- Visually check
the part of the
oscilloscope
in which
the fault
is
suspected.
In
this way,
it is possible
to
find faults such
as bad
soldering connections, bad interconnection plugs
and wires,
damaged
components
or transistors and
IC's that are not
correctly plugged
into their sockets,
-
Location
of the circuit part
in
which
the fault
is suspected: the
symptom often indicates
this part of the
circuit.
If the power supply
is
defective
the symptom will appear
in several circuit parts.
After having carried
out the previous steps, individual components
in
the suspected circuit parts must be examined:
«
- Transistors
and diodes.
Check
the voltage between base
and emitter
(0,7
V
approx,
in
conductive state)
and the voltage between collector
and emitter
(0,2 V
approx,
in saturation) with
a
voltmeter
or an oscilloscope. When
removed from
the p.c.b.
it is possible
to
test the
transistor with an
ohmmeter since
the base/collector junctions can
be regarded
as diodes.
Like
a
normal diode,
the resistance
is
very high
in one direction and
low
xn the other direction. When measuring
take care that the
current
from the ohmmeter does
not
damage
the component under
test.
Replace
the suspected component
by
a
new one
if
you
are sure that the
circuit
is
not in such condition
that the new component
will be
damaged
.
-
Integrated circuits.
In
circuit, testing can be done with an oscilloscope
or voltmeter,
A good knowledge of the circuit part under
test
is
essential.
Therefore, first
read the circuit descriptions
in sections
3.
,.10.
-
Capacitors.
Leakage can be traced with an ohmmeter adjusted
to its highest
resistance range. When testing
take care of
polarity
and maximum
allowed voltage. An open capacitor can be checked
if the
response
for
AC signals
is
observed. Also
a
capacitance meter
can used; compare the
measured value with
the value
and tolerance indicated
in the parts list