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Tektronix 502A

Tektronix 502A
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Maintenance
—
Type
502A
On
the
other
hand,
the
crt
display
can
often
be
used
to
isolate
trouble
to
one
particular
circuit
when
trouble
ob
viously
exists
in
that
circuit.
If
there
is
no
vertical
deflection,
for
example,
when
the
intensity
and
horizontal
deflection
appear
to
be
normal,
it
is
apparent
that
an
open
condition
exists
in
the
Vertical
Amplifier
and
this
circuit
should
be
investigated
first.
TROUBLESHOOTING
THE
LOW-VOLTAGE
POWER
SUPPLY
Proper
operation
of
every
circuit
in
the
Type
502A
Oscilloscope
depends
on
proper
operation
of
the
Low-Voltage
Power
Supply.
The
regulated
voltages
must
remain
within
their
specified
tolerances
for
the
instrument
to
maintain
its
calibration.
For
no
output
voltage
If
the
graticule
lamps
and
the
fan
do
not
operate
when
the
power
switch
is
turned
on,
check
the
power
switch,
the
fuse,
and
the
line
voltage.
If
your
instrument
is
wired
for
220,
234
or
248
volt
operation,
also
check
the
thermal
cutout
switch.
(If
your
instrument
is
wired
for
110,
117
or
124
volt
operation,
the
fan
will
run
even
though
the
thermal
cutout
switch
may
be
open.)
If
the
fuse
is
not
blown
and
the
line
voltage
is
correct,
next
check
the
primary
windings
of
the
power
transformer.
If
both
the
graticule
lamps
and
the
fan
operate
correctly,
the
primary
circuit
of
the
power
transformer
is
operating
nor
mally
and
the
trouble
lies
somewhere
in
the
secondary
cir
cuits.
If
only
one
of
the
outputs
of
the
Low-Voltage
Power
Supply
is
zero,
the
trouble
is
probably
due
to
a
defective
rectifier,
series
regulator,
or
power
transformer
secondary
winding,
although
this
trouble
can
also
be
caused
by
a
short
across
the
output.
To
determine
which
circuit
element
is
defective,
measure
the
secondary
voltage
of
the
transformer
and
the
voltage
at
the
output
of
the
rectifier.
The
cause
of
the
trouble
can
be
determined
by
voltage
readings
obtained.
For
failure
of
the
regulated
power
supplies
to
regulate
at
the
correct
voltages.
If
any
or
all
of
the
supplies
fail
to
regulate
at
the
proper
voltages,
first
check
the
line
voltage.
The
supplies
are
designed
to
regulate
between
105
and
125
volts
(or
210
and
250
volts),
with
the
design
center
at
117
volts
(or
234
volts),
rms.
(There
are
similar
line
voltage
ranges
for
the
other
nominal
line
voltages).
Improper
line
voltage
may
cause
one
or
all
of
the
supply
voltages
to
be
off.
All
the
low
voltage
power
supplies
are
dependent
upon
the
—
150
volt
supply
for
regulation,
and
consequently,
a
change
in
the
regulation
point
of
all
the
supplies
is
indica
tive
of
a
defective
—
150
volt
supply.
If
the
output
volt
age
of
the
—
150
volt
supply
is
off
by
only
a
small
amount,
it
may
be
possible
to
readjust
the
—
150
Control
for
the
pro
per
voltage.
In
any
event
it
will
be
necessary
to
recalibrate
the
instrument
according
to
the
procedures
given
in
the
Calibration
Procedure
section
of
this
manual.
Fig.
4-8.
Low
voltage
power
supply
test
points.
In
case
of
the
failure
of
a
single
power
supply
to
regulate
properly,
check
the
following:
1.
Line
voltage
2.
Transformer
voltage
3.
Output
voltage
of
the
rectifier
4.
Tubes
or
transistors
5.
Loading
Important
power
supply
voltages
are
marked
on
the
power
supply
schematic
diagram.
These
voltages
may
be
used
to
perform
checks
on
the
power
supply
operation.
One
cause
of
improper
regulation
by
a
power
supply
is
incorrect
load
ing
of
the
supply.
To
check
power
supply
loading,
shut
off
the
power
and
check
the
resistance
of
the
power
supply
out
put
bus
to
ground.
The
—
150
volts
bus
should
read
approxi
mately
3,500
ohms,
the
+
100
volt
bus
approximately
4,000
ohms,
and
the
4-350
volt
bus
approximately
10,000
ohms.
If
none
of
the
preceding
checks
determine
the
cause
of
the
trouble,
the
cause
of
the
improper
regulation
is
probably
a
change
in
value
of
one
or
more
of
the
resistors
or
cap
acitors
composing
the
voltage
divider
networks.
The
resis
tance
networks
in
the
grid
circuits
of
V746B,
V666B
and
V636A
and
the
base
circuits
of
Q644
are
particularly
critical
since
they
determine
the
output
voltage
of
their
respective
power
supplies.
Use
resistance
checks
to
isolate
the
defective
part
or
parts.
The
following
information
may
be
used
as
a
quick
index
to
troubleshooting
the
Low-Voltage
Power
Supply.
4-4

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