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Section V
CD
Check the dc
voltage
at the
output of
the Buffer
Amplifier Uunction of A2CR24
and
A2R44), this
should
be 0 +/-_IV dc.
Check the de power
supply voltages on
the A2
board
:
+31 +/-.5Vdeat A2Pin
I.
-26
+/-.5V dc at A2 Pin 2.
Check tile ac
voltage
at the output of the Buffer
Amplifier (junction
of
A2CR24
and
A2R44) with
an
oscilloscope;
this should be a
sine wave of 3
V
+/-1 V peak-to-peak. Notice that for the FAIL
situation
there are three possible branches (NO
SIGNAL, LESS SIGNAL, GREATER SIGNAL).
Take the
appropriate
branch to the next
check
point.
- - - - NOTE----
If the
output
from the Buffer
Amplifier is not
correct
then the
problem is probably in the ALC
loop; following the tree
should
be the fastest
way
of localizing
tJle problem. If after completing
tJle tree
you still
have not local-
iad the problem, then refer
to
Paragraph
5-79
which gives a
method for opening the ALC
loop.
A procedure for
checking
the
oscillator circuit
is
given
in Paragraph
5-81.
FAIL if
either
(or both)
supply
is not present.
Check the dc voltage on the lamp (A2DSVI) at A2
Pin 5. With
a
larger than normal
signal
level in the
Buffer Amplifier the
voltage at
A2 Pin 5
should
be
low, from 0 to +5V dc.
Check the dc voltage
on
the lamp (A2DSVI) at A2
Pin 5, normally this
is
+4 to +6V dc. If
the
voltage
is less. then the lamp voltage is trying to increase
the Buffer Amplifier signal level and the problem
is in the Buffer Amplifier.
If
the voltage is
greater,
the I<lmp is
causing
the low
signal
in the Buffer
Amplifier. The lamp is probably
open
if the
voltage is higher than + 15 V dc.
- - - - NOTE----
MUllIcntary
,I!Orting
or
the +31
+31 V
or
- 2b V
powcr
supply
may_
CJuse
a Locked
Curren
t
Linlit
condition
if the power
su
pplies
are still
loaded by
the
654A
circuitry.
III tile Locked
Currellt Limit
condition
the
dc
output
drops
to
less than
2
V
alld
docs
Ilot
return to normal
wircn
the
short
is
(cmoved.
This
docs
Ilot
damage
the
power
suppli es ..
Proper power
supply
voltage
can
be
restored
by
switclling the
IIlstrulllcnt off
alld
then on
again
.
5-20
Model
654A
II" Ille positive puwer supply
callnot
be adjusted
to
+31 V,
proceed
as I"ollows:
a. Veril'y tlut the positive pllwer
supply
is
not in a
current
limit cllndition by dis-
connecting. the power supply load
A
I
Pin
12 (+) and
A
I Pin 13 ( - ).
- - - - NOTE - - - -
Tum
AC
power off
while
dis-
CLlnnecting
the power
supplies.
1\3
circuitry
may be damaged if
only one
powcr
supply
is
eOIl-
nected
to it.
b. If the +31 V power supply is restored
by
discollnecting
the load
circuitry,
trouble-
shoot
the
load
circuitry
(Oscillator,
Buffer
Amplifin, Balanced Amplifier,
Average
Detector
,
Amplitudc Control
Integrator, and I\kter Amplificr)
.
c.
If tile +31 V power
supply
is not restored
by
disconnecting
tile
load
circuitry,
troublesllOot
tile positive
supply.
R.ekr to
schenlalic
4.
II' tilC
Ilcgative
ptlwcr supply
callnot bc
<ldjuSled to
@
-
26 V. use the
procedure given
ill
step
10 to
isolate
the prohlem area
.
Check T
I <llld the Iillc filter co IllPOIICIl!S:
also
@
chcck
Alerl thru AICR-I. AICI4
:Ind AICI5
.
®
Check the
dc voltagcs
at the Balanced Amplifier
J
output
which
should
be 0 +/-.1 V dc
at
the top
of
1
A2R74 and
the
bottom
of
A2R75 ('top' and
'bottom'
are
as viewed
011
Schematic No.2).
®
Check
for
0
pen
lamp
or
resistor
in
the
photo-resistor A2DSV I.
Troubleshoot the Amplitude
Control
Integrator
®
(A3Q6 through A3Q9) including the Amplitude
Current
Reference
and
the
lamp
of
the
photo-resistor (A2DSVI)
Troubleshoot the Buffer AmplifIer (A2Q8 thru
®
A2QI 0)
also
include
the
resistor
of
the
photo-resistor, A2DSVI.
Check
the
ac signal
level at the Balanced Amplifier
®
output
with an oscilloscope
.
The signal
should
be a
sine-wave of
4 + /-
.5V
peaK.-to-peak at the top
of
A2R74
and
at the bottom
of
A2R75, the two
signals should
be
180%
out of phase
with
each
other.
Note that in the FAIL
condition
there are
two possible paths.
@
Check
the ac signal with
an oscilloscope between
the attenuators (S4)
and C9
and C10, the
signal
at
both points should be
3
+/- I V peak-to-peak.
[f
signal
is
not
present, check C9, CIO,
A2R76 and
A2R77.
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