Alc Loop Assembly-Level Troubleshooting - HP 8340B Manual

ALC Loop
Assembly-Level Troubleshooting
To troubleshoot the ALC
loop,
you can break
the loop and inject
a
signal that you can trace
around the
loop.
In
the ALC loop troubleshooting mode, FET switches break the connection from the A25 ALC detector
assembly
to the
A26
linear modulator assembly).
A
resistor (A26R17)
is
switched across the integrator
capacitor
on the
A26
linear modulator assembly, converting
this stage to an inverting amplifier. In
this
way, the integrator output can be controlled via the reference DAC, and the signal traced from there to
the modulator driver, RF output, detector output, logger output, X5 amplifier, through the sample and
hold, level amplifier, and to
the analog-to-digital converter
driving
the level
meter.
1.
To enter the
test mode, press
[INSTR PRESET] [CW] [SHIFT] [METER].
The UNLEVELED annunciator
should be
on.
The POWER dBm display should indicate the
approximate RF output power. The ENTRY display should indicate
ATN:
-00,
MOD: 0.0
dB.
The modulators are controlled by the control voltage
MODLVL,
on the A26 assembly. This voltage
is programmed by
the
A27
level control
assembly ALC reference generator DAC. The DAC
can be
controlled using the RPG.
2.
Rotate the front panel knob
CCW
and note
that the
ENTRY display MOD
level changes.
3.
Continue
turning the front panel knob CCW
until the
POWER dBm display begins
to
change.
The
absolute
MOD
level indication is not important, but
a
further decrease
in
MOD
level should
generate a
corresponding decrease
in the
POWER dBm display, and the RF output power. For
example, a
10 dB MOD
level change should
generate
a
10 dB ±3 dB power change.
4.
Isolate
a
defective assembly by tracing the voltage from the MODLVL output to the modulators, or
by tracing the voltage from the high and low band detectors
to the level
meter
amplifier
output
on
the
A25 ALC detector assembly. The AM signal path (on the A26 assembly) must be checked
separately. Refer to Figure H-7.
5.
Attach
an
oscilloscope or DVM
to the
MODLVL
test point (A26TP5) and vary the front panel knob.
The voltage level should
vary between +0.5
(ENTRY display MOD
level at
0.0 dB)
and
—2.50 Vdc
(ENTRY
display
MOD
level at
-100 dB).
6.
Attach an oscilloscope or DVM to the MOD test point (A26TP5)
and vary the front panel knob.
The
voltage level should
vary between —0.25 (ENTRY display MOD
level at
0.0 dB)
and +1.00
Vdc (ENTRY
display
MOD
level at —100 dB). The
signal should
vary exponentially
with the front
panel knob.
7.
If
either MODLVL or MOD are incorrect,
the
A26
linear modulator assembly
is the most probable
cause. You can measure RF
levels at various points
in the RF path, and power
levels should
vary
with the front panel knob.
If
the
A26 assembly
signals
(MODLVL
and
MOD) are good, and the RF
measurements
are not
correct
(as indicated by the RF power display
on the front panel), refer
to
the RF path
trou¬
bleshooting procedures for microcircuit assembly
isolation.
HP 8340B/41B
ALC
Loop Assembly-Level Troubleshooting
H-21