HP 8340B Manual page 189

2.
Is
VSWP Correct?
Check
to
see
that
VSWP (A58TP10)
is
at zero volts at the start of each sweep. LVSX (low voltage
sweep
disable) is
high
in
swept mode for sweep
widths
>500
kHz,
but low in
CW/MANUAL.
Thus,
any offset in
VSWP would only affect the YO tuning
in
the
SWEPT
mode.
Because of the high
sensitivity
of the YO frequency to VSWP (500 MHz/V), an offset of as
little
as 50 mV added from
VSWP
to
PRETUNE
will result in the
YO
being tuned outside of the capture range of the
YO phase
locked
loop.
If
VSWP
is
zero, proceed
to question 3.
If VSWP
is
not zero, suspect the A58 sweep
generator.
3.
Is
the
YO Kick Pulse Correct?
Make sure the
YO kick pulse
is
being
generated
on the A55
YO
driver.
a.
Check the
LKICK signal by placing the channel A
probe of an oscilloscope
on
A55P1 -1. A TTL
low-going signal should be seen. If LKICK
is
present, suspect a
failure on the
A55 YO
driver.
If LKICK
is
not present, or
does not drop below
2
volts when
active, check
LKICK
at
A54TP2.
If
the signal
is
good at A54TP2, suspect
a
motherboard or connector
problem.
If
the signal at
A54TP2
is
bad,
suspect the A54 assembly.
4.
Is
HLEY High at Start of Sweep?
Check the
HLEY (high
lock enable
YIG oscillator, A50TP5) voltage at the start
of sweep.
If it is
low
or intermittent, the
YO
loop
will
not lock
-
suspect the A59 digital interface assembly.
HP 8340B/41B
YO
Loop Assembly-Level Troubleshooting D-19/D-20