HP 8340B Manual page 39

WARNING
|
When the
A19
power-on safety indicator
LED is on,
there are voltages
pre¬
sent inside the instrument that can
cause
personal injury
or death.
When the power switch is
set to
STANDBY,
only
the A52, A53, AND A56
regulators
are biased «ff.
Never short
a capacitor
with
a screwdriver
or similar direct
short. Either let
the capacitor bleed off via normal instrument loads or provide
a
discharge
path by applying
a
0.5W,
100^1
resistor
(via shielded clip leads) across the
capacitor terminals.
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CAUTION
j
To protect static sensitive components, troubleshoot
this instrument
only
at
a
work station equipped
with an
anti-static surface,
and
wear
a ground¬
ing
strap. When handling
a
printed circuit board, hold it by the edges; never
touch the finger contacts. Use low-static solder removal
tools when
deso-
Idering components. Use only soldering irons
that have
a
grounded tip.
DO NOT use silicone based thermal compound. Silicone based
oil
migrates
past element sockets, switch contacts,
or printed circuit
board edge
con¬
nectors, raising contact resistance, or electrically isolating the contacts.
Silicone
based
thermal compounds disperse into the air, depositing
them¬
selves anywhere
in the instrument. Heat increases the rate of dispersion.
Use only rosin
mildly activated solder when repairing
a
printed circuit
board. Rosin activated solder can
cause
reliability problems.
Cleaning solder flux from
a
printed circuit board after replacing
a
compo¬
nent causes serious reliability problems. When
you replace a
component,
solder flux remains
on
the newly soldered feedthrough pads. Solder flux
contains caustic rosin activating acids.
If
the residual rosin is left on the
board undisturbed, the activators remain encapsulated (and harmless), but
cleaning the
rosin off
(by any
means) releases
the caustic rosin activators
and spreads them over the
printed circuit board.
The caustic chemicals
get under
trace edges
(all
printed circuit board
traces have undercuts along the
edges
from the etching process) and can
not be washed out.
In
time,
these chemicals react
with
the dissimilar metals
in the
trace
(nickel, copper, gold), slowly dissolving the trace. The chem¬
icals also create an electrical path between traces, causing metal-ion
migration, which leads to high
impedance shorts and dendrite growth.
NEVER clean PC board fingers with an eraser. NEVER use tap water
in the
cleaning solution. Chloride contamination from tap water, from salt (skin
contact), or from any other source, can cause reliability problems. Always
wear
a
ground strap when handling any internal component or assembly.
HP 8340B/41B
Change
7
Assembly-Level Service
Overall Instrument Troubleshooting
A-33