HP 8340B Manual page 473

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,
ASS,
AND A56
regulators are biased
off.
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,
100^2
resistor (via shielded clip leads) across the
capacitor terminals.
•www^w
C.
CAUTION
3
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
Assembly-Level Service Overall Instrument Troubleshooting
A.33