Service Model
8903B
PRINCIPLES
OF
OPERATION---SERVICE SHEET
1
Relay
Drivers
U15 is
a
TTL octal peripheral driver capable driving the relays. The relays energize when the relay-driver
outputs
go
low. The diodes across the relay coils suppress the inductive transient generated by the
coil
when
the current is interrupted. U14 decodes the attenuation control lines
for
the 40 dB attenuator.
Humidity Sealing
The area surrounding the relays
of
this assembly have been coated with a humidity seal. The area
is
marked
on the circuit board.
If
components in the sealed area are replaced,
it
is recommended that solder
flux
no/
be
removed and that a humidity sealant (such as
HP
8500-3266)
be applied with
a
cotton swab over the affected
area.
8B-16
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