Keithley 7077, Instruction Manual

Page: 51 / 62

Service Information
4-13
4.5.3 Power up safeguard
To prevent relays from accidentally energizing and causing
possible damage during power up, a safeguard circuit has
been incorporated into the design. The protection circuit is
comprised of a dual NAND gate (U129) configured as a SET
or RESET flip-flop and an RC network (R102, C122 and
CR101). The time constant of the RC network keeps the out-
put of the NAND gate low during the power up sequence.
This low signal is applied to the OE input of the shift regis-
ters keeping the relays de-energized. After the capacitor of
the RC network charges, a STROBE signal will then force
the output of the NAND gate high, allowing configured re-
lays to energize.
4.6 Special handling of static-sensitive de-
vices
CMOS and other high-impedance devices are subject to pos-
sible static discharge damage because of the high-impedance
levels involved. The following precautions pertain specifical-
ly to static-sensitive devices. However, since many devices
in the Model 7077 are static-sensitive, it is recommended
that they are all treated as static-sensitive.
• Such devices should be transported and handled only in
containers specially designed to prevent or dissipate
static buildup. These devices will be received in anti-
static containers made of plastic or foam. Keep them in
their original containers until ready for installation.
• Remove the devices from their protective containers
only at a properly grounded workstation. Ground your-
self with a suitable wrist strap while working with these
devices.
• Handle the devices only by the body; do not touch the
pins.
• Any printed circuit board into which the device is to be
inserted must first be grounded to the bench or table.
• Use only anti-static type de-soldering tools and grounded-
tip soldering irons.
4.7 Troubleshooting
The Keithley Model 7070 Universal Adapter Card is an ex-
tender card that allows access to circuit components of the
Model 7077 during troubleshooting.
4.7.1 Recommended equipment
Table 4-4 summarizes the equipment necessary for general
troubleshooting.
4.7.2 Troubleshooting procedure
In order to service the matrix card, it may be necessary to re-
move the bottom shield. The bottom shield is secured to the
matrix card by eight screws. Remove these screws and sepa-
rate the shield from the pc-board. When reinstalling the
shield, make sure it is oriented such that the standoffs (staked
onto the shield) are positioned between the pc-board and the
shield.
Table 4-5 outlines troubleshooting procedures for the matrix
card.
CAUTION
Contamination will degrade the perfor-
mance of the matrix card. To avoid con-
tamination, always grasp the card by
the handle and side edges. Do not touch
the card edge connectors, board surfac-
es, or components. On connectors, do
not touch areas adjacent to the electrical
contacts.
If removing relays from the pc-board,
care must be taken to prevent traces
from breaking off the board. Using a sol-
der sucker, make sure all solder is re-
moved. Each relay pin must move freely
in the feed-through hole. Make sure
there are no burrs on the ends of the re-
lay pins.
Table 4-4
Recommended troubleshooting equipment
Description Application
DMM (Keithley 2000)
Dual-trace, triggered sweep
oscilloscope, dc to 50MHz
Extender card (Keithley
7070) Measure dc voltage
Check clock and logic pulses
Allow circuit access