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Maintenance
7-12
5. At this point, the instrument will enter the diagnostic
mode that switches the instrument among the zero
(common), calibration (reference), and signal phases of
its measurement cycle. The unit can be cycled through
these phases by repeatedly pressing the TRIG button.
The decimal point will indicate the range. During the
zero (common) phase, the display will appear as fol-
lows:
P=0
6. During the calibration (reference) phase, the display
will show:
P=C
7. Finally, the display will show the following message
during the signal phase:
P=S
8. To remove the instrument from the diagnostic mode,
turn off the power.
During normal operation, the instrument cycles through
these three phases in rapid succession. This cycling action
makes it difficult to troubleshoot the instrument utilizing
normal signal-tracing techniques. However, by using the di-
agnostic program to “freeze” the instrument on the appropri-
ate phase, troubleshooting is greatly simplified. Table 7-7
summarizes phases, display messages, and signals applied to
the A/D converter during each of the three phases.
Table 7-7
Diagnostic program phases
Phase
Display
message
A/D converter
input signal
Zero (Common)
Calibration (Reference)
Signal
P=0
C=0
P=S
Analog Common
-2V Reference
Output of Rang-
ing Amplifier
7.7.4 Power supply checks
All power supply voltages should be checked first to make
sure they are within the required limits. If the various oper-
ating voltages are not within the required limits, trouble-
shooting the remaining circuitry can be quite difficult.
Table 7-8 outlines the various power supply voltages that
should be checked. In addition to the usual voltage checks, it
is a good idea to check the various supplies with an oscillo-
scope to make sure that no noise or ringing is present.
WARNING
The electrometer board shield is con-
nected to analog common and can float
up to 800V above chassis ground, de-
pending on the input signal.
7.7.5 Relay configuration
Instrument functions are controlled by configuring the input
amplifier with a number of relays. These relays are them-
selves controlled by serial-parallel converter ICs that decode
control information from the microprocessor. Since each re-
lay must assume a given state for proper operation, it is pos-
sible to verify input configuration switching by determining
which relays are energized for every range and function.
Table 7-9 gives a summary of status for each of the twelve
relays associated with the electrometer input section. You
can verify proper relay operation for a given combination by
selecting the range and function in question and then mea-
suring the control voltage at the IC driver output. When a re-
lay is energized (on), the voltage at the output will be high,
while approximately 0V will be seen with the relay de-ener-
gized (off) (except K302 and K303, which are energized
when driven with 0V, and de-energized when driven to +5V).
Summary of Contents for 6512
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