Service 6
Keysight 34970A/34972A Service Guide 183
622 Frequency counter failed This test configures for the 100 mV AC range. This test
immediately follows test 621. With A4C301 holding charge from test 621 the AC
input is now switched to ground through A4K103. This produces a positive pulse
on the input to the frequency comparator A4U310. While C301 discharges, the
ENAB FREQ bit is toggled four times to produce a frequency input to the counter
logic in A1U205. A failure occurs if the counter can not measure the frequency
input.
623 Cannot calibrate precharge This test configures to the 100 V DC range with no
input. The ADC is configured for 200 ms measurements. The A1U205 pulse width
modulated (PWM) DAC output (C224) is set to about 4 volts. A reading is taken in
with A4U101 in the MC state. A second reading is taken in the PRE state. The
precharge amplifier voltage offset is calculated. The A1U205 DAC output is set to
about 1.5 volts and the precharge offset is measured again. The gain of the offset
adjustment is calculated. This test assures a precharge amplifier offset is
achievable.
624 Unable to sense line frequency This test checks that the LSENSE logic input to
A1U205 is toggling. If no logic input is detected, the meter will assume a 50 Hz
line operation for all future measurements.
625 I/O processor did not respond This test checks that communications can be
established between A1U205 and A1U305 through the optically isolated (A1U213
and A1U214) serial data link. Failure to establish communication in either
direction will generate an error. If this condition is detected at power-on self-test,
the instrument will beep and the error annunciator will be on.
626 I/O processor failed sel f-test A failure occurred when the earth referenced
processor, AU305, executed an internal RAM test.
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