Service 5
Keysight 34420A Service Guide 143
620 Ohms 500 mV voltage clamp failed This test configures the 10 V range. The 10 mA current
source is connected into an open circuit and the 500 mV ohms clamp voltage is selected
and measured. The result is checked for 425 mV ±175 mV.
621 Low impedance DC gain X100 failed This test configures the 100 mV range. The 10 mA
current source is connected into an open circuit and the 20 mV ohms clamp voltage is
selected and measured. The input gain is set to X100 through U307B, RP1A pin 2, and
U303C/D. The result is checked to 20 mV ±11 mV.
622 High impedance DC gain X1OO failed This test configures the 100 mV range. The 10 mA
current source is connected into an open circuit and the 20 mV ohms clamp voltage is
selected and measured. The input gain is set to X1OO through U307B, Ul02C pin 14,
U102C pin 31, and U304D. The result is checked to 20 mV ±11 mV.
623 Cannot calibrate precharge This test configures to the 1 V range with the input internally
grounded. The ADC is configured for 200 ms measurements. The U500 pulse width
modulated (PWM) DAC output (C512) is set to about 4 volts. A reading is taken in with
U101 in the MC state. A second reading is taken in the PRE state. The precharge amplifier
voltage offset is calculated. The U500 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 U500 is
toggling. If no logic input is detected, the meter will assume a 50 Hz line operation for all
future measurements.
625 I/0 processor did not respond This test checks that communications can be established
between U500 and U700 through the optically isolated (U506 and U704) 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 meter will beep and the error annunciator
will be on.
626
I/0 processor failed self-test This test causes the earth referenced processor U700 to
execute an internal, RAM test. Failure will generate an error.
627 Hard ware overload comparator failed This test configures the 10 V range. The +18 V
power supply is internally applied to the input. The overload comparator (U311A/B) bit is
checked for an overload condition (bit set to 0). Only the positive comparator is checked.
630 Bias current DAC failed This test configures the 1 V range with the input internally
grounded through R227. The bias current DAC, U301, is then programmed to both
extremes and a measurement taken at each. A check is made to ensure that O V is between
the two measurements.
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