Technical data

Readings are inaccurate
Inaccurate readings are normally caused by either invalid calibration or
by non-linear measuring circuits. If recalibration does not correct the
inaccuracies, the amplifier or solid state switches may be causing
measurement non-linearities. Non-linear operation of the ohms current
source due to failure of
Q202 to Q211 may cause reading inaccuracies in
the ohms function. If the input to the
ADC is shown to be linear, check
the
±10 V reference voltages. Check switch U411, U420, and U402 for
proper operation.
Readings show excessive noise
Reading noise is usually caused by the dc input amplifier or the a-to-d
converter circuitry. Remember that it is normal for the most sensitive
measuring ranges to show more noise than the least sensitive ranges.
Verify that the multimeter fails to meet its zero input specifications.
Low frequency ac measurements will experience more noise than higher
frequency inputs. Verify that the multimeter fails to meet ac specifications.
Ensure that the correct ac filter is selected for the applied input signal
frequency.
If calibration constants become corrupted, it is possible for the
multimeter to generate noisy results without a hardware failure.
Perform a new zero offset calibration and gain calibration before
attempting to service the instrument.
Readings indicate zero with any input applied
A constant zero reading is normally caused when either the output of
the dc input amplifier is stuck at a fixed level (like 0 volts) or when the
a-to-d converter integrator amplifier
U402 is stuck at a fixed level.
6
Chapter 6 Service
Troubleshooting Hints
119