METRON QA-90, User & Service Manual

Page: 62 / 112

6-2
All terminals for connecting the machine under test to are located on
the front panel. The mains contact is located on the rear panel adja-
cent to the mains switch marked Power QA-90.
6.2 Measuring System
Refer to drawing no. E160.20.2000. U1.
The measuring board covers the entire base of the cabinet housing.
Test currents are generated from the rear-left edge of the metering
board. The forward section to the left is the measurement amplifica-
tion unit with power to frequency conversion against the processor
component. Approximately two-thirds of the right-hand side of the
board comprises relay (sequence) drives and related relays for setting
up the various measurement modes and measurement inputs. All
communications with the CPU board occur via optical isolators,
which isolate the measurement and CPU components from one anoth-
er.
Refer to Schematic Diagram E160.20.2000. U1 (High Voltage Inter-
face QA-90) (Measuring Preparation QA-90). The metering system is
divided into six function blocks. The diagram contains the function
blocks, as well as the sequence functions for power and auxiliary ter-
minals on the rear panel of the QA-90. The T2070 Measurement
Transformer is used for "current consumption" measurements.
Refer to Schematic Diagram E160.20.2100.U3 (High Voltage Inter-
face QA-90). The high voltage (mains) interface component has sev-
eral functions. T2110 gives the power frequency and synchronization
signal to the High-Voltage Logic (U2210). U2210 generates a 50-60
Hz rectangular signal respectively to U2220A (to give 253 VAC to
VDE measurements and to Mains on applied part measurements) and
to U2220C (which gives 500 VDC to insulation resistance measure-
ments). 2220D emits a rectangular signal with a DC offset. The am-
plitude is adjusted by respectively R2224 for AC and R2228 for DC.
U2240 is a screening circuit with a center frequency set by D2240 de-
pending on the mains frequency. The screen converts the DC signal to
almost a sinus signal. This then goes to an amplifier/buffer, referred to
as the "High Voltage Drives" (refer to Schematic Diagram
E160.20.2300.U3 (High Voltage Drives QA-90)), which in turn feeds
T2150 (page 2).
For measuring mains on applied part (253 VAC) the signal goes from
T2150, through the S2151 relay and 47 kOhm series resistor (R2150,
R2151), to the measuring object. For measuring insulation resistance
(500 VDC), the signal passes through the S2151 relay to the rectifier