Inside the 1902
47
You will need:
• A digital voltmeter capable of resolving to 1 microvolt, with
a cable to mate with the internal test connector, a Radiall
SMB snap-on socket (see page 51)
• A dual-channel oscilloscope with bandwidth of at least
10 MHz and sensitivity of 5 mV per division, to attach to the
same test connector
• A 1-pt cross-head screwdriver for undoing the outer case
• A small, flat-blade, insulated trimming tool for adjusting
potentiometers
• A sinewave generator capable of generating 50 or 60 Hz at
some ±5 V, for adjusting the mains notch filter
• BNC cables and (for the 1902 MK IV fitted with digital
filters) a BNC to 1902 transducer input cable
The 1902 circuit board is mounted in an inner can, enclosed by
an outer case. You have to remove the inner can and take off its
lid to do the calibration.
1. Take due precautions against electrostatic discharge. Earth
yourself to the 1902 with a wrist strap and earth the 1902 to
mains earth.
2. First, remove power and all cables from the 1902.
3. At each side of the back panel of the outer case is a small
black screw. Undo these with a 1-pt cross-headed
screwdriver.
4. Push the BNC connectors on the front of the case so that the
inner can slides out by a couple of centimetres, dislodging
the back panel. You can now see that the inner can is also
restrained by an earthing strap of green & yellow wire,
attached by a small screw.
5. Undo the screw and lock-washer from the inner can. Make a
note of the order of these fixings, since it is important to
replace them the same way (see also Reassembly, page 53).
Slide the bright-metal inner can out of the black outer case.
6. Remove the lid. There are four small screws near the corners
of the inner can that hold the lid to the base, clamping the
Calibration:
tools required
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