Fluke 900 - Manual (Page 85)

FLUKE 900 SERVICE MANUAL Maintenance
5.3.8 Interface Buffer Board
1. With the unit turned off, disconnect the J1 and J2 connectors which attach the
Interface Buffer to the main unit
2. Remove the cover from the Interface Buffer.
3. Remove the 4 screws securing the board to the bottom cover.
4. Place new board in position and fasten it to the bottom cover with the 4 screws.
5. Replace the top cover.
5.4 Calibration Procedures
The Interface Buffer and the High Speed Board are calibrated during the manufacturing process.
After service repair of one of the signal channels, it should be calibrated again in a known-good
machine.
5.4.1 Interface Buffer Calibration
Remove the cover of the IB and connect J1 and J2 ribbon cables to the HSB. Turn the power
on and wait for completion of the selftest. Select debug mode and run test 46
(Freq_bias_short). This may be run in looping fashion as well. Two offset values appear on
the screen. They will be within + or -10 when the IB is calibrated properly.
Resistor R57 determines the offset values. Trial and error selection of a value between 350K
and 450K is the recommended procedure. First remove R57 so it is open circuit and gives a
large negative offset value. Then connect jumper leads to U25 pin3 and U26 pinl (these are
the endpoints of R57). While test 46 is looping, connect various resistors to the jumper leads
until an acceptable offset value is achieved.
5.4.2 High Speed Board Calibration
This section describes the HS Board fault mask calibration procedure, the preliminary
conditions and the final criteria to be met to consider a board calibrated. The main
requirement is that the absolute value of the offset for all pins be less than or equal to 4. Also
described is the way to isolate faulty chips in some special cases related to finask calibration.
5.4.2.1 Calibration Data Format
The calibration data printout consists of seven blocks. The first one’s header is "Limits
table", and this name will be used to refer to the block.
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