TB 9-6625-2293-35
18. DAC REF and Input Capacitance
a. Performance Check
(1) Press ∆t pushbutton for a ∆t display on crt.
(2) Connect digital multimeter LO to chassis ground and HI to J118 (fig 1) pin 2.
(3) Rotate ∆REF OR DLY POS control ccw until digital multimeter indication
remains at a constant value (approximately -1.25 V dc). Record indication.
(4) Rotate ∆REF OR DLY POS control cw until digital multimeter indication
remains at a constant value (approximately 1.25 V dc). Record indication.
(5) Add the absolute values of indications recorded in (3) and (4) above
(approximately 2.500 V).
(6) Subtract the total in (6) above from 2.500 V, then divide the difference by two.
(7) Adjust DAC REF R2127 (fig 2) to add the (signed) number obtained in (6) above
to the reading in (4) above.
(8) Repeat (3) through (7) above as necessary to obtain a total DAC range of 2.500 V.
(9) Disconnect digital multimeter.
(10) Press ∆t pushbuttons to off (no cursor on display).
(11) Connect oscilloscope calibrator SOURCE/MEASURE CHAN 1 to TI CH 1
using a 5-80 pF standardizer.
(12) Set oscilloscope calibrator for a CHAN 1, EDGE mode output of 600 mV at
1 kHz frequency.
(13) Rotate oscilloscope calibrator knob below EDIT field pushbutton to adjust
waveform to 6 divisions of vertical deflection on TI.
(14) Adjust 5-80 pF standardizer for a square front corner over the first 40 s (0.4
division) of the positive portion of waveform. Note waveform front corner for use in (17)
below.
(15) Change oscilloscope calibrator EDGE mode output to 300 mV at 1 kHz.
(16) Set VOLTS/DIV switch to 50 mV and rotate oscilloscope calibrator knob below
EDIT field pushbutton to adjust waveform to 6 divisions of vertical deflection on TI.
(17) Adjust CH1 50 mV C ADJ (C105) and CH2 50 mV C ADJ (C205) for CH 2 (fig 1)
for same waveform front corner as noted in (15) above.
(18) Set VOLTS/D IV switch to 100 mV.
(19) Repeat (13) through (18) above until no change is observed in the waveform
front corner.
(20) Press VERTICAL MODE CH 1 to off and CH 2 to on.
(21) Repeat technique of (11) through (19) above for CH 2.
