Fluke 900 - Manual (Page 92)

Maintenance FLUKE 900 SERVICE MANUAL
Check ifabs((U-H) -(U-L)) = 0,1,2,3,4,5
abs((D-H) -(D-L)) = 0,1,2,3,4,5
abs((U-H)-(U-L)-(D-H)+(D-L))
= 0,1,2,3,4,5
If not, it means that for some reason there is too large a difference between the
propagation time for a rising and falling edge from point A to point B (see fig. 2).
SELFTEST BUFFER
fig.2
The difference may be caused by one of the following:
- selftest buffer 74AS244
- logic comparator 74AHCT86
A bad component may be isolated with an oscilloscope as follows:
Scope setup:
chan A -1 V/div
B -1 V/div
timebase -10 ns/div (100 ns/div and xlO scale)
horizontal
marker -+ 1.4 V
Fluke 900 setup:
Bring up the F_Mask Calibration menu. From the main screen, press
tcNTRj and the hidden key found at the point between inext ) and itesti .
Press Ei(calib) EI)(f_mask).
- Cormect probe A to the enable input on any 74AS244 selflest buffer (pin 1 or 19)
and GND lead to the GND pin of this chip (pin 10).
- Connect probe B to the output of a buffer on this chip and the probe’s GND lead to
pin 10.
- Press fD(h_fault) and measure tl and t2 as indicated on figure 3a. Next, connect
5 - 1 4