Troubleshooting; Displayboard; Nanovolt Preamp Noise Troubleshooting; Display - Keithley 181 Service Manual

MAINTENANCE
MODEL 181
Figure 5~18 is the A/D Buffer Output Wavefom The 181 is on
the 2OmV range with 1OmV applied to the inputs Thts
waveform begins with a 200msec delay and then a signal ins
tegratc takes place. After thesignal integrate, the preamp zero
takes place, Then a 2V cal is performed. When the 2V cal IS
compleio, another 200msec delay occurs. The 2OOmscc delays
are 10 allow sufficient time fix the preamp to settIe After Ihe
delay and another preamp zero, an AID zero takes place.
Notice the two zemes are different in value Another 200msec
May occurs ;ind rhc whole phase starts over again
5-18. Display Board
The Display Board c~rcwtry has two dwe c~rcuils [hiit operate
the display. The segment drive circuitry iQ201 thru 02081 is
connected 1" all displays in parallel. The digit dwe circuitry
crrablos all segments of a single digit one at a time. AlI signals
are referenced to dIgital common. Check these circuits per
Table 5-5
5.19. Nanovolt Preamp Noise Troubleshooting
BegIrl the troubleshooting procedure by checking thp 2V
range. If this range is noisy, the AID or digital circiriiry zxe
most likely at fault If the 2v range is quiet, the nan"v"lt
Pr"aKlP IS l""St Ilk+ al fault C"",,,"r,ents suspected o,
generating noise should be checked. That is, the noise level of
the device should be measured and verified as out of spec
before replacing the device.
,tep
I
2
3
4
5
6
I
8
9
IO
II
12
Item/Component
U203. PI" 14
U203, Pin 8
U203. Pin I
U203, Pi11 3, 4, 5, 6,
10. 11, 12. 13
UZOl. Pin I, 13, 9, 7
11202. I'ln 1, 13, 9. 7
0201 208
US202~20/
Plrl 1. 2. 4. 6. /.
0, 10
.11008, Pin 13
J1008. Pin 12
.11008, Pin i,
.11008, P,r, 10
shunt.
B.
MonitorU404 Pin 7forareadingof lessthan%X. If PinGis
within +2V, proceed to Step 2. If Pin 7 is greater than ?2V
reselect Fi423 and Fi424 per the following procedure.
TABLE 5-5
DISPLAY
Required Condition
l~urn on P"WCi
Sdf!Cl zomv DC range
Slrorl l""",
irgrncnt Dwcrs
3111,x11 of SC(~II,P,,,
lrlvers, collcclois 0,
~2O?~Q208
'IPSS s201, s207. Szot
'rf?ss s202. S206, S20$
'ross S203, S?O5. S21(
i
3
1
Prc?ss S204. S211
5-8