SECTION 7
Maintenance
CHECK’ indicates that the zero check relay (K110) is acti-
vated (dosed). The absence of the star (3 in the label indi-
cates that a high logic level (-SW will activate the relay or
switch.
7.7.4
A/D Test Mode
The instmment can be placed in the A/D test mode (DE-
BUG) which allows the A/D multiplexer (U144) to be
stopped in any one of its A/D phases. This feature sim-
plifies signal tracing through the instrument. The three
states of the multiplexer (mux) are as follows:
A/D SIG - In this mux state, the amplified and con-
vertedinputsignalisapplied totheinputof theA/Dcon-
verter.
A/D ZFXO -In this mux state, analog common (OV) is
applied to the input of the A/D converter.
A/D REF - In this mux state, the +2.8V reference is ap-
plied to the input of the A/D converter.
Perform the following steps to place the instrument in the
A/D test mode (DEBUG):
1.
Keep pressing and releasing MENU until the follow-
ing message is displayed:
DEBUG
NO
2. Use the rotary knob to display the following mes-
sage:
DEBUG YES
3. Press MENU to place the instrument in the A/D test
mode. The multiplexer will stop with the condi-
tioned input signal applied to the input of the A/D
converter as indicated by the following message on
the right display:
A/D SIG
4. To select one of the alternate A/D test states, simply
use the rotary knob to display one of the following
messages:
A/D ZERO
or
A/D REF
5. To exit from the test mode and return the instrument
to normal operation, press SHIFT and then EXIT.
7.7.5
Troubleshooting Procedures
Tables 7-5 through 7-8 summarize the troubleshooting
proceduresforthevariousdrcuitswithintheinstrument.
In general, the procedures should be performed in the or-
der given. Figure 7-5 shows the various test points for the
troubleshooting procedures:
WARNING
Lethal voltages may be present on compo-
nents and shields. The ac line voltage is pre-
sent on the pc mother board. Fmrcise ex-
treme caution when troubleshooting a par-
tially disassembled unit that has power ap-
plied to it. Never touch components, shields
or the pc boards while power is applied.
7-13
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