Table of Contents
Advertisement
5.7 Trouble Analysis
5.7.1 Overview
CAUTION
Only well-qualified personnel should
attempt trouble analysis. Be sure
power is OFF during disassembly and
setting up for tests. Carefully observe
the HANDLING PRECAUTIONS given
at the beginning of Section 5.
Resources: Refer to Section 4 for a good understand-
ing of the theory of operation. Section 4 provides
block diagrams and discussion for the necessary
background, which can generally save time in trouble
analysis.
Abnormal digital signal levels. Most digital signal
levels in this instrument are normally near zero (logic
low), about +3.5 to + 5 V (logic high), or rapidly
switching between these states. Failure of a digital
source often produces a dc voltage of about +2 V on
a signal line. Use high-impedance probes in measur-
ing. Use a scope as well as a voltmeter, because an
average of 2 V may be normal for a digital signal that
has a duty cycle near 50%.
Duplicated circuit and resistor networks. Some
circuits, as in the display board for example, are
duplicated several times. The ICs can usually be
exchanged' between a faulty circuit and a functional
one, to identify a "bad" IC. Notice, also , that the re-
sistor networks DB-Z2..DB-Z10 are simply compact
packages of 220 ohm resistors. If one resistor is open,
it is not necessary to replace the entire package. Use
a 5% resistor.
Circuit board replacement. Refer to paragraph 5.3
for recommended procedures to obtain replacements.
Service and Maintenance
Power-Up Self Check. Upon power-up, the Digibridge
performs a self-check sequence as explained in para-
graph 3.13. If a self-check failure occurs, the instru-
ment display will indicate the nature of the failure.
For more detail, refer to paragraph 5.7.2 below.
Telltale symptoms. Scan the following group of symp-
toms for a preliminary analysis of trouble and sug-
gestions for more detailed procedures if applicable.
Display. A perpetually blank digit or decimal point
may be caused by a fault in the directly associated cir-
cuit on the display board. (Refer to comments above.)
D Error. A large D error may be caused by faulty
"protection" diodes in the analog front end. Check
MECR30, CR31, CR32, CR33 on 1689-4702 Analog
& Control board.
Reactance Error. If C or L measurements are not accu-
rate, the test signal source may be at fault. In checking
it, verify that the frequency is within ±0.01% of the
specified nominal frequency. Refer to paragraph 3.4.1.
Keyboard Malfunction. If the instrument passes its
power-up self-check and proceeds to measure DUT
has no (or only a few) keyboard indicators lit, and
the keys on the keyboard don 't function normally:
the fault is probably in the 30-wire cable connection
between keyboard and main board. If the keyboard
has been disconnected, check for misalignment of the
connector that plugs directly into the keyboard. If only
one key or indicator malfunctions, check that portion
of the keyboard assembly (1687-4200).
Test Signal. To check performance of the test-signal
source, use a scope to look at the open-circuit signal
at the IH terminal of the test fixture (right front con-
tact - be sure there is no DUT) . The signal on each
range should be an undistorted sine wave at the se-
lected frequency, with the top of the waveform about
+3.4 V and the trough about + 0.85 V. Range should
be locked to Range 4 if checking signal level accuracy.
1693 RLC Digibridge
123
Advertisement
Table of Contents
