Tektronix 485, Instruction Manual

Page: 35 / 197

Maintenance—485/R485 Service
Integrated circuits can be checked w ith a voltmeter, test
oscilloscope, or by direct substitution. A good under­
standing of the circuit description is essential to trouble­
shooting circuits using integrated circuits. In addition,
operating waveforms, logic levels, and other operating
information for the integrated circuits are given in the
Circuit Description section. Use care when checking
voltages and waveforms around the integrated circuits so
that adjacent leads are not shorted together. A convenient
means o f clipping a test probe to the 14- and 16-pin
integrated circuits is w ith an integrated-circuit test clip.
This device also doubles as an integrated-circuit extraction
tool.
Special Troubleshooting Information
General. The following information provides a step-by-
step procedure to aid in the troubleshooting of some of the
more complex circuits and/or systems in the 485. A
thorough understanding of the circuit operation is essential
before troubleshooting in these areas. Read the applicable
portions of the Circuit Description in Section 3 before
proceeding with troubleshooting. This troubleshooting pro­
cedure refers to the diagrams, operating voltages, and
waveforms in Section 8 and Fig. 4-2. Specifications for the
troubleshooting equipment referred to in these procedures
are given earlier in this section under Troubleshooting
Equipment.
Troubleshooting the Power Supply. Table 4-4 gives a
guide to troubleshooting the power supply of the 485.
A short on a semi-regulated supply may lower the trans­
former voltage so far that no supply can perform. An over­
loaded supply often discharges very fast when the inverter
shuts off. Isolate shorts and overloads by withdrawing and
replacing all connectors one by one; however, comb con­
nectors BB, CC, DD, S, T, U, V, and W must not be removed
with power on. With power off, any or all of the comb con­
nectors can be removed for troubleshooting with an
ohmmeter.
To detect a short in the —2950 V supply, perform the
following: 1) Turn power o ff: 2) Remove comb U, and
bend pin 3 to prevent contact; 3) Re-install comb U and
turn power on.
If other supplies now perform correctly, locate and
repair the short in the —2950 V supply. Turn power off,
straighten pin 3 and re-install comb U.
W A R N I N G
Line AC and stored DC potentials are present on the Inverter c irc u it board and on the
transformer c irc u it board. The stored DC remains long a fte r the instrum ent is disconnected
from the pow er line. V erify that the line cord is disconnected and that the tine storage
capacitors, C1822 and C1823, are com pletely discharged before attem pting any repairs or
ohmic measurements. The stored DC voltage can be measured between TP1834 (POS) and
TP 1844 (NEG). I f manual discharge is necessary use a 1.5 k 2 W insulated resistor w ith
insulated leads. USE EXTR EM E CAUTION.
TABLE 4-4
Power Supply Troubleshooting
Trouble Symptom Checks to Make Comments
Inverter starts then shuts o ff at slow
rate (3 Hz or less).
Check Balance Node TP1951 during
20 ms "sampling period" BAL T.P. more positive than +200 mV
means short on negative supply or
overcurrent in H.V. winding of T1960.
BAL T.P. more negative than —200 mV
means short on positive supply or high
beam current.
Error on BAL T.P. stops inverter. SEE
BALANCE NODE NOTE preceding Fig.
4-2.
Check inverter current TP1926 during
20 ms "sampling period" If current remains at lim it level for entire
sampling period, U1910 w ill stop
inverter. Possible short on semi-regulated
supply.
Check Line Stop TP1918 during 20 ms
"sampling period". Check Line Selector
switch for correct position.
If proper AC voltage from T1801 does
not reach pin 4 of U1910, TP1918 w ill go
positive stopping the inverter.
REV. D, AUG. 1975 4-7