HP 5334B Service Manual page 169

8-347. The READ LEVELS-Trigger Levels function may display an incorrect voltage or polarity as the Trigger
Level control is adjusted over its
full
range of
+
5.1 to
-
5.1 volts. If the display is faulty and both the A and B
channel trigger levels exhibit the same symptoms, the fault may be with the Measurement microcomputer
(MCU), U29 or Measurement Data Bus (U29,IlO Port 4). Check for enabling pulses from the Measurement
MCU.
8-348. Set the equipment as follows:
HP5334B Under Test
FREQ A
.......................................
ON
AUTO TRIG
...................................
OFF
SENS
.................................................
OFF
READ LEVELS
.............................
Trigger Level Mode
a. Use an oscilloscope to check for strobe pulses from Executive MCU (U29 pin 7) to pin 13 of both
DACs U33 and U34 (the WRITE lines). Also, check for strobe pulses from U29 pin 32 to pin 12 of
U33 and from U29 pin 30 to pin 12 of U34 (the CHIP SELECT lines). Observe a
+
5 volt peak-to-
peak pulse train at each measurement point. If the strobe pulses are missing, U29 may be defective.
b. Check the Measurement Data Bus at the DACs for TTL levels on pins 4 through 1 1 . A bent or
broken pin may be the source of the problem.
8-349. From this point on, it is assumed that a failure has been isolated to the DAC A circuit. The following
troubleshooting information refers to only DAC A circuit components. The two circuits are similar enough so
that the troubleshooting techniques can easily be applied to the DAC B circuit if it fails.
8-350. If the DAC A Zero adjustment will not calibrate, check that the adjustment pot, R67, has the correct
k
10 millivolt range. This voltage can be measured at pin 3 of op-amp U25A. If the voltage is correct but the op-
amp cannot be adjusted for offset, replace U25. If the adjustment voltage is not correct, check the resistor
divider, R68 and R86, at the op-amp input as well as the trimmer potentiometer, R67, for opens, shorts, or incor-
rect values.
8-351. If the DAC A Gain adjustment will not calibrate, check for the correct reference voltages at U35. The
+
5 volt Linear supply voltage should be present at pin 1 and
+
2.5 volts should be present at pin 2 and TP10. If
the reference voltage is missing or incorrect, check for a short at capacitor C77 on the reference output line
before replacing U35. If the reference voltage is correct, the GAIN potentiometer, R102, should provide an ad-
justment range of approximately
+
2.5 to
+
2.1 volts at pin 15 of DAC U33. If this voltage is incorrect, either
R102 or U33 is defective.
8-352. To check the operation of the DAC A circuit, set the equipment as follows:
Settings of 5334B
FREQ A
...........................................
ON
AUTO TRIG
...................................
OFF
No Input Signal to Counter.
HP 5334B
-
Service Manual
8-49