Troubleshooting The Dac Switching Section - HP 5334B Service Manual

a. Short the inverting input of U25A (pin 2) to the +2.5 volt reference at TP10.
b. Measure approximately
-
3 volts at the output of the DAC voltage circuit, U25D pin 1 4 .
c. If the DAC voltage is not correct, measure the voltage at the output of the current-to-voltage con-
verter, U25A pin 1 and at the output of the Polarity Switch U26B pin 15. Each pin should measure
approximately
-
3 volts.
d. If the current-to-voltage converter output is incorrect, replace U25. If the Polarity Switch output is in-
correct, verify that the control signal from the Measurement MCU at U26 pin 10 is a TTL HIGH
level. Also check the feedback resistors, R69 and R70, for opens or shorts before replacing U26.
e. Remove jumper from U25A pin 2 and TP10.
3-353. Troubleshooting the DAC Switching Section
8-354. To check Polarity Switch, U26A, set the counter as follows:
Settings of 5334B
READ LEVELS-Trigger Level mode ("L" appears at extreme right side of display.)
No Input Signal to Counter.
a. Rotate the Trigger Level Control over its full range. The input voltage at U26A pin 12 and 14 should
range from
+
2.5 to
-
2.5 volts.
b. If the input voltage is incorrect, resistor-pack R119 or the Trigger Level Control may be defective.
c. If the output voltage is incorrect, verify that the control voltage from U29 pin 28 to U26A pin 11 is a
TTL LOW. If the control voltage is correct, replace U26.
8-355. To check Trigger Level Switch, U27C, set the counter in the READ LEVELS-Trigger Level mode, with
no signal connected to the Counter's input.
a. With the SENS key OFF, the output at U27C pin 4 should range from
+
2.5 to
-
2.5 volts as the Trig-
ger Level Control is adjusted.
b. With the SENS key ON, switch output should remain at zero volts, regardless of the Trigger Level
Control setting.
c. If the switch is not operating correctly, check for the correct TTL levels at control pin U27C pin 9,
before replacing U27. When the SENS key is OFF, the TTL level should be LOW; when the SENS
key is ON, the TTL level should be HIGH.
8-356. Check the READ LEVELS comparator, U36B, with no signal connected to the Counter's input.
Settings of 5334B
AUTO TRIG
...................................
OFF
READ LEVELS
.............................
OFF
SENS
.................................................
OFF
a. Rotate the Trigger Level Control over its full range. The output of the comparator U36B pin 7
should toggle between
+
5 and
-
5 volts as the Trigger Level Control is adjusted through the mid-
range position.
HP 5334B
-
Service Manual
8-50