HP 5334B Service Manual page 194

8-449. Troubleshooting the Peak Detector Circuit
8-450. The purpose of the Peak Detector circuit is to sense when an input signal is present or not present.
When an input signal is present, the Peak Detector enables the HP 5334B to count, and when no (or insuffi-
cient) input signal is present, it disables counting.
8-451. TROUBLESHOOTING THE PEAK DETECTOR DIODE
8-452. When the Peak Detector goes bad, the most common symptom is that the signal going to the com-
parator (U306-2)
will
have a large offset voltage. To troubleshoot the Peak Detector, perform the following:
a. Remove signal from INPUT C.
b. Check for a voltage of about
+
0.2V at the junction of the Peak Detector diode CR302 and resistor
R309.
c. Check for a voltage of about
-
0.2V at the junction of the CR302 and resistor R308.
d. If you measured O V at either of the junctions in the previous steps, b and c, check the node(s) for
shorts to ground.
e. If there is no obvious shorts, umolder CR302 and remove it form the the circuit board.
f. Using a ohmmeter, measure for shorts in CR302. If CR302 checks out good, then check the bias cur-
rent circuits in the following step.
g. Check for about +0.W at the anode of CR304 and for 0.7V at the cathode of CR305.
h. If all the above checks are good, replace CR302.
8-453. TROUBLESHOOTING THE THRESHOLD COMPARATOR
8-454. Comparator U306 output (pin 1) depends on the rela.tionship of its two inputs; that is, when the voltage
at pin 2 is less (more negative) than the voltage at pin 3, pin 1 should be at
-
1.2V
a
0.4V. When the voltage at
pin 2 is more (less negative) than the voltage at pin 3, pin 1 should be at -5.OV +0.4V.
.
8-455. To determine if the Threshold Comparator circuit is functioning properly, perform the following:
a. Connect a 90 MHz.,
-
30 dBM signal to INPUT C.
CAUTION
If your HP 53348 contains a Revision A, B, or C A1 Main Board, read
the the following instruction:
To prevent shorting out resistor R319, you must not leave poten-
tiometer R328 (Threshold Adjust) in its fully clockwise position
too long.
b. Adjust R328 so that the voltage at U306 pin 2 is less (more negative) than the voltage at U306 pin 3.
c. Verify that U306 pin 1
is -
1.2V
2
0.4V. If the voltage at pin 1 is not
-
1.2V
+
0.4V, the comparator
circuit is malfunctioning; go to step f.
HP 5334B
-
Service
~ a n u a l
8-74