HP 3455A Operating And Service Manual page 151

8-C-6 Absolute Amplifier Circuitry.
8-C-7. If the waveshape at TPS appears to be incorrect, try the following checks (except where
noted, the input signal should be a 10 V, 100 Hz sinewave).
a. To check if other circuits on the Al4 board causes failures in the absolute amplifier, the ab-
solute amplifier can be isolated. This can be accomplished by lifting R52 and R53. The amplifier
should now be operating correctly. Troubleshoot the amplifier circuit, if defective.
e. The cathode/anode junction of CR5 and CR4 should have a sinewave with slight distortion at
the zero crossover point. Replace CR4 or CRS, if necessary.
f. If the signal at TP5 is distorted, CR4 may have leakage. CR4 and CRS can be interchanged.
g. If Q14 or QI5 appear defective, check with an ohmmeter and replace, if necessary.
8-C-8. Squaring Amplifier, Integrator, and Antilog Circuitry.
8-C-9. The squaring amplifier, integrator, and antilog circuits are connected by feedback paths.
Isolation of these circuits may be difficult. There are, however, some checks used to help
troubleshoot these circuits (except where noted, the input signal should be a 10 V, 100 Hz sinewave).
a. In some cases it is possible to isolate the integrator from the other circuits on the A1S board.
This can be accomplished by shorting TP6 to the cathode of CR3. The reading at TP1 should be ap-
proximately zero. If there are great offsets at TP1, troubleshoot the integrator.
b. Apply a 100 mV, 100 Hz sinewave to the input terminals of the 3455A. The instrument should
be in the 10 V range and display approximately .1000 V. Check for dc readings of + .82 V at TP4, -9
V at TP3, -.48 V at TP2, and + .067 V at TP1. These readings may be helpful in isolating the squar-
ing amp, integrator, and antilog circuits.
c. Reapply a 10 V, 100 Hz sinewave to the input of the 3455A. The instrument should be on the 10
V range. Check for dc readings of approximately + .66 V at TP4, -1.15 V at TP3, -.6 V at TP2, and
+6.7 V at TP1. Again, these readings may be helpful in isolating the squaring amp, integrator, and
antilog circuits.
e. Check for a voltage drop of approximately + 3.3 V across R36. If this voltage drop is incorrect,
Q12 or Q13 and their associated circuitry may be defective.
f. For parasitic oscillations at TP3 change R36 to 649 2 (part number: 0698-4460).
g. If the display of the 3455A indicates an overload condition with a good waveshape at TP5,
Model 3455A SERVICE GROUP C Section VIII
8-61