HP 3455A Operating And Service Manual page 159

f. For no A/D waveform at TP1, go to paragraph 8-E-4 for troubleshooting. For an incorrect
waveform go to paragraph 8-E-6.
8-E-4,. Mo A/D Waveform.
8-E-5. Since the A/D waveform is dependent on various circuits in the 3455A (input, main amplifier,
etc.), isolation of these circuits is necessary. The method used is simply a signal tracing method with
limited operational checks.
a. Set up the 3455A using the procedure of paragraph 8-E-3a, b, and c.
b. Measure the voltage at the multiplex node (sources of A10Q1, Q2, Q3, and Q4). If the voltage is
not -10 V dc, the input circuit may be inoperative. The multiplex node may also be loaded down by
one or more FETs.
c. Measure for a -10 V dc voltage at AIOTP4. If the voltage is incorrect, troubleshoot the main
amplifier circuit. Make sure A10Q19 is turned on.
d. Measure the instrument's reference voltages. A10TP8 should be +10 V + 100 nV and TP7
should be -10 V + 20 mV. If the reference voltages are incorrect, troubleshoot the reference assembly
{A11 or A20) and/or U7. The reference voltages are used on the A/D board and should be correct for
proper A/D operation.
e. Short across capacitor Al4C2 and measure the voltage at A14TP1. The voltage should be ap-
proximately zero. If there are any great offsets, troubleshoot A14U3 and associated circuits. If the
voltage at TP1 is good, remove the short from C2 and continue with this procedure.
f. Short A14TP1 to ground and measure the 0 detect, 10 V detect, and .2 V detect levels. The table
below gives the correct detect levels. Remove the short from TP1 and apply -10 V to TP1. Measure
the levels of the 0 detect, 10 V detect, and .2 V detect. See the table below for the correct levels.
A14TP1 Shorted | A14TP1 at-10 V
O Detect Level =5V =z0V
10 V Detect Level =OV =z5V
-2 V Detect Level =O0OV =OV
If the levels in the table are incorrect, troubleshoot A14U4, 5, 6 and their associated circuits.
g. Other circuits on the A/D board may affect A/D operation. These circuits are the input circuits
and diode array #1 and #2 and their associated circuits. Also make sure Al4Q3, Q4, and their
associated circuits are operating correctly. If these circuits appear to be working correctly, the in-
guard logic may be at fault. Go to paragraph 8-E-10 for further troubleshooting.
8-E-6. Incorrect A/D Waveform.
a. Check for a leaky Al4C2, Q5, U3, or US and associated circuits. Circuits past U4 may
also be defective.
b. Circuitry preceeding the integrator may also cause an incorrect A/D waveform. Make sure
BAO? Q4, and their associated circuits are operating correctly.
c. Check for correct operation of the detect circuitry. Paragraph 8-E-Sf may be helpful in
troubleshooting these circuits.
8-E-7. Correct AID Waveform.
a. If the A/D waveform is correct and the A/D board is still inoperative, check the zero detect cir-
cuit. Make sure the zero detect signal is stable with the correct voltage levels (approximately 0 V or 5
Vv).
Model 3455A SERVICE GROUP E Section VIII
8-69