HP 3455A Operating And Service Manual page 150

Section VIII SERVICE GROUP C Model 3455A
g. If all the above checks are good and the 3455A displays approximately 10 V ac (with 10 V, 100
Hz input), the RMS convertor should be ready for calibration. If the reading is incorrect, A10Q3 may
not be turned on or may be defective.
§-C-4. Preamplifier and Input Attenuator Circuitry.
8-C-5. The waveshape at TP8 appears to be incorrect, try the following checks (except where noted,
the input signal should be a 10 V, 100 Hz sinewave).
a. Check for the correct power supply voltages at U6 pins 4 and 7. Pin 4 should have approximate-
ly -15 V and pin 7 should have approximately +15 V.
b. Set the 3455A on the 10 V range and short the input terminals. Make sure TP8 can reach zero
volts, when adjusting R65. If unable to reach zero, try changing R77 to 412 kQ (part number: 0698-
4540). If R77 is a 412 K resistor already, replace U6.
c. If the signal at TP8 is riding on a high de level, make sure CR7 and Q20 are not touching any
shielding. Also make sure Q19 is not touching the heat sink of U6. Check CR7 and Q20 for shorts.
d. If the zero reading at TP8 is good on the 10 V and 1000 V ranges and bad on the 1 V and 100 V
Tanges, try the following checks.
1, Check the gain switching circuitry of QI6 to QI9, and U5. Make sure TP7 reads approx-
imately zero volts on the 10 V and 1000 V ranges. On the 1 V and 100 V ranges TP7 should read
approximately -15 V.
2. If the gain switching is correct, lift the drain or source of Q19. Check for a zero reading at
TP8 with the 3455A in the 1 V range. If the reading is good, replace Q19.
3. If the reading at TP8 is still bad, short the drain to the source of Q18. If the reading is then
good, replace Q18.
4. If the reading at TP8 is still bad, short TP8 to the junction of R86 and R87. If the reading
is corrected, troubleshoot the feedback network.
e. If the 3455A has a history of bad Q19's replace K1, K3, and Q18. Q18 may be damaged if Q19
has been damaged. The timing of K1 and K3 could be incorrect, causing Q19 to be destroyed by a
1000 V input. Check ac calibrator output for any spikes and make sure the 107 V Hz limit has not
been violated.
f. If it becomes necessary to replace the matched set of resistors R76, R86, and R91, the new set
should be properly aged. Do the following procedure.
1. Set the 3455A to the 10 V range and apply a 10 V, 100 Hz signal to the input. Note the
reading on the display.
2. Apply a 1000 V at 100 Hz signal to the input. Leave the 1000 V connected for about two
minutes.
3. Remove the 1000 V signal and reapply the 10 V at 100 Hz signal to the input. After a cool-
ing down period (less than 2 minutes), the reading on the display should have not changed more
than 25 counts from the reading in 1 above. Replace R76, R86, and R91 if necessary.
g. If it becomes necessary to replace the matched set of resistor R91 and R93, they also need to be
aged. Use the procedure in f above. The only exception to the procedure is to have the aging done on
the 100 V range rather than the 10 V range. A 100 V at 100 Hz signal should also be applied in place
of the 10 V signal.
h. Other circuits on the A15 board may cause preamplifier malfunctions. The preamplifier can be
isolated from the other circuits by lifting R52 and R64. If the preamplifier is working correctly, after