HP 3455A Operating And Service Manual page 167

Model 34554 SERVICE GROUP G Section VII]
8-78
nearest -hp- Service Office, if inoperative. The only checks that can be made are the following.
1. If the reading on the display jumps 10 or 100 counts when adjusting any adjustments on
the reference assembly, the wiper of the pot may be dirty. Work the adjustment screw of pot
back and forth to clean the wiper.
2. Make sure an oxidation layer has not formed on any pins of the assembly printed circuit
board. The pins can be cleaned with a soft lead eraser.
3. Typically, the maximum noise allowed on the reference voltages (use a DVM with input Z
> 1010) are 20 nV for the + 10 V reference and 30 yV for the -10 V reference. Replace the
assembly if excessive noise is present.
4. The + 10 V reference voltage at AIOTP8 should be adjustable to +10 V + 100 pV and
the -10 V to -10 V +20 mV. Replace the assembly if both the + 10 V and -10 V are not ad-
justable. Replace A10U7 if only the -10 V reference is incorrect.
8-6-4. Turn-Over Errors (Schematic 1, 5, and 6).
8-G-5, Turn-over errors are present when, for example, a positive input reading is good and the
negative input reading is out of tolerance. This can be checked by taking a positive reading and then
reversing the input leads. The following are a few turn-over checks and hints.
a. When checking for turn-over errors, the 10 V range and zero offsets should be the first things to
check.
b, Check the A/D converter (A14) if turn-over differences are observed. Replace, if necessary.
¢. Turn-over errors on all ranges: Unsolder CR} and CR2 from the A10 board. Ifthe error disap-
pears, CR1 and/or CR2 may be leaky. Make sure the + 10 V and -10 V references are good (A10TP8
should be + 10 V + 100 2V and Al0 TP? should be.-10 V + 20 mV).
d. Turn-over differences on the 10 V range: Check A10U1, U2, Q7, or QI8. Other possible causes
may be KI, Al, Q2, Q4, QI9, and their associated circuits.
e. Turn-over errors on the top three ranges: Check A10K6 and Q15.
f. Turn-over differences on the 100 V and 1000 V ranges: Lift Al0Q9 and CR29. If the error
disappears, CR31 and/or CR29 may be leaky. Do the same with CR16 and CR17. Q8 may also be
defective.
g. The FETs connected to AIOTP2 may cause turn-over errors, if leaky. Q40 may also be leaky.
h. If the negative readings are good and all positive readings above 20 V are unstable on the 100 V
range, check A10Q36,
8-G-6. Other Troubleshooting (Schematic 8 and 11).
a. If the instrument fails to sample in the de volt, high resolution mode and the ac volt normal
resolution mode, check A1C29 or U48.
b. If the HP-IB operation is intermittent with the instrument's LED's dim, make sure the 50/60
Hz switch is in the correct position.
c. If the fan refuses to spin after repeated turn-ons, change AIR1I5, 19, and 24 from 11.8 kQ to
13.3 k@ (0757-0289). This change should not be made on operating fan circuits.
d. A good fan measures approximately 30 2 between the brown and yellow wires of the fan. A
defective fan will usually measure between 10 2 and 15 9.