Section
VIII
SERVICE GROUP E Model
3455A
b. If the 0 detect signal of older instruments is unstable, modify the instrument in the
following
way:
1. Change A I4R44 from a 10 M ohm resistor toa 2 M ohm resistor (part number 0683-20S5).
2. Change
A12R7,
R8. R16, and R17 from 4.99 K
to
10 K ohm (part number 0757-0442).
c. Check for the correct
operation
of
AI4US and
its associated circuitry. Since U5-and its
associated
circuitry is an
absolute amplitifer,
the
output
of the
amplifier
(emitter of U6) should be the
same as the signal at TPl. Therefore, pins
14
and 15 of the A 14 board should also show
the
absolute
value
of the signal at TPl. The correct signals for a
-10 V
input to the 3455A are shown in Figure
8-E-l. If the signals at
A14P1
pins 14, IS, and S are incorrect, troubleshoot the detect circuits.
d. Make sure any oxidation layers
have
not formed on the pins of the AI4 board. The pins can be
cleaned
with
a soft lead eraser.
B-E-8.
AID Noise (Schematic
6).
8-E-9. A/D noise
will
usually
show
up in all ranges and ail functions of the 34S5A.
Two
circuits to
check for noise on the
A/D
board are the integrator and the input circuits.
a. Check for a noisy
A14Q5 or
U3.
Make sure there are
not
oscillations present
at
TPl.
b.
Check
for a stable zero
detect
signal at
A14U6
pin 1. If the
signal
is unstable, U4 or U6 may be
noisy.
c. Check for a noisy
AI4Q3.
B-E-10.
Inguard Logic Troubleshooting (Schematic 7).
8-E-l 1. Before troubleshooting the inguard logic make sure the outguard logic is operating correctly.
Use the half-splitting technique of paragraph
8-195.
a. Check the
back
gate bias
voltage
(B.G.) of the processor (A10U26 pin 387).
The voltage
should
be within
±
.25 V of the voltage marked on the processor. If
the
voltage is incorrect,
check
for the
correct
value
of pad resistor A10R105 (pad list in parts list). If the pad resistor is the correct value and
the bias
voltage
is incorrect, replace the processor.
8-E-12.
Inguard Logic Troubleshooting with no AfO Waveform.
a. The signals at A10U26 pins 34 to 37 should be the same as those on A3TP4
to
TPl. The
only ex-
ception is the signal at A10U28 pin 9. If the signals do not agree, check for malfunctions in the in-
guard light isolators. A10U34 and U35, plus their associated circuits. Line F0 and FI transfer data
from outguard
to
inguard (F0
is the
data transfer
line and
FI is
the
data
transfer
rate
line).
b. If pin 9 of AIOU28 is different than pin 37 of
U26
and the HAZ line (pin 1 of U27)
is
low, the
pulse transformer and/or associated circuits
may be
defective. This
interrupt circuit can be checked
by
manually clocking A IOTP
10.
and can be achieved by pulling TP 10 low and then releasing
it.
If
no
toggling is taking place, troubleshoot the interrupt circuit consisting of A10U32A and
U27.
If there is
toggling, check
T2
or the outguard
section
(Al).
c.
The interrupt request lines
at
pin 29
of A10U26
must
be high. Troubleshoot the interrupt cir-
cuitry if necessary.
d. Data lines D0 to D7 (pin 18 to 35 of U26) and program address lines (PA0 to PA7 pins
1
to 8 of
U26) should
have voltage levels
from approximately zero to approximately + 4.5 V. Check
for
any
circuits causing these lines to be loaded
down.
It is possible and normal to observe sharp peaks of
3
V
to 3.5
V
on
some
lines
of
U26. These peaks
are present when
the line is in a tri-state mode. This is a
possible and normal operation.
8-70
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