HP 5245L Service Manual

Section
II
Paragraphs
2-103
to
2-
116
2-103.
HOLDOFF. A
secondoutput
fromthe
sample-
rate multivibrator is
amplified
by
Q10
to drive the
holdoff
one-shot multivibrator
(Q11-Q12).
The holdoff
multivibrator
output consists
of
a
positive pulse
which
normally
begins
about
1 microsecond
after
the end
of
the
gating
signal
and
ends
55
milliseconds
after the
end
of
the
sample-rate
pulse.
A holdoff
pulse
is
also
generated
as
a result
of manual
reset
(-15
volts
from
manual
reset line
through
CR6 and
Q9
to
Ql1-Q12)
so
that
counting
cannot
start
unti.l
after reset
switching
transients have
ended.
2-L04,
CIRCUIT
DETAILS.
Refer
to the schematic
diagram,
Figure
7-20,
for circuit
details.
Inhibit
amplifier
Q3
is
in
series
with the
Q4
collector;
inhibi-
tion of the
sample-rate multivibrator
occurs
whenQ3
is
cutoff by a
positive
inhibit
signal
(whicheffectively
disconnects
theQ4 collector).
At
theendof the
multi-
vibrator cycle,
recovery
amplifier
QT
conducts
heavily
to discharge
C4
rapidly
in
preparation
for
the
next
cycle.
RefertoParagraphs
z-Lg
thru2-22 for a
basic
one
- shot
multivibrator
discussion.
2-105.
OSCILLATOR AND
OVEN A24
A25
A26
2-106. GENERAL.
Crystal
oven assemblyA24,
oven
control
assembly
A25,
and
the
first
portion
of
oscil-
lator assembly
A26
provide
an
extremely
stable
1
MHz
signal.
The
secondportion
of
4.26 amplifies
and shapes
either
the
internally
generated
tMHz, an externally
supplied
1MHz, or,
for
scaler
operation,
the output
of the
decimalcounterAlT
or A16.
Refer
tothe
sche-
matic
diagram,
Figure 7-21,
during
the following
explanation.
2-t07.
CRYSTAL
OVEN ASSEMBLY
AZ4.
The crys-
tal oven assembly
is
a
thermally insulated
chamber
which contains
a
heating
element, a
temperature
sensing circuit,
and a
1
MHz
piezo-electric
crystal.
2-108.
O\mN
CONTROL ASSEMBLY
A25.
The
oven
control
assembly includes
oscillator
A2SQl
which
pro-
duces
a
3 kHz
output
whose
amplitude
is
controlled
by
the
temperature-sensing
element
in
the oven. The
oscillator
output
is
amplified
by
A25q2, detected to
produce
a DC
level
whose
amplitude
is inversely
pro-
portional
to oven
temperature.
The DC
level is amp-
Iified
and applied
to the heating
element in
the
oven.
The
oven control
assembly
contains its
own
power
supply
which operates
continuously
whenever
power
is
connected
to
the counter.
2-109.
OSCILLATOR ASSEMBLY
,4,26.
The
oscilla-
tor assembly
includes
the
Q1
oscillator
cireuit
which
is
corurectedtothe
l
MHz
crystal
inA24.
The
1
MHz
oscillator
output is
amplified
by A26Q2,
A26Q3, and
A26Q4.
A
portion
of
the
A26Q4
output
is detected and
applied
to
A26Q1
as
AGC
so that
power
dissipation
in
the crystal
can
be held at
a constant
low
value.
An-
other
portion
of
the A26Q4
output
is
supplied to the
rear
panel
MODE
switch
where
it
usually is
switched
through
to amplifier
A26Q5
and
trigger
A26Q6-A26Q?.
Outputs
of 1
MHz
are
taken
from
both A26Qb
and A26Q6-
A26Q7.
2-22
Model
5245L
2-110.
CIRCUIT
DETAILS.
Refer
to
the schematic
diagram,
Figure
7-2L,
for
circuit
details.
The oscil-
lator
A25Q1 is
controlled
by the Wien
bridge in
A24.
Positive
feedback
from
the
A25Q2
emitter
through
A24R1-424C6
to the
A25Q1
emitter maintains
oscil-
Iation
at
the frequency
of maximum
feedback,
which
occurs
at
that frequency
for
which
the
A24R1-A24Co
phase
shift
equals theA24R2-A23C7
phase
shift(about
3kHz).
Degenerative
feedback is
provided
from
the
425Q2
emitter and
the A24RT1-A24R3-A24R4
divider
to
the
A25Q1
base; an
increase
in
oven temperature
lowers
the resistance of
RT1,
thus
increasing
degen-
erative feedback
and lowering
oscillator output; like-
wise,
a
temperature
decrease
results
in increased
degenerative
feedback
to
produce
increased
oscillator
output. The
detector
is
a
voltage doubler circuit;
425C2
charges
fully
through
the
negative
half cycle,
and
then
discharges
through A25CR2 in
series with
the driving
source during
the
positive
half
cycle.
Capacitor
A25C4
between the collector
and base of
A25Q4
dampens
sudden
DC
voltage
swings and filters
any
AC
component
from
the detected
signal.
The
A26Q1
oscillator
is
a modified Pierce
oscillator; its
base is
maintained
at a DC level from
the A26R1=
A26R2
junction;
AGC
current is
supplied
to
its
emit-
ter from
the A26CR1-A26CR2
detector
(voltage
doubler)
which is referenced
to the
A26R1-A26R2
junction.
Limiter
A26CR5 holds
signal
extremities
to
within
about
0.6
volt of
ground potential (see
Para-
graph
2-8).
Operation
of
the trigger circuit
(A26Q6-
A26Q?)
is
discussed in
detail in Paragraphs
2-18.
z.LLT,
MULTIPLIER
ASSEMBLY
A2?.
2-L12.
The
1 to
10 MHz
multiplier
assembly
consists
of X2
multiplier
Q1,
X5
multiplier
Q2
and
amplifier
Q3.
Circuit
details
are
shown
in
the
schematic dia-
gram,
Figure 7-22.
Divider
R4-R5
and
emitter
resistor
R6
provide
optimum DC
bias
toQl for
efflc-
ient
frequency
doubling; likewise,
divider
RB-R9 and
emitter
resistor
R10
provide
optimum
DC
bias to
Q2
for
efficient fifth-harmonic
generation.
The inter-
stage
coupling
transformers
(L1
-L2,
L3-L4,
and
L5-
L6)
include
capacitive
dividers in
their secondary
circuits
(CB-C9,
C13-C14, and
Cl9-C20)
for
inter-
stage impedance
matching.
2-113.
5-MHz DECADE DMDER ASSEMBLY
A28.
2-LL4.
The
5
MHz
decade
divider
assembly
reduces
the frequency
of
its input
by
a
factor
of
ten
(normally
l
MHzto
100kHz). Basic
operation
is
identical
to that
described
for
the
5
MHz decimal counter
(Paragraphs
2-76
through
2-?9),
except that there is no
displayed
count.
Refer
to
the
schematic diagram, Figure 7-23,
for
circuit
details.
2-IL5.
LOW.FREQUN
NCY
DECADE
DIVIDER
A
2-116.
A
block
diagram
of a
typical decade
divider
is
shown in
Figure
2-L48.
A
decade
divider
is
an
arrangement
of four
cascaded
binaries
(flip-flop)
so
that for
every ten
input
pulses
there
is
one
output
02349-1

HP 5245L Specifications

General

Brand	HP
Model	5245L
Category	Cash Counter
Language	English