Keithley 237 Service Manual

SECTION 4
Principles of
Operation
allel output
combination shift
register/drivers.
They in-
terface directly to the
I/O controller
VIA ciock
data inter
face. A strobe is provided
by software to
bad
the outputs
of
the
drivers with
data.
The
displays
are indexed at a
100Hz
(lms) rate.
Circuit components
Ui and
U2
provide
a
timer ftmction
that allow
the selection of either
a
bright or dim
digit and
is used for
flashing cursor digits. A counter
(U2)
times
out after 256isec and bit 16
of the
two
serial bank
source
drivers
is
fed back into Ui pin 12
and
Ui
pin
6.
Depend
ing on the
states
of these bits, the LEDs
wffl
be
on
for
the
entire
imsec duration or 1 /4 (256jisec)
duration
The col
umn
drivers
are
selected
by a
one-of-ten
decoder
(U3)
which is directly
driven from the
VIA of the I/O
control
1er.
These columns,
as they are selected,
also select col
umns in the front panel
key switch
matrix through di-
odes
CR1 through CR 10.
The column drivenlow,
and the
four limes fed back
to
the VIA of the
I/O controller
iden
tify which
key is pressed.
45 POWERSUPPLY
The
schematic diagram for the
power
supply is located
on page 4 of drawing
236-106.
The power supply
consists of
an
offline transformer (T1)
which has
selectable 115/230V operation. lt is essentially
two
115 volts
primaries either in series or parallel. In par
allel
with one of the primary
windings is the cooling
fan
allowing it
to
always
operate
on 115V. The line select
switch
(S2)
allows user
selectable line
voltages.
Switch S1
is the main
power switch, F1 is the system fuse,
andJ2
isa
rear panel
mounted line filter connector. The secondary
of the transformer
provides all of the power supply volt-
ages.
The
digital power supply
(gray transformer secondaries
on
schematic)
is capable
of delivering
+5V at
1,2A
to
power alldigital and
front panelcircuit components. Full
wave
rectification is provided by
the bridge rectifier CR1.
Capacitors
C106 and
C107 provide
spike
suppression,
while C4
provides ifitering.
Voltage regulation is pro-
vided by
VR1 with capacitor
C24 on its output
for
stabil-
ity.
The ±15V
and
±30V
analog
supplies are derived from the
orange
colored secondaries.
The±l5Vpower
supply
con
sists
of
a
bridge rectifier
CR4, ifiter capacitors C53 and
C55,
±15V regulators VR3
and VR4, and stability capaci
tors
C91 and
C92.
A
voltage doubler
circuit consisting
of
diodes CR2,
CR3, CR6 and
CR7, and storage capacitors
C52,
C54,
C56
and C57
combine with
adjustable
regula
tors VR5 and VR6, and
resistors
R6, R7,
R8
and R9
to pro-
vide the ±30V
outputs.
The +5V analog supply (brown
secondaries)
consists of
the
bridge rectifier
CR5,
filter
capacitor C7,
regulator
VR2, and output
capacitors C93
and
C15.
This
power
supply provides
+5V
for
analog functions.
The ±150V
supply (yellow
secondaries)
is provided for
the bootstrap
amplifier. A bridge
rectifier is configured
from four diodes; CR8, CR9,
CR10 and CR11. These di-
odes rectify
and use capacitors C58
and C59 as storage
elements.
A ±15V analog
supply is derived from a shunt
regulator
circuit consisting of
resistors R73 and R74, and
regulators
VR7 and VR8 in
parallel with capacitors
C95
and
C96.
The
+50V
supply
(blue
secondaries)
is used to
drive the
high voltage converter
of the Model 236. The
Model 237
uses
this
50V
supply
to
derive
the ±1200V output. Bridge
rectifier
CR12 and CR27 provide
full
wave rectification,
while C110 and R80 suppress
diode spikes.
Thyristor Q7,
zener diode VR9, and resistor R90
provide an SRC clamp
ing circuit
such to
guarantee that
fuse F1 will clear when
the instrument
is
set
for 115V operation while connected
to
the 230V
line. Minimal bad is guaranteed
by
resistor
R69 and filter function is
provide
by
capacitor C10.
4-11

Keithley 237 Specifications

General

Brand	Keithley
Model	237
Category	Measuring Instruments
Language	English