RADIOMETER TCM3 - Page 73

TCM3
and
TCM30
Service
Manual
2.
TCM3
Main PCB, Continued
Heat Control
Audio System
5V Supply
199808-1
The
electrode
heating voltage
is
controlled
by
the
switching
regulator,
the
amplifier
QA3c,
the
switching
elements
Q6,
L1,
CR9,
C34
and
the
D/A
converter
QD23.
The
circuit
constitutes
an
inverting
power
supply
system,
delivering
a
negative
DC.
voltage
to
the
Analog
PCB.
QAI
contains
an
oscillator
(output
at
pin
3),
a
“battery-low”
detector
output
terminal
(8),
a
disable
input
terminal
(6)
and
a
feed-back
input
terminal
(7).
The
oscillator
is
timed
by
C51
to
31
kHz.
The
switching
duty
cycle
and
hence
the
heating
voltage
is
controlled
by
the
feedback
voltage
at
pin
7
via
the
amplifier
QA3c
and
the
D/A
converter
QD23.
With
an
electrode
connected
to
the
monitor,
the
heating
voltage
ranges
from
-11
to
-15
V,
controlled
by
the
temperature
termostatting
system.
Even
without
an
electrode
there
will
be
a
negative
10
V
supply
to
the
Analog
PCB.
The
“killer
circuit”
Q14,
R98,
Q15
discharges
C34
when
the
monitor
is
switched
off,
thus
removing
the
negative
voltage
from
the
Analog
PCB
and
the
electrode.
The
Heat
Control
System
includes
a
so-called
“watchdog”
circuit
to
switch
off the
heat
power
in
the
case
of
a
software
failure.
QD29
(74HC123),
a
mono-stable
.
multi-vibrator,
requires
a
trigger
pulse
every
10.24
ms
to
prevent
its
Q-output
to
switch
the
heat
voltage
low
(-10
V)
and
thus
switch
off
the
heat.
The
watchdog
circuit
timing
signal
is
controlled
by
the
software
which
in
turn
is
activated
by
the
interrupt
signal
RSTA
every
10.24
mS
(QD11
pin
5).
The
audio
signal
from
QD3
(TO
out)
is
attenuated
by
the
analog
switch
QD28
and
R21-R24
according
to
the
control
signal
from
PA2
and
PAS
(QD3).
The
audio
signal
is
amplified
by
the
complementary
driver
Q4/Q5
to
provide
the
audio
transducer
with
sufficient
power.
The
power
supply
is
a
linear
series
regulator
consisting
of
Q8,
Q9,
QA3a
and.
associated
components.
In
addition
to
being
the
supply
for the
Main
PCB
circuits,
the
regulator
constitutes
a
stable
reference
for
the
Analog
PCB
monitor
circuits.
Q13/R99 limits the regulator max. current to 200 mA.
The
regulator
is
started
when
the
input
voltage
exceeds
6
volts,
thus
activating
Q12
and
supplying
Q9
with
base
current.
The
RAM
is
powered
via
Q10.
When
the
monitor
is
switched-off,
Q10
will
be
cut-
off
and
the
RAM
is
supplied
via
terminal
J7/14
directly
from
the
NiCd
battery.
Continued on next page
287