Service
In
normal
operation
(OVERLOAD LED
off),
the
voltage
at
C416
is
higher
than
the
reference voltage
at
U401a/pin 2
(duty
cycle
at
the
base
of
0421
is
now lower, thus C416
cannot
sufficiently discharge).
The
output
of
U401a goes
to
maximum
and
cuts
off CR427, allowing C419
to
be charged via
R452
to
the
Zener voltage
of
VR428.
If
the
C419 voltage
is
greater
than
the
reference voltage at U401 b/pin 5,
the
output
at
pin 7
is
switched
to
minimum
which
turns
0426
off.
The
voltage
at
TP8
is
then
adjustable
between
~
8 V and
~
20
V via
the
AMPLITUDE VERNIER R460.
Screen
grid protection
A second
function
of
the
overload
detect
circuit
is
to
provide screen grid
protection.
Should
tubes
V4011V402 be
operated
without
load,
the
entire
cathode
current
would
flow into
the
screen grid (G2)
and cause damage.
To
avoid this,
the
anode
voltage
is
sensed. With no load,
the
anode
voltage
drops
to
approximately
-155
V,
this voltage
drop
being
routed
to
monostable
0432
/
0433
/C432/
R473
/
R478
causing
it
to
change state.
Transistor
0433
conducts
and
the
voltage
drop
across
R475
turns
0434
on
.
The
threshold
of U401a
is
thus
shifted
to
approximately
-133
V,
and
the
comparator
switches
off
the
output
stage
(as already described).
Note:
the
monostable
is
employed
as pulse-stretcher
to
ensure a pulse width
adequate
for
the
' U401 a
response time, and
thus
secure switch-off
of
the
output
stage.
Output Amplifier
The
output
amplifier comprises
two
identical stages working
in
parallel
(anode
s
of
tubes V4011V402 are
connected
together).
For
this
reason,
only
one
stage need be described
in
detail.
The
signal from
the
timing board
is
routed
via
the
input
Schmitt
trigger and push-pull stage
0403
/
0404
to
the
base of
0405.
Transistor
0405
then
functions
in
a
saturated
Schottky
configuration,
whereby
diode
CR404
is
used
to
reduce
the
storage time of
the
saturated
0405.
With
0405
turned
on
,
0407
is
switched
off
and
0406
is
conducting
. A
potential
-
155
V
is
then
applied
to
the
gate
of
FET
0408
, which
cuts
off
0408
and
interrupts
the
current
through
tube
V401.
With
0405
turned
off
,
the
voltage
at
the
gate
of
0408
is
approximately
equal
to
that
applied
to
the
collector
of
0428.
(Note:
this
voltage
depends
on
the
R460
AMPLITUDE
VERNIER
setting).
The
amplifier
output
current
is
then
determined
by
the
difference between
the
gate
and
gate-source
voltages
of
0408
together
with
the
0408
source resi stors lOUT =
VG
- VGS
RS
In
the
30-100
V
amplitude
range K401 shorts R425,
thus
changing
the
output
current
by a
factor
1 :3.
Additionally supply voltages -
155
V and
-133
V are changed
to
-
260
V and -
238
V respectively,
via
relay K501.
Power-on Circuit
The
output
tubes
require
about
30
seconds warm-up time
after
power switch-on. During this
time,
no
current
flows. At power-on,
0431
is
turned
on
and holds
the
0432
base
at
approximately
-155
V.
The
monostable
0402
/
0403
is
then
in
the
unstable state (as when
the
output
amplifier
is
operated
without
load).
Approximately
-155
V
is
then
applied
to
the
gr
ids
at
V4011V402, and
to
the
gates
of
FETs
0408
/
0418,
thus
switching
off
the
amplifier. After 1 minute, capacitor
C430
is
charged via
R467 such
that
0431
turns
off.
The
monostable
0432
/
0433
is
released and
returns
to
the
stable state,
thus
enabling
the
output
amplifier.
8-30
Model
2148
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Scans by Artekmedia => 2009
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