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4-19.
ONE NOTE
IS
WEAK. -
4-21.
FULL
VOLUME
PEDAL
CIPHER.
-
4-20.
a.
Trace
the note
as
described
in
the
pre-
ceding
section to see
whether weakness
is
due
to
dirty
contact,
poor
connection,
defective
filter
or
reduced output of
magnet
coil.
Check
at
each
point
by
comparing
intensities
with
higher
and
lower frequency numbers.
b.
It
is
possible that
one
or
more
notes
may
be
acoustically
weak, due
to
the
room
and
the furnishings,
although the
actual
signal
level
is
equal
to that
of
adjacent
notes.
This can be
checked
by
reading
voltages
of
the various notes
on
an output meter
connected
to the
voice
coil
terminals
on
the amphfier.
All
notes
will
not
give
equal
output, but voltage
should vary
smoothly
from
note
to note. In
this test
variations
of
less
than
30%
should be
ignored.
c.
Each magnet
is
set at
the factory
by
tapping
it
gently,
with
the
set
screw
partially
loosened, while observing
an
output
meter.
Experience has
shown
that the
magnets seldom need adjustment
and
that setting
them
without proper
equipment
involves
danger of
damaging
both magnet and
wheel.
Therefore
it is
not
recommended
that the service
man
attempt
this
adjustment.
PERCUSSION
NOTES WEAK, DO
NOT
PLAY,
OR DO NOT DECAY
PROPERLY.
-
a.
Adjust percussion cutoff
control
with
expression pedal
wide
open,
VOLUME
SOFT
tab up,
upper
DRAWBARS
tab
down, and
drawbars pushed
in,
THIRD
HARMONIC
tab
down, and
FAST
DECAY
tab
down. Hold
down
first
C
key
on
upper manual and
adjust percus-
sion
cutoff control to the exact point
of cutoff
or
silence.
b.
Check
tube
VII.
c.
Always
adjust
percussion cutoff control
after
replacing
VI
1
(
1
2AU7).
a.
It
is
possible to
make
a
black pedal lock
down
and
play continuously
by
striking
it
very hard
at
an
angle so that
it
moves
sidewise
as
well
as
downward.
This
will
never occur
if
the pedals
are
used
pro-
perly, as
only
slight
pressure
with
the
toe
is
required
for playing.
b.
A
pedal locked
down
in this
way
will
remain
visibly
depressed
and
tilted.
It
can be
released
by
pressing
it
down
very
hard,
with
a
slight
pressure
opposite to
the direction
of
tilt.
No
permanent
damage
will result.
4-22.
HUM.
-
a.
A
loud
60
cycle or
1
20
cycle
hum
in
the
speaker
may
come
from some
nearby
electrical
appliance,
as
explained
on
the
instruction card
which accompanied
the
instrument.
It
may
be picked
up by
the
matching
transformer or the console
wiring.
Hum
from
this
source
will
disap-
pear
if
you
remove
the
BK
plug
from
the
AO-41
chassis.
It
may
be
eliminated
by
moving
either
the console or the
appliance.
b.
Any
other
hum
must
originate in
the
amplifier
circuit,
and
can
generally
be
cured
by
replacing
one
or
more
of
the
electrolytic
condensers.
c.
In case
hum
originates
in
either amplifier
but
is
not due
to the electrolytic con-
densers,
its
source can
be
isolated
by
successively
removing
tubes or
by
ground-
ing successive points
in
the
signal
circuit.
4-23.
REPLACING
TUBES.
-
a.
The vacuum
tubes
are
all
standard radio
types
and
can be
tested
in
the usual
way.
Figures
5-15
through
5-18
show
the
location
of tubes
in
the
amplifier.
b.
If
tube
VI
1
is
replaced,
check
percussion
cutoff
adjustment
as
explained
in
para-
graph
4—
4h.
4-6
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