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maintenance
BIAS
&
BALANCE ADJUSTMENTS
In
the
HF89,
fixed
bias
Is
employed
In
the output
stages.
Each
of the
two
amplifiers
is
provided with a
BIAS
ADJ.
potentiometer
and a
BALANCE
ADJ.
potentiometer.
It
is
essential that
these controls
be
adjusted
exactly
as
in-
structed before putting
a completed
kit
amplifier
into use,
or
at
any
time
thereafter
when any
of the
output
tubes
are
replaced
or
itis
suspected
thatadc unbalance
in
the
output tubes has occurred
in
the course of
use.
In
the
Final Steps of
the Construction
section,
it is
instruc-
ted that both
BALANCE
ADJ.
pots
R23 end
R24,
and
both
BIAS
ADJ.
pots,
R25 and
R26,
be
set to
mid-rotation
before
connecting
the unit
to
the
AC
line.
Be
sure
this
is
done.
Immediately afterthe
unit
is
turnedon, perform
balance and
bias
ad]ustments
in
each
amplifier
accord-
ing
to
the
methods and schedule given below.
Instrument Required:
VOM
or
VTVM
of
at least
±3%
accuracy on
DC
voltage
measurement
with a lowest
DC
voltage range
not
exceeding 3
volts
full
scale.
a)
BALANCE
ADJUST METHOD.
(Use the
BALANCE
ADJ.
pot
and
METER
jacks
for
the
particular amplifier.
1
or
2,
being
adjusted.
)
Set the
VOM
or
VTVM
to
the
lowest
DC
voltage raige
end
at either
the
plus or
minus
DC
voltage
function.
Connect
the
meter
leads
to
the
pair
of
METER
pin jacks
for
amplifier
1
or amplifier
2 on
the
rear chassis
apron.
If
the
meter
deflects
to
the
left
of zero,
switch the function
to
the opposite
sign or reverse
the
leads.
Adjust the corresponding
BALANCE
ADJ.
control
for
a zero
or
minimum
reading.
This
completes
the bal-
ance
adjustment,
whereupon
the
meter
leads
can be
re-
moved
from the meter
pin jacks.
b)
BIAS
ADJUST METHOD.
(Use the
BIAS
ADJ.
pot
and
a
METERjackof
the
pairforthe particular
amplifier,
1
or
2,
being
adjusted.) Set the
VOM
or
VTVM
at
the
plus
DC
voltage function
and
the lowest
DC
voltage range
(not
exceeding 3
volts
full
scale).
Insert
the "hot"
meter
lead
into either
one
of the pair of
METER
pin jacks
for
the
particular amplifier
(1
or 2)
being
adjusted,
and connect
the
common
or
ground meter
lead
to either
of the speaker
connection
terminals
marked
"G"
(ground).
Set the cor-
responding
BIAS
ADJ.
control
forameter readingof 0.63
volt.
This
completes
the
bias
adjustment,
whereupon
the
meter
leads
can be removed.
BALANCE &
BIAS
ADJUSTMENT
SCHEDULE
TIME
PROCEDURE
Immediately
upon
turning amplifier
on
for
the
first
time.
(Either after
completing
the
kit
or
after
replacing output
tubes.
1.
Adjust
BALANCE
amplifier
1
2.
Adjust
BIAS
amplifier
1
3.
Adjust
BALANCE
amplifier
2
4.
Adjust
BIAS
amplifier
2
Repeat
steps
1,
2,
3,
and
4
15 minutes
later
Repeat
steps
1,
2,
3,
and
4
2
hours
later
Repeat
steps
1,
2,
3,
and
4
2
weeks
later
Repeat
steps
1,
2,
3,
and 4
TROUBLE-SHOOTING PROCEDURES
Your
amplifier
should
require
little
service
except
for
normal tube replacement.
We
recommend
no
substitutions
for
the tube types used
in this
amplifier.
The EL34
type
is
distributed
nationally
by
the
Amperex
Electronic
Corp-
oration (230
Duffy Ave.
,
Hicksville,
L. I.,
N.Y.
)
and
Mu
Hard
Ltd.
(International Electronics
Corp.,
81
Spring
St.,
N.Y.
12,
N.Y.)
If
necessary,
replacements
at
any
time
can be obtained
directly
from
EICO.
To
facilitate servicing,
remedial
and
trouble-shooting
procedures
have been
provided
in
the
TROUBLE-SHOOT-
ING CHART
that follows:
A
VOLTAGE
AND
RESIST-
ANCE
CHART
is
also
provided
as
an
aid
in
locating
de-
fective
components and
to
permit
a
careful,
stage-by-
stage
check
of the amplifier.
DC
operating voltages
are
given both
at
no
signal
and ata
signal
developing 50watts
output per channel
as
well
as
the corresponding lkc
signal
voltages.
To
isolate
the source
ofunusual
humor
noise
in
your
sys-
tem,
first
turnoff
theACpowerand
thenunplugthe
audio
cable connecting
to
the amplifier
input.
Then
turn
the
AC
power
on again
and
note
whether
hum
or noise has
de-
creased.
If
it
has,
the
fault
is
in
the preamplifier or
associated
equipment and
measures should be taken
to
correct
it
as
described
in
the service notes
for
these
units.
4
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