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SYMPTOM
Excessive
phono
hum
Excessive
phono
noise
Sustained
oscillations
CAUSE
12AX7
(VI) defective
FIL
leads dressed too close
to
grid lead,
C27
defective
Tube
shield
not
making
electrical
contact
to
base
or
base not
making
electrical
contact
to
chassis.
Outer
braid
connection on
phono
1
or
phono
2
Input facks accidentally
shorting to chassis at
the |ack.
Outer
braid of shielded lead
going from
phono
1
and
2 Jacks
to
switch not
grounded
at switch as
required.
Shielded lead contacting
chassis
ground
at
other)
than required
point.
VI
socket
and
contacts
dirty.
Poor
dress
of output
transformer
T2
leads.
Sustained
phono
microphonics
I
12AX7
(VI) defective
Hum
on
all
Inputi
12AX7
(V2) defective, not properly shielded,
or
dirty
socket
and
contacts.
Dress of
power
transformer T1 leads.
REMEDY
Replace
Dress
fll.
leads
away
from
grid
lead.)
replace
C27.
Check
and
correct
clear
short
at Input Jack
repair
repair
clean thoroughly with carbon
tetra-
chloride.
Dress
all
InpotJeads
and
12
leads
a-
way
from
each
other.
Keep
T2
leads
away
from Input
lacks.
replace
12AX7
(VI) with selected
new 12AX7
or
with
premium 12AD7.
see
below
correct
!f
*
troubl
»
I,
no
output
"J™
check ac
signal
voltages
starting
at the Input
and working
step-by-step toward
l
h
?rSf/
PU
'i
LEVEL
*
LOUDNESS
controls
to
maxtmum(10)and BASS and
TREBLE
controls
to their
mid-points
(0).
Use
a
l!
9
™
1
'
"f
a
l
w,
PP
l,ed
hy
E.CO
377
Sine
&
Square
wave
Audio
Generator,
^addition,
use
out of
0 4
viS
whtaK
P
Tr
.L
0b
,,
*i
a
leV
"'
°-
004volt
M
to
PHONO
Input
1
from
an
audio generator out-
frHe°nerator1
'
Lhl
J
h!"hT"
J
"
ea,ur
^
lowe
*'
AC
volts
range
of
your
VTVM
(also
Improves
signal to
hum
221
or
214^
S*
n
P
Uf
.
?'
P
®^
,nce
"VM
for
0,1
AC
*
,
9
nal
voltage
measurements
(such as the
EICO
232
,
249,
221
,
or 214).
This
procedure should
suffice
to
localize the defective stage.
°
n
"T'Z'j f
St
?
rted
OUtput
'
to*
*
b ®
replacement,
signal tracing, or
procede
directly to
voltage
and
any
circuit
cTangeT*"
1 *'
?
de
'
fred/
prem,um
5881
»«
b
«
ma
y be
u*ed
as direct
replacements
for
the
6L6
GB
tubes without
When
the defective stage
1,
localized,
procede
to
a
resistance
and
voltage
check
of the stage, using the data
In
the re-
?«^n
e
c
ret)SaSorto?'
te
^
Disconnect the
amplifier from the
power
line
and
discharge
capacitors
prior to
making
any
o,
bo*
of
6L6
GB
ootpo,
K,b«
V3
and V4.
Do
nor
him
the
o»p.mo, J,
TROUBLE
-
SHOOTING
TYPICAL TUBE
STAGE
1
•
Check
tube
2.
Check
plate
and
cathode
resistors.
3.
Check
coupling
capacitors
for
leakage
or
short.
c
0U
*P
U
*
|
t
Q 9 G
*
check dc
resistance
of
transformer
windings.
5.
Check
grid leak
resistor
for
open.
6.
Check
cathode by—
pass
capacitors
for short.
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