Eico HF-20 - Page 20

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
*
|tQ
9
G
*
check
dc
resistance
of
transformer
windings.
5.
Check
grid
leak
resistor
for
open.
6.
Check
cathode
by—
pass
capacitors
for
short.