Heathkit DX-40 - FINAL AMPLIFIER STAGE DIAGNOSIS; Testing and Component Checks

3.
If
little
or no
grid
drive
is
available,
there
is
the
possibility
that
the
small
buffer
coils
have
been
interchanged.
A
rough
check
can
be
made
by
noting
the
size
of
the
wire
and
apparent
number
of
turns.
Fine
wire
and
more
turns
are
used
on
the
low
frequency
coils,
with
pro-
gressively
larger
wire
and
lesser
turns
as
10
meters
is
approached.
Check
each
coil
for
a
broken
winding
which
would
check
"open"
on
an
ohmmeter.
Also,
be
sure
the
coil
leads
do
not
short
to
each
other
or
to
the
switch
frame.
4.
WITH
ALL
POWER
REMOVED,
test
the
1.1
mh
RF
choke
which
is
connected
in
the
plate
circuit
for
an
open
winding.
5.
With
the
line
cord
plugged
in,
high
voltage
will
be
indicated
on
all
pins
except
4
and
5
(if
measurements
are
taken
from
the
chassis).
This
is
due
to
the
buffer
stage
being
operated
in
series
with
the
oscillator
stage.
Voltage
readings
on
the
buffer
stage
should
be
taken
from
the
tube
cathode
which
is
tied
to
lug
#1
on
terminal
strip
FF.
CAUTION:
Both
meter
leads
will
therefore
be
"alive".
Plate
voltage
read
from
lug
#1on
terminal
strip
GG
should
be
approximately
350
to
400
volts
(TUNE
position),
depending
on
the
adjustment
of
the
drive
control.
Screen
voltage
should
be
about
230
volts,
also
depending
on
the
adjustment
of
the
drive
control.
Approximately
6.3
volts
AC
should
be
indicated
between
filament
pins
4
and
5
on
socket
D.
FINAL AMPLIFIER
TUBE F 6146
1.
Test
the
6146
tube
G
by
substitution.
2.
Grid
drive
should
be
approximately
3
ma.
for
proper
operation.
3.
WITH
THE
LINE
CORD
DISCONNECTED,
check
the
.425
mh
RF
choke
inthe
final
plate
cir-
cuit
with
an
ohmmeter.
4.
Tighten
the
nuts
on
the
feedthrough
insulators
(top
and
bottom).
5.
Inspect
the
antenna
tuning
condenser
and
the
final
amplifier
condenser
for
shorts,
such
as
solder
drips
from
capacitor
terminals
shorting
to
the
chassis.
6.
Check
the
68
upf
capacitor
between
the
bandswitch
terminal
A8
and
the
grounded
lug
on
the
terminal
strip.
A
swollen
capacitor
will
indicate
that
the
component
has
been
damaged
by
excessive
RF
current
due
to
trying
to
load
an
improper
antenna
impedance.
The
swelling
of
the
capacitor
will
have
decreased
in
its
capacity
and
it
will
require
replacement.
This
capacitor
is
only
used
in
the
circuit
on
the
80-meter
band.
7.
Inspect
the
connections
of
the
final
tank
coil
tothe
bandswitch.
Make
sure
each
is
connected
to
the
proper
terminal
on
the
switch.
GRID
DRIVE
to
the
final
amplifier
stage
is
essential
to
supply
safe
operating
bias
to
the
6146.
The
final
amplifier
should
not
be
on
for
more
than
a
few
seconds
if
no
grid
drive
is
indicated
on
the
panel
meter.
THE
DRIVE
CONTROL
tunes
only
the
upper
and
lower
extremes
of
the
band,
no
(outside)
har-
monics.
KEYING
is
accomplished
by
grounding
the
cathode
of
the
Final
Amplifier
and
the
Oscillator
stage.
Poor
solder
connections
in
this
circuit
can
cause
erratic
keying.
DIPPING
the
final
is
accomplished
by
tuning
the
stage
to
resonance
or
to
minimum
plate
current
indication.
The
antenna
tuning
or
loading
capacitor
should
be
closed
at
"0"
position
before
be-
ginning
to
load
an
antenna.
Dip
the
final
to
read
minimum
plate
current,
open
the
loading
capa-
citor
a
little
at
a
time,
successively
redipping
the
final
until
the
final
"dip"
is
at
the
recommend-
ed
current
value.
If
the
dip
is
too
low
on
the
plate
meter
and
it
is
impossible
to
load
it
with
the
Page 38