Heathkit DX-40 - SPEECH AMPLIFIER AND MODULATOR STAGES; Tube and Component Testing; Meter Circuit Analysis

Antenna
Tuning
Condenser,
the
antenna
impedance
is
not
proper
and
should
be
checked.
This
is
also
true
if
it
will
not
dip
at
all.
The
easiest
test
for
this
condition
is
to
substitute
a
dummy
load,
(a
115
volt,
60-watt
bulb)
for
the
antenna.
NOTE:
Feeding
a
very
low
impedance
antenna,
especially
at
the
low
frequency
end
of
the
80-
meter
band
may
require
additional
capacity,
such
as
a
.001
pf
(large
size
mica)
connected
a-
cross
the
coax
output
connector
to
allow
a
proper
"dip"
in
plate
current.
This
may
be
done
with
a
coax
"T"
connector
if
required.
CAUTION:
Remove
this
capacitor
before
attempting
opera-
tion
on
any
other
band
as
it
will
make
it
impossible
to
"load"
the
antenna.
CREEPING
PLATE
CURRENT
will
usually
indicate
a
"soft"
6146
final
amplifier
tube.
Loss
of
grid
drive
usually
accompanies
this
condition.
This
is
usually
caused
by
consistent
operation
of
the
6146
tube
in
an
overloaded
condition.
Plate
voltage
reading
will
be
the
total
B+
voltage
which:can
be
read
at
Pin
#1
on
the
5U4GB
rectifier
socket
E
and
should
be
near
600
volts.
Once
again,
use
caution
in
testing
high
voltages.
The
screen
voltage
(with
no
modulation)
should
be
about
100
volts.
SPEECH
AMPLIFIER
AND
MODULATOR
STAGES
TUBE
A
--
12AX7
Dual
Triode
TUBE
B
--
6DE7
Dual
Dissimilar
Triode
1.
‘Test
the
12AX7
tube
A,
and
the
6DE7
tube
B,
by
substitution.
2.
With
the
line
cord
disconnected,
test
the
resistance
between
the
ungrounded
end
of
the.1
yf
paper
capacitor
(ug
#10n
terminal
strip
BB)
and
the
chassis.
It
should
read
approximately
500
KQ
on
the
OFF
and
PHONE
positions.
The
other
switch
positions
should
show
no
read-
ings.
3.
Inspect
all
pin
connections
on
the
tube
sockets
for
poor
solder
connections.
4.
Recheck
the
step-by-step
wiring
and
be
sure
that
components
and
leads
are
connected
to
the
correct
numbered
pin
connections
on
the
sockets.
5.
With
a
crystal
plugged
in,
the
power
on
and
the
transmitter
tuned
for
CW
conditions,
switch
to
PHONE
position.
Measure
the
voltage
from
lug
#1
on
terminal
strip
BB
to
the
chassis.
It
should
read
approximately
230
volts,
as
read
on
a
Heathkit
VIVM.
The
plate
voltage
on
the
first
triode
section
of
the
12AX7,
read
from
pin
#6
to
the
chassis
with
the
VTVM,
should
be
about
70
volts
on
the
150
volt
scale,
and
the
grid
voltage
near
-.6
voltson
the
1.5
volt
scale,
read
from
pin
#7
to
the
chassis.
The
second
triode
plate,
from
pin
#1
to
chassis,
should
read
about
90
volts.
The
firsttriode
cathode
is
grounded.
Voltage
reading
on
the
12AX7
second
triode
cathode
should
be
about
.9
volts,
measured
across
the
2.7
KQ
resistor,
from
pin
#8
to
the
chassis.
6.
Plate
voltageon
the
7
watt
triode
section
of
the
6DE7
should
be
approximately
580
voits.
This
is
read
from
pin
#1
of
the
6DE7
to
the
chassis.
This
same
voltage
read
from
plate
to
cathode
(pins
#1
to
#9)
is
about
460
volts.
The
plate
voltage
of
the
1.5
watt
driver
triode
is
near
20
volts
(with
no
modulation),
read
on
the
50
volt
scale
of
the
VIVM,
from
pin
#6
to
chassis.
The
first
triode
cathode
is
grounded.
Screen
voltage
for
the
6146
final
amplifier
is
obtained
(through
the
10
KQ
resistor)
from
the
heavy
duty
triode
section
cathode,
which
is
pin
#9.
The
voltage
at
pin
#9
to
chassis
is
approximately
115
volts
and
at
the
6146
screen
grid
pin
#3
is
near
100
volts
with
no
modulation.
The
grid
voltage
read
from
pin
#7
to
chassis,
with
the
VITVM,
should
be
near
-.9
volts.
METER
CIRCUIT:
A
1002
one
milliampere
D'Arsonval
movement
meter
is
switched
to
read
plate
or
grid
current
to
the
6146
final
amplifier
tube.
To
avoid
extreme
excursions
of
the
meter
pointer
while
operation
on
CW,
it
may
be
advisable
toleave
it
switched
to
the
lower
reading
grid
position.
The
meter
shield
can
serves
to
mount
the
meter
as
well
as
shielding
any
RF
radiation
through
the
meter
hole
which
could
cause
TVI
Page 39