Heathkit DX-40 - FINAL AMPLIFIER STAGE ISSUES; Testing and Troubleshooting the 6146

an
open
circuit.
This
is
the
power
transformer
center
tap.
The
white
cable
wire
at
KK5
should
connect
to
terminal
4
ofthe
operating
switch
and
through
terminal
5 to
ground
but
should not
be
grounded
in
the
STANDBY
position.
Check
this
wiring
to
be
sure
it
is
correct.
CAUTION:
The
following
tests
are
made
with power on;
use
extreme
care.
With
a
low
impedance
load
(near
52
ohms)
connected,
the
operation
switch
in
the
PHONE
position,
the
meter
switch
in
PLATE
position,
depress the
key
momentarily.
It
should
read about
1/2
scale,
when
not
tuned
to
reso-
nance.
If
the
meter
shows
little
or
no
indication,
check
the
screen
voltage
at
pin
3
of
the
6146
tube
with
a
(VOM)
Volt-Ohm-Milliammeter
using
a
DC
range
of
at
least
150
volts.
The
screen
should
read
about
100
volts.
If
much
less
than
this,
trace
back
through
contact
3
of
the
oper-
ation
switch,
through
contact
2
to
pin
9
of
the
6DE7
tube;
all shouldread
near
100
volts.
Switch
the
meter
to
a
higher
range,
1500
volts,
and
check
pin
1
of
the
6DE7.
This
should
read
about
580
volts.
If
the
voltages
are
missing or
very
far
from
normal,
using
the
voltmeter
on
the
1500
volt
scale
withthe
schematic
diagram,
trace
the
voltage
back
to
its
source
atthe
rectifier
pin
1.
No
voltage
at
E1
indicates
an
open
power
transformer
primary,
open
power
transformer
center
tap
outlined
in
the
steps
above,
defective
5U4GB
rectifier
tube
or
a
short
circuit.
The
latter
would
probably
have
made
itself
known
by
now.
Lack of grid drive may be traced back to the oscillator. Turn to the TUNE position, make sure
the key circuit is closed, and listen witha receiver for the oscillator. Try different crystals. If
no signal is heard, check the plate voltage of the oscillator at pin 6 and screen voltage at pin 8.
There
should
be
about
250
volts
on
the
plate
and
about
150
on
the
screen
in
the
TUNE
position.
es
If
low
grid
drive
is
experienced
oneither
the
15-meter
or
the
20-meter
band
positions,
discon-
nect
the
80-meter
(black
dot)
coil
from
the
band
switch,
turn
it
end
for
end,
reform
the
leads,
and solder back in place.
As
the
two
6CL6
tubes
are
in
series,
no
voltages
on
the
oscillator
may
be
due
toan
open
circuit
or
defective
6CL6
in
the
buffer
stage.
Consequently,
both
stages
must
be
operating
properly
to
obtain
grid
drive.
The
plate
voltage
of
the
buffer
stage
will
be
about
580
volts
to
ground
and
its
cathode
should
be
about
250
volts
to
ground
(in
the
TUNE
position).
If
the
voltages
differ
greatly,
remove
the
power
and
completely
check
the
circuit.
In
certain
rare
cases,
keying
is
completely
normal
in
the
TUNE
position
but
erratic
keying,
or
no
keying
at
all,
occurs
in
the
CW
position.
This
condition
is
generally
corrected
by
carefully
removing
exactly
23
turns
of
wire
from
the
40-meter
slug
tuned
coil,
Part
#40-104.
Begin
at
the
red
dot
terminal,
which
is
the
outside
of
the
winding.
This
changes
this
coil
to
a
20-meter
coil.
Use
the
same
procedure
as
prescribed
on
page
31,
tuning
the
slug
for
the
first
peak
in
grid
current.
Sufficient
drive
should
still
be
obtained
on
all
bands
using
proper
crystals
or
VFO.
FINAL
AMPLIFIER
STAGE:
The
6146
final
tube
can
be
damaged
in
a
few
seconds
if
the
final
is
not
tuned
to
resonance.
If
you
wish
to
test
other
stages
with
the
plate
current
on,
switch
to
the
TUNE
position.
This
will
enable
you
tooperate
the
transmitter
up
tothe
final
amplifier,
includ-
ing
the
indication
of
grid
current
on
the
panel
meter
without
any
RF
output.
The
TUNE
position
removes
the
plate
and
screen
voltage
from
the
6146
and
grounds
the
screen
grid,
however,
use
caution
as
the
high
voltage
is
still
present
in
other
sections
of
the
transmitter.
Failure
to
ob-
tain
a
dip
in
plate
current
when
tuning
the
final
amplifier
may
be
due
to
a
lack
of
grid
drive,
in-
correctly
wired
final
tank
coil,
loading
capacitor
advanced
too
far
(insufficient
capacity),
shorted
coax
line,
the
crystal
switch
in
the
wrong
position,
or
with
an
improper
crystal
or
one
out
of
the
tuning
range
of
the
buffer
stage.
A
low
impedance
antenna
system
on
the
80-meter
band
will
also
give
the
same
effect
and
may
even
result
inthe
meter
being
"pinned"
at
the
full
scale
reading
until
sufficient
capacity
has
been
added
in
parallel
to
the
2-gang
output
loading
capacitor.
See
Final
Amplifier
section.
NOTE:
When
using
the
DX-40
on
CW
with
the
VF-1,
the
VF-1operation
switch
should
be
placed
in
the
STANDBY
position,
and
the
key
plugged
into
the
DX-40.
For
best
results
with
either
crystal
or
VFO,
the
grid
drive
should
be
adjusted
as
near
as
possible
to
3
milliamperes.
In
some
cases
and
on
certain
bands,
slightly
more
than
the
indicated
3
milliamperes
will
result
in
added
efficiency.
Page 36