It is suggested
that the
drive be taken
from the
feedthrough insulator
(point
X
in the
DX-100 or
DX-100B
Manual) through a length
of RG-62/U
coaxial cable to a connector mounted on the
rear
apron ofthe chassis
in
the high voltage
power
supply compartment. Drill a hole inthe interven-
ing
partition
to allow
passage
of the cable.
Discoffiect
the lead from the tinal amplifier
grid
circuit side of
the
feedthrough insulator
to the terminal
strip,
thus breaking the RF
path
at this
point.
Mount
a
second coax connector
on the
rear
apron of the transmitter
directly behind the
rearmost 6146 tube
socket. Connect
the
centgr
pin
of this
connector
directly to the
grid
buss
with
#16 bare wire.
This modi.fication
will
require a
jumper
of RG-62/U
coax between
these
added connectors
for AM and
CW
operation
in order to
restore the.normal
RF
path'
RG-62/U
cable is recommended
in this
application
to minimize
loss of
drive at the
higher frequencies'
The switch can be
mounted on
the front
panel
in the area beneath
the
"Heathkit"
nameplate'
The
VR tubes
and bias control
car be
mounted
on a
small
aluminum
bracket
which can
be
secured
under one of the
power
translormer
mounting
screw nuts near the
LV filter choke'
After
re-
placing
the
2.2 ia L
watt
bias
string
resistor
(between
terminal
strip
DD-l
and
EE- 1)
with a
3 t<O controt, reconnecting
the
end
of the
2.2
KQ
L/2 watt
resistor
(connected
to
DD-1)
tothe
control center
lug, and
running
an extension
of the
ground
end oJ the bias
string
(previously
connected to
DD-2) to the
switch,
the wiring
is straj.ghtforward'
If the
DX-100
or
DX-1008,
intended
lor
use as
a companion
unit with the
sB-10, has
been
pre-
viously
modified to
employ
grid
block
keying
of
the
buffer
(12BY?)
stage, the fact that
the
vFo
is on it all times
may cause
objectionable
interference
during
standby
periods.
If SSB and
AM
are
the
modes of operation
most frequently
employed, a
return to tlre original cathode
keying
cj.rcuit is
recommended. To
retain the
grid
block
method of
keying
but
render the VFO
inop-
erative during SSB
standby
periods,
a
suggested
solution
j.s
shown in dotted lines on the
sche-
matic.
By
using a
3PDT switch
of the
rotary
or level type
in
place
of
the
DPDT
mentioned
previously,
the extra
switch
section canbe
employed
to
loute
the VFO
and buffer cathode
return
to
pin
1 of the accessory
socketdirectly
lor
external
grounding during transmitting
periods
only'
thus bypassing the
plate
switch,
which
is on
continuously
during SSB operation.
A
variation
of
this
solution
to the
problem
would
be to
accomplish
switching of the
plate
transformer
primary
power
externally by
placing
an external
SPST
switch across
pins
3 and
4 of the accessory
socket.
Then,
leaving the
plate
switch on
the
front
panel
in
the
OFF
position,
the cathode
return
lead
appearing on
pin
1 of
the accessory
socket can
be
grounded
externally during transmitting
periods.
The external
plate
power
switch
can be
mounted
at any convenient
location on the
operating
desk.
To
accomplish external
grounding of the
cathode
feturn
lead,
Iefel
to
"Operation"
on
page
56.
once the modiJication
is complete,
check
your
wiring once again.
connect
a temporary
iumper
between
pin
5 and
pin
8
of the
accessory
socket.
Now with the SB-10
disconnected
from the
transmitter, turn
the
power
on.
PIace the
"CW-Phone"
switch ln
"CW"
position
and turn
the
drive
control
fuII
CCW.
After
warmup,
with
the switch in Class
AB1
position
and
no
jumper
between the adapter coaxial
connections,
apply
plate
voltage
watching
the
plate
current
closely'
Quickly
adjust the fixed bias
control
until
55 ma
of
plate
current is
indicated.
IJ the
plate
cur-
rent is excessive or cannot
be
brought
to this
value, turn ofl the
plate
power
and
check
your
work once again.
Once 55 ma
of
plate
current
is indicated, check
to make
sure the
two
OBZ
voltage
regulators are
showing a
soft blue
glow.
If
a VOM
or VTVM
is available,
the
grid buss
shout-d show approximately
-50volts
DCandthescreens
ofthe6146's(pin
3) shouldshow
210VDC'
Now turn the
plate power
off,
place
the
switch
in Class C
position,
and connect the
short
coaxial
jumper
between the adapter
coax
fittings
on the back
ofthe transmitter.
Using a
dummy
load,
check for
normal AM or CW operation.
A
slight
decrease in available
grid
drive can
be attri-
buted to the length of coax
added
to the
RF
signal
path.
Once
the
modification iscomplete
and the
above
tests are
made, connect
the SB-10to
the trans-
mitter as
described
previously. If required,
as outlined above,
provide
a
ground-when-trans-
mitting connection for the VFO
and buffer
cathode
return. Now
proceed
with testing
and
adjust-
inent of the SB- 10 Adapter.
Page
52
To Next Page
Loading...