Gift to

ed to try a hotter tra nsistor
(suc h as the 2N3866), but if
you do you m ay have osci
lla tion p r o b l e m s. At
5/$1.00 from Jam es, t he
2N22 19 does just fi ne!
The output transistor and
co il we re isolated in a sma ll
hal f-box of br ass to provide
shie ld ing. A small ho le in
one wa ll passes the ba se
lead wh ich wa s insu lated
w ith a small pi ece of spag
he t t i st r i p p e d fr om
ho oku p w ire . Th e b ase
resistor is gro unded to the
outside wa ll, and a shor t
w ire connects to the rf ou t­
pu t of t he O X oscill ato r.
Since the fi nal has f ar more
o utput tha n need ed (it
d raw s 200 mW, as shown],
the output is capacitively
coupled to a 47-0hm load
resisto r. The top of th e
resistor is connected to the
driver output via a va riabl e
capacito r w hich serves as a
drive level control.
Pho to D. Mike WBB/XF, on e o f the original " d ynamic duo" in Lansing A TV. The pa th is
about B mil es, and he was loafing his amplifier along at about 100 Watts peak ou tp ut
when the p ic tu re was taken.
The modul ato r is a circu
it designed by WB8JXF.
Th e only modi ficati on invo
lves an adap tation for
use w ith ac-coupled cameras-
fa r mo re co m mo n
than the more expensive deco
u p le d c a m e ras with
w hich the original ci rcuit
wa s used . Modulator lay out
is non-crit ical. The shielded
enclosure and feed through
capacito rs show n in Photo
B we re a holdover from
earlier use of the mod ulator
with various 432 transm itter
strips, and such elabo rate
packaging is not requ ired
here.
Tune-up is qu ite simple,
bu t a few regulatory cautio
ns are in order. The
essential point is that we
a re no t a uthor ized t o
broadcast TV signals on
channel 2 or 3, even if the
channel is not in use locally
. The litt le drive r do esn't
put out muc h rf co mpared
to the transmitter strip, but
if you connec t it to a w ire or
ante nna, it wi ll rad iat e and
that co uld be an invitat ion
to trouble. If the uni t is bu ilt
in a shielded box and coax
is used to make the interco
nnections, you w ill be
hard pressed to pick up the
V H F signal o n the TV in the
shack and yo u w ill not be
rad iat ing the signal.
A grid-dip met er makes
for easy tun e-up , although
it is not essential. If one is
ava ilabl e, switc h th e dipper
to the w avem et er mod e
(assumi ng the proper coil is
in place) and co uple the
co il to th e OX osci llator
co il. T un ing around the
proper fr equ ency should
show an rf peak . Tune to
the pea k and you w ill be on
frequency regardl ess of the
usually poor calibrat ion of
most d ippers (the o ne I used
turned o ut to be off by 10
M Hz) . Now sw itch to the
d ip mode and co up le to the
f inal output co il. With the
lead from th e modulato r
disconnected , tun e the coil
for a dip. At this point you
can con nec t the modulator
lead and you sho uld be o n
frequen cy . If yo u sw it ch the
dippe r to the wavem et e r
mode, yo u should see a
very substantial rf peak
from th e fi nal.
If no d ipper is av ail ab le,
you ca n proceed as follows.
Tempor aril y remo ve the
co nnection between the
fixed output capac itor and
the 47-0hm load resist or.
Sold er th e free le ad of th e
capacitor to the base of a
#49 lamp and gro und th e
shell. D isconne ct the modulator
lead and connec t the
1DO-Ohm resisto r to 12 V dc
and tune the output coil for
maximu m brilliance on the
l am p . Re co nn ec t t he
capa cito r to t he lo ad
resistor and connect the
1DO-O hm resistor back to
the modul at or output.
Use a piece of coax to
connect the output of the
dr iver to the input of your
TV and tune the latt er to
the i-f channel. Set the drive
ca pac ito r t o minimum
(p lates co m p l e t ely unmeshed)
and turn the driver
on . The TV screen should
go blan k w ith a st ro ng
signal. Connect a properlyadjusted
camera to the
video inpu t and run the
came ra inpu t pot throu gh
its range. At one extrem e,
w hite areas w ill begin to
smear, eve ntually spreading
to cover and obscure
any v ideo d isplay. This is
call ed " w hit ing o ut ." At the
other end of the range, the
contrast w ill increase and
yo u wi ll begin to lose sync.
The prope r sett i n g is
A1- l 00 Ohm s
A2-1000 Ohms
R3-47 Ohms
A4-100 Ohms
A5- 100 Ohms
R6-10k li nea r-t ap er. panelmounti
ng pot
R7- 47 Ohms, 2 Wa tts
R8- 10 Ohms, 2 Watts
A9- 560 Ohm s
Al0- l 00 Ohm s
All - 2200 Ohms
C1- .001-uF ceramic disc
C2-5·30-pF variable (DAIVE)­
value not critical
C3- .001-uF ceramic disc
C4-100-uF , 16.v alum inum
electrolyti c
D1- 9·V, 1-Watt zener
0 1-2N2219
0 2- 1306 (CB dri ver)
0 3-1307 (CB final)
L1-15 turns of #28 ena mel on a
If. inch slug-tuned form
Table 1. Component values
fo r Fig. 1. All resistors are
1/4 Watt unless o therwise
no ted.
73 Magazine . Octobe r,1980 63