CIRCUITS
AB
+ 12 VDC
5900 100
BC548
.001 F
220
BC548
100 pf
3.579
MHz
XTAL
27K
1K
100 pf
1K
ANTENNA
470 47 pf
CD
Fig. A: Opened TV with 2 circuit boards. Thick, red wire
drives the tube from a high-voltage power supply, lower
right. Fig. B: Circuit boards snapped out of the opened
television without any unscrewing.
Fig. C: Colpitts oscillator circuit uses TV components to
generate a sine wave you use as a broadcast carrier.
Fig. D: The flyback transformer, which pulls the tube’s
beam along its scan pattern, has lots of winding wire.
voltage wire connected the power supply to the
tube’s anode cap. Again, be careful here. Cracking
the case and liberating the PC boards required
only a Phillips screwdriver. The boards themselves
were “snap fit” into place without screws.
The PC boards yielded far more parts than I
expected. There were only 3 proprietary integrated
circuits; the rest of the circuitry was all discrete,
standard components. At the time of this set’s
design, surface-mount technology had not yet
kicked into high gear, so all the parts are easily
usable for hobby projects, except fora few large
SMT resistors.
I found 18 BC548 (2N2222) and BC558
(2N3906) transistors; both are commonly used in
simple oscillators and transmitters. I also found
15 diodes and half a dozen semiconductor components of other types, plus an array of ¼-watt
resistors, and ceramic disk and electrolytic
capacitors. The RF chokes and slug-tuned inductors showed promise for future experiments, as
did the infrared sensor for the remote control.
The power supply circuit included a mains-to-12V
transformer and a pair of heavy-duty voltage
regulators, and the flyback transformer on the
back of the picture tube had lots of wire the perfect size for winding RF coils.
There was one great prize for any radio hobbyist, a “color burst” crystal with a frequency of
3.579MHz, which lies in the CW (continuous wave,
used for Morse code) portion of the 80-meter ham
band. There were also 4MHz and 8MHz crystals
that have potential for other projects.
Using just the parts from this TV, there are many
designs for radio transmitters you could build. One
example is based on a Colpitts oscillator, which
uses two BC548 (2N2222) transistors, a color
burst crystal, and some resistors and capacitors.
Add a Morse code key or other momentary switch
at the oscillator’s power point and a ¼-wavelength
stretch of wire at the antenna point ( 65' 3" for
a 3.579MHz signal), and you have a low-power
radiotelegraphy station that will broadcast your
dot-dash signals 25 miles or more, depending
on conditions. Just remember to get your amateur
radio license before you press the key down.
T.J. “Skip” Arey N2EI has been a freelance writer in the
radio/electronics hobby world for over 25 years and is
the author of Radio Monitoring: A How To Guide.
Make: 139
