CRT Test Rig Produced Lissajous Curves

Last night’s CRT exploration adventures with [Emily] produced beautiful Lissajous curves on-screen that looked great to the eye but were a challenge to capture. (Cameras in general have a hard time getting proper focus and exposure for CRT phosphors.) Here’s a picture taken with exposure time of 1/200th of a second, showing phosphor brightness decay in a simple curve.

Lissajous curve with shorter exposure

Due to this brightness decay, more complex curves required a longer exposure time to capture. This picture was taken with a 1/50th second exposure but only captured about half of the curve.

Lissajous curve with longer exposure

Our test setup was a jury-rigged nest of wires. Not at all portable and definitely unsafe for public consumption. It required a space where everyone present are mature adults who understand high voltage parts are no joke and stay clear. (And more pragmatically, if an accident should occur, there will be other people present to call for immediate medical attention.)

CRT Test Rig angled view

Our beam power section consisted of two subsystems. The first is a battery that supplies low power (8 volts and less than 1 watt) to heat the filament. Using a battery keeps it electrically isolated from everything else. The second subsystem supplies high voltage to drive the CRT, and we keep a respectful distance from these parts when powered on.

CRT Test Rig beam power system

Connected to the tail end of the tube is the connector we freed from its original circuit board, wired with a simplified version of what was on that board. Several pins were connected to ground, some directly and others via resistors. The two wires disappearing off the top of the picture are for the heated filament. Two wires for experimentation are brought out and unconnected in this picture. The red connects to “screen grid” (which we don’t understand yet) and the black connected to an IC which we also don’t understand yet.

This is a rough exploratory circuit with known flaws. Not just the two wires that we haven’t yet connected to anything, but also the fact when we connected its ground to transformer’s ground, the tube flared bright for a fraction of a second before going dark. We only got a dot when connecting transformer ground to the filament heater negative, which was unexpected and really just tells us we still have a lot to learn. On the upside, something in this circuit allowed our “focus” wire to do its job this time, unlike our previous session.

CRT Test Rig tube wiring

But that’s to be figured out later. Tonight’s entertainment is our beam control section, which sits safely away from the high voltage bits and we can play with these while our tube is running.

CRT Test Rig beam control system

Controlling vertical deflection is an old Tektronix function generator. This is a proper piece of laboratory equipment producing precise and consistent signals. However, its maximum voltage output of 20V is not enough to give us full vertical deflection. And since we only had one, we needed something else to control horizontal deflection.

That “something else” was a hack. The big black box is a “300W” stereo amplifier, procured from the local thrift store for $15. Designed to drive speaker coils, tonight it is driving a CRT control yoke’s horizontal deflection coil instead. It was more than up to the task of providing full deflection. In fact, we had to turn the volume down to almost minimum for tonight’s experiments. A cell phone running simple tone generator app provided input signal. Not being a precision laboratory instrument, the signal generated was occasionally jittery. But enough for us to have fun producing Lissajous curves!


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