Oscilloscope Built From an Old Monitor

In my senior year of high school, I ripped apart an old CRT monitor and replaced the horizontal drive signal with the output of an audio amplifier. The result was a rudimentary audio oscilloscope.

March 16, 2007

Here's an old IBM PCJr color monitor from 1985, that I've decided to turn into a simple oscilloscope! The diagram at the right demonstrates how a CRT display works:

  My old IBM PCJr monitor

My old IBM PCJr monitor

  CRT functional diagram

CRT functional diagram

The emitter, or "gun" at the back of the screen shoots out a beam of electrons towards a fluorescent screen, and two electromagnetic deflector coils move the beam up and down (usually at 60Hz), and side to side (usually at 31500 Hz), whilst changing the intensity of the electron gun (or guns, with a color screen), to form an image on the screen.

So here is the back all opened up on our IBM monitor:

  IBM PCJr monitor guts

IBM PCJr monitor guts

There are four wires leading up to two deflector coils in the neck of the CRT. In this case, I found by trial and error that the red and blue control the horizontal sync, while yellow and brown control the vertical sync. (Try disconnecting one wires while it's running. If you get a vertical line, you probably just cut one of the horizontal signal wires, and vice-versa.)

  Oscilloscope wiring diagram, vertical configuration

Oscilloscope wiring diagram, vertical configuration

After some trial and error, I figured that by keeping the vertical sync connected, and replacing the horizontal sync with an audio signal from an amplifier, you get a nice little oscilloscope (see left). I tried it reverse too; replacing the vertical sync with the audio input and keeping the horizontal sync connected to the monitor supply, but this didn't work as well. I ended up with a horizontal line that would just move up and down with the amplitude of the music (see right).

  Vertical Configuration (1 height = 60Hz)

Vertical Configuration (1 height = 60Hz)

  Horizontal Configuration (1 width = 31.5KHz)

Horizontal Configuration (1 width = 31.5KHz)

This is because the horizontal coil is moving the beam left-to-right at 31.5kHz, as opposed to 60Hz with the vertical configuration. To remedy this, I tried powering the horizontal coil with the vertical 60Hz supply, but things started producing smoke, so I promptly stopped. I also tried rotating the coil to deflect horizontally instead, but it was epoxied on pretty well. Thus, I settled on the vertical configuration.

I also looked up the pinout of the PCJr proprietary monitor connector so I could change the colors. This is achieved by shorting the intensity pin to the color that you want.

Here are some shots of the oscilloscope that I took in a dark room. (Sine waves at 60, 120 and 240Hz)

And there you have it. It's not a replacement for an actual calibrated scope by any means, but it certainly looks cool when you drive it with music.

A few observations:

  • The beam appears as a dotted-line. This is due to the CRT blanking periods.
  • The scope is much more responsive to bass sounds. It appears that the properties of this particular coil cause it to act as a low-pass filter.
  • The waveform will sometimes drift upwards/downwards. I think this represents the phase difference between the input signal and the 60Hz vertical sync.

Below is video of the oscilloscope in action: