Sanity Check - CRT TV Mus-O-Scope

Hello all,

I’m working on converting a 70s CRT TV to display audio input as a waveform on the screen. The design consists of a power supply, two 10W audio amplifiers, pots for volume adjustment and three 6.35 mm audio jacks for input. The jacks, pots and power LED will be mounted into the side panel of the tellie.

For ease of use, I’ve opted for two mono jacks and one stereo. The plan is to have the two mono jacks disabled if the stereo jack is in use. The tip of the leads is the switched contact, with the other one or two fixed. I am assuming the tip will be -ve or 0V with the shaft +ve. Thus, the only way I can see to connect the jacks together and have only the stereo connected if it is in use is as follows:

I would be most grateful for a sanity check to ensure what I’ve assumed is reasonably likely to be OK?

Note: The X and Y signals are only combined for the sake of simplicity; they’re not joined before the amplifiers.

Many thanks,


Not sure I follow your drawing, but wouldn’t it be something like this?


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Thanks, @analogoutput.

Yes, I think so, however, only the tip pin is switched, thus having to rely on it as a shared ground. I struggled to find a symbol for the part myself, however, I link the parts here:



If I understand what you’re saying you’re talking about putting ground on the tip and signal on the sleeve. No, don’t attempt to do that. That’s completely nonstandard and you’ll end up connecting ground on one piece of equipment to signal on the other, a recipe for disaster. Ground must be on the sleeve, which connects to the cable’s braided shield.

What you’re saying is you have the wrong jack for the job. You need a stereo jack that’s switched on both tip and ring. This would work: P0072 6.35mm DPDT Non-Insulated Stereo Jack Socket - Altronics

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Thanks so much, @analogoutput. To be honest I don’t recall why I thought the tip was ground versus the sleeve. Probably after dealing with PSU issues with audio gear where the polarity of the DC jacks is opposite to everything else I’ve come across.

I’ll grab the other jack and test before going any further!


After many delays and three different amplifier designs and two power supplies, I’ve finally got this project up and running. Quite happy with the result.


Have you adjusted the TV’s High Voltage in order to prevent burn in of its phosphor layer?
Often times there is a trimpot which you can use to adjust that.

Your initial post suggest that you are using audio on the x and on the y axis.
If both signals are the same you will only see a straight line at an angle of about 45 degrees the length of which is proportional to the amplitude of the signals. If you use a stereo signal, you might see some more interesting things, but only if there is a difference in the left and right channels. So, I wonder, have you considered using a diferent signal on the y-axis? You could e.g. use a 50 or 60 Hz AC (line derived) signal. In that way you will get a flat horizontal axis and see the signal like you would on an oscilloscope and whenever the audio signal contains a frequency that is a multiple of the line frequency you will see nice lissajous-kinda pictures i.e. rotating curves.

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Hey Jos,

Good point regarding burn-in. Conveniently, the channel select knob brightens and dims the ray, being an unintentional but useful control feature.

The input is two mono inputs, and I may add a stereo input as well, so the XY is always on separate signals. I do like the idea of adding an internal signal generator and mixer to have more of an oscilloscope looking output.

Thanks mate!