Analog Oscilloscope: Turning electron beam on/off

[allenwp]

Hi there,

I just got my first oscilloscope and I plan to use it fairly unconventionally: I am making a video game engine that renders to an audio stream that is intended to be displayed on an analog oscilloscope. Here's what I have so far: https://twitter.com/AllenPestaluky/status/1233447852262772737

To do this, I am using my oscilloscope in XY mode and using the audio output to directly control where the electron beam of the scope's CRT is pointing.

Is it possible to control when the electron beam is firing by using the EXT TRIG input? (As in, is it possible to do "blanking" with this type of setup?)

For example, say I had X input as a sin wave and Y input as a cos wave. This would draw a circle like this:


Would it be possible to use the EXT TRIG input to change the display to look something like this?


Here is a better picture of the scope where you can see all the switches and knobs I have:


I have tried using the EXT TRIG with a sin wave of different magnitudes, but I can't seem to get it to do anything useful... I might be entirely misunderstanding the bits of information I've been able to dig up.

Thanks for your help!
Allen

[TimFox]

I don’t recognize that model.  Some CROs have a “Z axis” input, often on the rear panel, that modulates the beam current.

[james_s]

If your scope doesn't have this it's likely not going to be too hard to add but the task will be much easier if you can find a schematic for your scope.

[mikerj]

Schematic here.  Connector J701 is shown as a Z mod input so hopefully this is present on the rear panel.

[T3sl4co1l]

Right, you need a 3rd channel, typically called the "Z axis".  Which annoyingly doesn't fit inside stereo audio.

Note that blanking is subject to the same bandwidth limitations as X and Y, though this isn't usually as much of a problem as it is for them.

If you can get an unfiltered audio DAC, you may be able to overcome both by simply stepping instantly between points.  Downside is your figures can't be smooth anymore, they will look like a string of dots.

Tim

[allenwp]

Y’all are fantastic! Great job finding the schematic, I’m sorry I didn’t post a picture of the back! There is a Z Input. I’ll give that a try sometime later this week!

Also, in terms of generating signals, I think I have that covered: I can send out 192kHz worth of samples for each of the 4 output channels of my audio interface and it has DC coupling, so no low frequency filtering and no DC offset correction. This is how I’m drawing the cube in that video without any problems that you’d have on a normal sound card that a computer would have.

I will try hooking up my third channel to the z-input on the back and see if I can get it doing the blanking that I’m trying to do.

Thanks again! I’ll post back with results

[james_s]

Something to keep in mind is that the Z input is usually inverted so that applying a voltage blanks the beam, I suppose so that with nothing connected the scope works normally. At least in the case of my 465B the voltage needed is considerably higher than for a video monitor, IIRC it takes something like 15V to blank the beam fully.

[allenwp]

Perfect, thanks for the extra info! I have no idea how high of a voltage my audio interface can go to, so I probably will need to get a separate amp for it to do full blanking...

[james_s]

You don't need anything fancy. A single transistor and a few resistors ought to do it.

[Wallace Gasiewicz]

Sounds like great fun.
A great use for these old scopes.
Perhaps your postings will boost the prices...I think I might have to stock up HA!
I think at one time that I had an old Heathkit scope that did not have a timebase.. only X and Y inputs...

Thanks for the post

Wally

[allenwp]

Hi again,

I've tried hooking up a 3.5V (from my DAC), 26V, and 44V DC signal (a bunch of chained 9V batteries) and it seems to make the beam on the scope brighten and dim for the briefest moment before settling right back at the same brightness it had to start. On the back it reads "30V pp" on the z-input. If I were to guess what behaviour I'm seeing, I would say it looks almost like the z-input has DC offset correction (AC coupling), which is preventing me from directly controlling the beam brightness. But I'm really just taking a guess here -- why would there be AC coupling on this when I have the rest of the scope set up to be operating in DC mode?

Any thoughts? Is there a configuration I should be using on my scope to make it operate the way I'm wanting?

Thanks!
Allen

[T3sl4co1l]

Not uncommon on cheaper scopes: the CRT cathode and control grid are at -HV (typically 1-2kV), so you need two power supplies to get DC coupling.  Tek did this (most of their scopes having two -HV windings and rectifiers, one supplying cathode, one supplying grid), many did not (using a coupling cap instead for blanking).

Once you get a scan going, it should be okay, give or take a little tweak of the brightness to adjust the baseline level.

The relatively high voltage makes me wonder if it's tied into the grid directly (the LV side of the coupling cap I mean)...

Tim

[Paul Rose]

On that schematic, z-mod (J701) is connected to a 0.02 uF 2kV capacitor (C706).

So yes, it is permanently AC coupled, regardless of your input settings.

[allenwp]

Great, thanks for your help with this!! I‘ve got a lot more work to do on the software side, but I’ve definitely learned a bit and I feel I know how to approach this when I have time to tackle blanking in the future.

Thanks again!
Allen

[allenwp]

Hey, thanks again for your help getting me up and going with this. I've made some good progress after getting a new oscilloscope that has a DC-coupled Z-input!

Here's a video -- skip ahead to see different shapes, and eventually, a little game demo that I created: https://youtu.be/t2i9rZ1DvGo

Unfortunately, I do have an issue with a wobble on the X channel, as if it's not properly DC coupled, but I don't quite know what to make of the behaviour, or if this is normal on an oscilloscope: https://www.eevblog.com/forum/beginners/wobble-on-x-channel-of-oscilloscope-in-dc-xy-mode/

Cheers,
Allen