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I got an old CRT oscilloscope someone had laying around unused in his garage (an ATAIO AI 751A which seems pretty obscure, not getting any meaningful Google results). Unfortunately the user manual is missing, I couldn't find it online and this is the first CRT oscilloscope I work with, so I'm not sure if I'm operating it properly.

It powers up but there's nothing on the display when the probe is not touching anything, even when it is set to GND. Touching the probe's tip triggers a (very flickery) 50Hz ~10mV peak to peak signal (yes, I'm european) but setting it to GND to center the signal vertically stops showing the signal. At dual display mode both channels are shown when I touch the tip in channel 1 (channel 2 disconnected is shown at ground level), but as soon as I release the finger, both channels are gone too.

Touching a 9V battery gets a single sweep when the tip touches or releases the battery, while I'd expect at least that channel to be shown at a constant value.

Setting a high period for the time division and touching the tip triggers a single sweep, but as soon as it reaches the far right it stops. I can trigger another sweep touching or releasing the tip again.

I pressed a switch labelled "Auto Level" which I think should make the trigger level automatic based on the last samples (which seems plausible since the trigger controls don't do anything when auto is on) but still nothing is shown on the display when touching a 9V battery.

Fiddling with the controls (e.g. pressing the "Auto Level" switch) triggers a single sweep too.

Is the oscilloscope broken or am I doing something wrong? Where should I start troubleshooting?

Images: (don't pay attention to the settings)

Full oscilloscope enter image description here

Triggering controls close-up enter image description here

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To downvoter: what can I do to improve this question? –  DuckTyped Mar 17 at 17:34
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Picture of the scope's knobs and buttons would be helpful. –  Phil Frost Mar 17 at 18:05
    
@PhilFrost oh, of course. Now they're linked at the bottom of the question. –  DuckTyped Mar 17 at 18:27
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It sounds like the trigger is set to "normal" mode when you want "auto" mode. –  Olin Lathrop Mar 17 at 18:51
    
@OlinLathrop exactly, I failed to understand that "PULL AUTO" meant pulling the knob would set the mode to "auto". It's so obvious now, but I didn't even think you could pull it! –  DuckTyped Mar 17 at 18:58

2 Answers 2

up vote 5 down vote accepted

Pull out the trigger level knob. This will put it into "auto" trigger mode, which basically means it will automatically start a new sweep a short interval after the previous one ends. This will give you a continuous trace even with a DC input signal.

On modern scopes that have microprocessors, "auto" now means something completely different, in which the micro searches for a set of vertical and horizontal settings that give a "reasonable" display (YMMV).

The "Auto Level" button does something different: It uses a pair of peak detectors to determine the swing of the signal, and places the trigger level at some intermediate (50%?) value.

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I almost couldn't believe it would be that easy when I pulled the knob and it actually worked! Thanks. Now I see what "PULL AUTO" meant, I can't believe I didn't realize it before. The rate of triggering is controlled by the sweep time, right? –  DuckTyped Mar 17 at 18:54
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Generally, yes. Usually, there's a fixed delay between sweeps, so at slow sweep speeds, the sweep time dominates, while at the highest sweep speeds, the delay time dominates and the trace gets dimmer as a result. –  Dave Tweed Mar 17 at 19:01
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@DuckTyped some scopes have a "hold off" setting which adjusts a delay after the sweep until the trigger is armed again. –  Phil Frost Mar 17 at 19:49
    
No fair figuring out the answer with a photo to work from ! ;) –  Scott Seidman Mar 17 at 21:46

I suspect your scope is fine, and you just need to manually set the trigger mode, channel, and level. Even on today's modern digital scopes, hitting an AUTO button does things you don't want it to do.

Further, pretriggering on these dinosaurs is difficult or non existent, so the trigger point will be at the far left of the screen, and not the center, which you're probably used to.

You'd be better off starting your experience with a sine or square wave than with a DC battery. Is there a little doohickey coming off the front somewhere that provides a square wave output for adjusting your probe? That would be a convenient source to play with.

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Yes, there is a square wave output for calibration! Completely missed that, very convenient. My trigger settings are set to: "Slope +", "Source CH1", "Coupling DC" and a "LEVEL - PULL AUTO" knob which does what it should with AC as far as I can understand, but nothing when touching the battery. It still seems weird I can't get the flat output even with input set as GND. –  DuckTyped Mar 17 at 17:45
    
Often there's a "find trace" button that can help figure out what the scope is seeing. Maybe your trace is just out of range. Start by setting the trace on GND and push "find trace", and setting the trigger to auto. Play with the offset controls and see if you can find your trace. –  Scott Seidman Mar 17 at 17:48
    
You want the scope on DC to start. –  Scott Seidman Mar 17 at 17:48
    
What are your choices for source? –  Scott Seidman Mar 17 at 17:56
    
Yes, the scope is on DC. There is no "find trace" button, but I'm sure it's in range since it shows fine for a single sweep when I fiddle with the controls and I managed to center it setting it to GND and using channel 2 to trigger the display touching the tip with my finger. It even traces an horizontal line at 9V if I touch the battery and trigger with my finger in channel 2... it just seems to refuse drawing unless I do the "channel 2 finger" trick. –  DuckTyped Mar 17 at 17:58

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