heyrick1973 -at- yahoo -dot- co -dot uk

Oscilloscope

Sunday, a vide grenier (quel surpris!) and I bagged myself a little oscilloscope for €15. A Metrex OX710B. I suspect it dates from 1985. Here it is, as bought, prior to cleaning it up:
The reading on the screen is the Earth humming in B flat. No, seriously, I stuck the probes into the ground and twiddled the display until I saw something. It is hard to capture a 'scope spot with a digital camera, but it looked like what you see, only across the entire display.

I took the scope apart to give the casing a good clean. It's a far cry from the defunct Tektronics valve 'scope that I used to have. In fact, this part of the circuitry seems remarkably simple. There is more underneath in a shielded metal box, but I didn't take the thing apart any more than getting the lid off.

The serial number begins "85" so I would guess that is perhaps the year of manufacture?

The equipment isn't quite good enough for examining video signals. The bandwidth is 15MHz (but 10MHz on certain ranges). This is what a video signal looks like:

But if I switch to TV triggering (the 'scope will start tracing upon detecting a TV sync) and bump up the sweep rate, we can fiddle the view until we can see a single line of view on the screen. From the far left, we do not see any sync pulse as that is what is used to trigger the scope's sweep. The splat on the left is the colour burst. The disjointed rubbish following is the line of video data. It is a mess because it is live video, and running at a rate of 50 half frames per second, this may be more than one I'm looking at. An engineer would typically use a test colour bar generator that provides a disinctive and steady pattern on the screen. On the right, it all goes quiet with a dip into netagive. That's the sync pulse for the next line, which is what follows.

What you can see here is the Pi writing IIC data to an OLED. Again, while the trace isn't perfectly sharp and there seems to be no distinction in the up/down swings (a setting to adjust?) - it is not as bad as this photo suggests; the camera didn't want to focus on an ephemeral trace in the dark. However, you can see the start of the IIC pulses.

The scope has dual inputs, so a more useful fault-finding situation would be to hook up, say, the clock and data lines, and watch their behaviour respective to each other.

It is old, and it is absolutely no comparison to a digital storage 'scope that has the ability to "record" a waveform at high speed and look around to examine it in detail. However... sometimes you really need to see what is happening, and in situations faster than you could observe on, say, a multimeter. The probes that I was supplied with are a BNC adaptor to banana plugs, this connecting to a touch-probe hooked to a length of unshielded wire. Accordingly, the leads are rather liable to pick up interference. I suspect this 'scope found most of its use in a classroom looking at boring stuff like sine waves. I don't know if I should buy a proper shielded probe. I really should, but I worry that it might cost more than I paid for the 'scope!
The image is a bit fuzzy, and sadly at the fastest speeds (1 and 0.5 µs across the screen) it is practically invisible. However I hope it has some life in it yet, especially when I'm poking around my Beeb. I'll try hooking up an Econet clock box and see what that looks like.

 

Key labels

Two weeks ago, I got myself a Bluetooth keyboard. I sent off for some labels, and they arrived today. Absolutely NOT what I had ordered.
What I had ordered, seen here in a screenshot from the website, was this:
Large, clear, US layout (because for some reason, iOS in British English retains a US keyboard layout), and in lower case. I thought the lower case was cool, it reminded me of those Ladybird books I had when I was a child. [if you have never seen a Ladybird book, click here and here]

What I received was GIANT uppercase labels for a full-sized keyboard!

Because it did not cost much, I decided that returning it would be more hassle than justified, so I found a pair of scissors and did a bit of Blue-Petering.

When the job was done... Let's just say that Mom could read it from the other side of the room. I'd have preferred lower case, but for now, there's no excuse for not being able to find the right key!

 

What would be nice - if 4keyboard consider it viable - would be to have labels to match these Bluetooth keyboards - for iOS presents a variety of additional/extended keypresses by pressing a key whilst holding Alt/Option, or pressing a key whilst holding Alt/Option and Shift, as my diagram indicates (I have not tested if this is common or specific to the English layout):

The "(...)" means press that, then another, and the appropriate accent will be added. For instance, Alt/Option + N (for ~ accent), then 'n' to create 'ñ'.

 

 

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Zerosquare, 5th September 2014, 03:04
Nice stuff. You're most likely correct about the "classroom" comment; this brand and kind of oscilloscope were (are?) quite common in French high schools, for physics lab work. 
 
Regarding the "invisible" swings: this is normal for a low-end analog scope like this one. Selecting the shortest time division and setting the brightness to the max may help, but don't expect miracles.

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