Sunday 26 January 2020

Synthesizer Build part-19: CHEAP OSCILLOSCOPE for the synthesizer.

A cheap but good functioning little oscilloscope that you can buy for cheap on eBay and use for your synth.

As a vital addition to my synthesizer I bought this little oscilloscope so I can easily check the signals my synth produces, so I can keep an eye on things and easily check if something is broken. I couldn't include it by giving it its own panel because I had no more room for that in the synthesizer so I decided to make a special holder from aluminium that goes over the case and holds the scope and has a mono 3,5mm input jack to which I can connect a cable from any output I choose.

The scope I bought is the little DSO138 that you can find on eBay or AliExpress or many other sites for around $20 dollars. If you order it, make sure you get it with an acrylic case because the buttons are on a lower PCB than the screen so it's not easy to make your own case for it. You can order acrylic cases for this scope for about $5 dollar.

If you are new to DIY synthesizer building I strongly advise to get one of these!! You are going to need a scope for trouble shooting so get one! If you can, buy a good multi channel digital oscilloscope, a really good one, like the Rigol DS1054Z 4 channel oscilloscope that I use, or a Siglent for instance. But if you can not afford to invest that much, just get this cheap scope. It'll do for 90% of the testing you're going to have to do. For tuning filters and testing VCO's you can't do without one.

Here are some pictures of how I use the scope:

As you can see I used a Banana plug to BNC adapter on the input and then some thick copper wires to the 3,5mm mono jack input. The holder is made from 1.5mm thick aluminium which I had left over and I decorated it with that circle pattern using a Dremel tool because it was scratched. Looks really cool I think. This is an ideal set-up because I can move the scope over almost the full length of the synth so it's never in the way. All the buttons are within easy reach and it is powered from the power supply from the synthesizer itself. There's a little wire coming out of the back of the synth that plugs straight into the scope.
[Edit: Room to move it around? Ha, the next article will take care of that. No more room for anything after that, LOL]

The oscilloscope menu is not very clear and user friendly. The documentation doesn't explain it in depth at all so here is a little explanation of the scope-menu:

Use the  [SEL] (select) button to go between the different options. There's no need to confirm settings with the OK button.
The option you selected will have a square around them, at the bottom of the screen, but there are a few options that are hidden from normal view. I'll explain those here:
With the 'trigger slope' selected, press [SEL] one more time to change the trigger level. You can see it rise or fall by the little arrow on the right side of the screen. (Use + or - to move it up or down.)
With the 'trigger slope' selected, press [SEL] two more times to change the zero level. This changes the position of the waveform on the screen up or down. (Use + or - to move it up or down.)
To display all the parameters like frequency, voltages, duty cycle, etc. long press the [OK] button (the top button) and the text display will turn on. At the same time the 'Hold' function will engage so you'll have to short press the 'OK' button again to get the scope running again. Long press again to turn the text display off and then short press 'OK' again to turn the waveform display on again.

The scope is delivered with lots of documentation to show you the different modes for triggering and what all the switches are for so I'm not going to go into that here. If you have any questions about this scope you can always put them in the comments below and I will answer them for you as far as I can.

Recently I also took delivery of an other small scope, the FNIRSI-138 Pro. This is a little square scope, very much like the one we discussed earlier but this one has a battery on board. The accompanying documentation says it will last you about 4 hours on a full charge. It comes with a BNC to Alligator clips probe and a short USB-C cable and a printed quick start manual in English and Chinese. 
It works very well and this has an 'Automatic' function. You press it and it automatically sets the scope to the best settings to display the waveform. This works very well although I find the knobs on this scope even more difficult to figure out than the DSO138. Their functions are again not very intuitive or obvious. The scope goes for about €30,- and measures 6 x 6½ Centimeters and 2,2 CM deep.
Here's a picture of mine. It has the protective screen cover still in place.

This scope has the advantage that you don't need to buy an extra case for it. It already has a very practical shape with all the knobs located underneath the screen. The ON/OFF switch is a minuscule little thing located on the righthand side. Above it is a tiny little reset button which comes in handy if you fiddle around with the knobs and the waveform disappears from the screen, which is what happened to me. Not even pressing the 'Auto' button helped, but the reset button did the trick. I think you could easily make a panel and mount this scope inside it and figure out a way to override the battery and feed it from the synth's own powersupply. This should not be too difficult. The scope is held together with 4 little nuts and bolts with standoffs for the circuitboards so it can easily be disassembled to make alterations. The battery is connected with a standard mini Molex connector. But if you decide to alter it you do it at your own risk! I haven't tried it myself so I'm not giving any guarantees.

You can have the display show some measuring values like frequency and voltages (Peak-to-Peak, Vrms, Vmin, Vmax etc). To display this info you long press the right button. Long press it again to clear the screen. The second button from the left will select different settings like V/division, Speed, Trigger Threshold, etc. You click it and the item that is selected is displayed in blue. Then you can use the third and second button from the right as up and down button to set the parameter to your liking. The left button is the Auto function. If you press it the scope will automatically select the best settings to display the waveform.

Beware of the advert for this scope on eBay. It shows a full size digital desk oscilloscope with a price of about € 30,- but you have to click a few boxes and then it shows that this page advertises 3 models of scope and you have to choose which one. If you go straight to 'buy now' and you think you're getting a full size scope for 30 euro's forget it. The picture shows the most expensive model with the price of the least expensive model. Beware of that.

If you are really serious about your DIY synthesizer hobby then I strongly advise to fork out some cash for a digital oscilloscope. I myself use a Rigol DS1054Z 4 channel oscilloscope which cost me €400,-- but you can get a reasonably good 2 channel one for under 200 US dollars these days. Here's a link to a Hantek scope that'll do the job nicely and will serve you well for a long time.

So that's how you can add a little scope to your modular setup for a minimum amount of dosh. I hope this was of use to you and you enjoyed this article. Please leave me a comment below and I'll see you on the next one.

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