DSO 138 Oscilloscope

The first thing that I noticed about this kit is how good the documentation is that comes with it. Instructions from Chinese based manufacturers are usually poor and that’s if you get any, the only good point is the laugh you get from the translations. In this kit though you get two pages and in colour with perfect English and clear photographs.

As you can see from the picture below everything is included apart obviously from the solder.

parts of the DSO 138 oscilloscope kit

The complete kit, apart from the solder, obviously!

what you get in the DSO 138 oscilloscope kit

Once you open the kit up you’ll see the two sheets of instructions, one shows the steps to construct the oscilloscope and the other is about using it.

the instruction sheets

Instructions are good, in full colour and understandable English.

You will also notice the test leads included, the bag of components, the display PCB (which is best left in its protective bubble wrap until needed) and the main red PCB.

One thing to note is that this is a partially constructed kit. All the surface mount components are already soldered. This makes it a lot easier.

If you follow the instructions, which I would recommend with this kit as they actually are correct. You should fit the resistors first. They are quite small and I used a component tester to measure them and then simply fitted what the resistor value said in step 1, all my resistors and values were correct so I soldered and trimmed them to end up with the picture below.

DSO 138 oscilloscope kit with resistors soldered

The PCB with the pre-soldered surface mount components and the resistors soldered into place.

Step 2 of the instructions is to fit the HF chokes, there are three of these and they look similar to the resistors. You can identify them from the picture below.

DSO 138 kit HF chokes

These are HF chokes not resistors.

Solder them in place and trim the leads and you should end up with something like the picture below with L1, L3 and L4 fitted.

HF chokes

HF chokes soldered in place.

Step 3 is to fit the diodes, take care here as the diodes are not the same. D1 is a 1N5819 and D2 is a 1n4004 or 1N4007. They are also polarised so you need to get hem the correct way round. Fit them and you should have something similar to the picture below.

DSO 138 with D1 and D2 fitted

Diodes D1 and D2 in their correct positions.

Step 4 is to fit the crystal, this is easily identified and once soldered into place it should look like the picture below.

dso 138 constuction

The 8Mhz crystal soldered in place.

Step 5 is to fit the mini USB socket, as mentioned in the instructions this component is optional and as I couldn’t find any use of it I didn’t bother fitting it.

Step 6 is fitting the push button switches. You do need to give them a bit of a push to get them to sit properly on the board, they will fit flat against the PCB when pushed all the way down and you will end up with the board looking like the picture below.

The five push button switches soldered on the DSO board.

Step 7 is the fitting of the ceramic capacitors, these are different values so you need to be careful. There are eleven capacitors that are 0.1uF so solder these in first, then C12 and C13 are the same value of 22pF so identify and solder them. C7 and C8 are 120pF so solder those in next. That just leaves three values left. Easy, just solder those in and you will have a board looking like the picture below.

Ceramic capacitors all fitted.

Step 8 is simple and just involve soldering in the LED but just be careful to solder it in the correct way round.

The LED soldered in place.

Step 9 is the soldering of the pin header, as the instructions show have the opening towards the edge of the board as shown in the picture below.

dso 138 power connector

Power connector for 9V DC.

Step 10 is the fitting of the two transistors, again you need to be careful as they are different. One is a NPN and the other a PNP and you must not get them the wrong way round.

DSO 138 transistors

PNP and NPN transistors soldered in place.

Step 11 is the fitting of the regulators. Again they are different, a 78L05 for positive 5V and a 79L05 for negative 5V. Don’t mix them up.

78L05 and 79L05 regulators shown on the left and right of the transistors.

Step 12 is to solder in C4 and C6, the capacitor trimmers, these are easily identified and both the same value and can be fitted either way round, you can’t go wrong with these.

Capacitor trimmers C4 and C6.

Step 13 is to fit the power inductor L2, easy to identify and you simply solder it in either way round

Step 14 is to mount the electrolytic capacitors, there are six of them and they are all the same value 100uF 16v but they are polarised so have to be soldered in the correct way round. The square pad is also marked with a + sign to aid you.

The polarised electrolyic capacitors.

Step 15 is simple, you just solder in the power connector

power connector

Step 16 is to solder in the three pin headers J5 and J6. Again this step is optional so I didn’t bother to do it.

Step 17 is to solder in the pin headers of J7 and J8 which are two pin, and J3 which is a forty pin one, they don’t have to be soldered in any particular way round but be careful when you solder in the two pin ones as you can get them to twist slightly, try to get them at right angles to the big forty pin one as they do have to line up with the display board.

The headers soldered in place, the bottom left one was slightly off on the first attempt as it has to line up to the display board that plugs into the top.

Step 18 is to fit and solder the slide switches. They take a bit of jiggling around to get to fit properly and to get them seated properly in the printed circuit board. Make sure they are properly in place before you solder them.

Slide switches mounted, make sure they are pushed flat down before you solder them.

Step 19 is to solder in the BNC connector. I had to put in a larger tip in my solder station and turn up the heat to get this to solder. One of the reasons why a soldering station is a good investment, you can read here about the one I bought. The BNC connector pins for the ground form the whole metal body of it so this will heat up while you solder it and also it will stay very hot for a while afterwards. You really need to get this hot to solder it properly as it will also form a mechanical connection that will take a fair bit force upon connecting the test lead up to it.

Step 20 is to simply solder a loop of tinned copper wire or an offcut of a component lead. This is the test signal square wave output. You don’t get this in the kit so don’t waste time looking for it LOL.

Step 21 is to solder JP3, this is marked on the board and it’s just two pads adjacent to each other. You just have to put a blob of solder on them to make sure they are joined.

JP3 has a blob of solder to join together the two adjacent pads.

Step 22 is to assemble the display board. J1 is the forty pin header and J2 and j3 are the two pin ones. It’s important and also fairly obvious that these solder onto the back of the board, not the side with the display on.

Make sure the connectors are soldered onto the back of the display PCB.

Test and use

You have now completed the soldering stage. Apply a 9V supply to either J9 of J10. You need to be aware of the polarity. I used J10 and using this the conventional centre pin is the positive. With a voltage or multimeter the voltage at TP22 should be around 3.3V. Mine was so it’s looking good so far. As the instructions mention if you get 3.3V at TP22 you can disconnect the 9V voltage supply and put a blob of solder on JP4 and make sure that the adjacent pads are shorted together.

The display board should now be plugged in, if you remember earlier I mentioned making sure the two pin sockets were soldered in square. If this is the case it should be a simply line up and plug in.

You can then reapply the 9vV supply and hopefully the LCD will illuminate and you will see the boot up screen. The green LED should also blink and you should be in possession of a working oscilloscope.

The completed DSO 138 oscilloscope kit, powered up and booted.

The button functions are printed in white on the circuit board. If you press the “SEL” select button you will see the various options flash as you press it, pressing the + and – buttons increases or decreases the values.

Mine worked first time without any problems but there is a comprehensive troubleshooting section with voltage references shown on a clear picture. There are also instructions for entering test mode. Because mine worked I didn’t have to try any of this but it does look good and not many other kits have any instructions or help if it doesn’t work.

How to use

That completes the first construction sheet. The second sheet covers “how to use”, the display and controls.

First thing to do is to calibrate the probe and the instructions cover this. I used a standard oscilloscope probe like this one here instead of the supplied crocodile clip one. These instructions are very good and will get you up and running. The back of the “up and running” sheet also contains a full circuit diagram. The only thing I found that didn’t work correctly was the factory reset. Holding down the + and – buttons didn’t do a factory reset on my unit. Maybe the software has been changed?

I personally think some of these entry level devices can get a lot of criticism, this is generally from people who have, and are comparing them with machines costing hunreds if not thousands of pounds. A lot of people as much as they would like can’t afford that sort of money for a hobby and although this type of oscilloscopes is limited it does offer you an amazing piece of test equipment and having one of these is better than not having anything at all.

Let’s not forget that you can do pretty much most fault finding on audio equipment. Having just built a voltage controlled oscillator I was able to see all the waveforms working, check the pulse width control was working and tweak the pre-sets to make the sine wave more siney. My second job of sorting out why a breadboard monostable wasn’t correctly functioning was also a breeze with this oscilloscope, also made easier by replacing the test lead with a proper probe. Simply following the trigger though to where it stopped and I’d found a faulty capacitor.

If you’ve got a few hundred pounds for an oscilloscope then read this and buy one of the excellent scopes you can get for that money. If you can’t afford that sort of money then this is a brilliant piece of test equipment that will enable you to see all sorts of electronic stuff you wouldn’t otherwise be able to.

DSO 138 Oscilloscope case

Once you've built the DSO 138 oscilloscope you may want to enclose it in a case for protection. If this is the case (pun intended) there are kits specifically made for the job.

DSO 138 Oscilloscope case instructions

Although the case is excellent unfortunately it doesn't come with any assembly instructions and I certainly didn't find it easy and I had to take it apart after making mistakes a few times so I've put together my own instructions of how to put it together in one attempt.  

dso138 oscilloscope case kit

The dso138 oscilloscope case kit.

The kit contains everything you need to construct the case enclosure for your DSO 138 oscilloscope. The first thing you need to do is peel off the brown protective paper from the perspex panels. you may find a few remaining bits of squares that haven't been removed, take them all out before you start assembling anything.

dso138 case first panel

The first perspex piece that you need.

Carefully unplug the display board from the main board. This fits over the panel with the connectors going through the cut outs. The four smallest nuts and bolts in the pack are then use to fix the display board in place.

dso138 case display from back

This is the display board bolted to the panel and shown from the back.