2020-05-16

Build of a Small PCB Based Oscilloscope - and Another Oops.

A little while ago I purchased a kit for a small, very simple oscilloscope.
This is the JYE Tech DSO138, a kit with SMD components soldered, so only through-hole components need to be mounted.
The scope is a single channel device with a bandwidth of 200kHz, using a sample rate of 1Ms/s, and has a small, but readable 2.4" screen, if I recall correctly.
The kit went together all right, even if soldering was tricky, even with a quite hot soldering iron. The tip had trouble heating up the PCB pads, probably because they had a very narrow edge around the through-holes.
Otherwise, when I finally found the assembly instructions for the correct version of the board, the check/test went well, and after adjustments the screen shows a relative good square wave, using the built-in test generator.
This test was done with a USB cable connecting a 5V power supply to the scope.

Now came the test with the "normal" power (only) connector on the analog board (with all the switches etc). I found a battery and a red/black wire set mounted on the connector, got it connected, and - <snif> <snif> - something gets hot, and nothing on the screen.
A check revealed that the connector had reverse polarity .... Oops.

A correctly connected cable was found, and phew! The screen showed something. But not all was good.
I checked the voltages on the board, and the -5V voltage was essentially absent - a few 100s mV. Not good. I disconnected the output of the ICL7660 DC/DC converter IC, still no negative voltage output.
The 7660 has definitely gone to the eternal IC fields. The quad OP-amp could also be defective, so I have ordered spares of both.
The arrival of the spare parts is expected in a few weeks, so the project is now on hold. Not a huge problem, it is not a critical item that I need right now, and I have more than enough other projects to get to.

So, what can we learn from this? Oh, yes, when building kits and/or connecting power  to kits or modules, check, then double check the polarity of the power supply leads, and the polarity on the PCBs.
A pity that there is no standard for this, but what can we expect from stuff coming from different parts of the world?

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