Strange test signal, shape and behaviour

[vinnieb]

Hi,

Put together my DSO138 a few weeks ago and really love the kit! Initially had some problem with the -5V supply due to soldering skills, but that seems pretty stable now.

But, you can see in the attached images I get some strange test signal behaviour. On DC, 0.5V the wave is far from square, and off the scale, DC 5V is more visible in another image. With AC 0.5V, its off the scale again, but looks OK-ish at 5V? It's a cheap wall socket power adapter which might explain the noise.

I guess its probably some more dry solder joints somewhere but I'm struggling to know where to focus, I don't want to mess up joints that are OK, so I wondered if you could help where might introduce this kind of problem?

Thanks!

[MNTech]

I see in your 2nd and 3rd photos the 3.3 volt test signal appears to be 16 volts.
5 volts/division times 3.1 divisions = 16 volts.
R6, R7 and R8 divide the signal by 5 when the switches are set for 5 volts per division.
If R8 is open the signal would not be divided and would appear be 5 times higher as we are seeing.

[vinnieb]

Thanks for the suggestion! By open you mean, if I check the resistance of it while its on the board, it would be zero? I've checked those 3 and they're all matching the schematic values, and I tried cross checking to see if I got the incremental values measuring across them, and that value came out to R6+R7+R8

One other thing I noticed though, if I move the switch down to the lower interval, the display stays the same i.e. way oversampled for that I'm trying to fit on the screen. It's like the switch is just going straight to the furthest right position rather than registering its in the middle? I haven't dropped the unit or anything so a little surprised if its a mechanical switch issue?

[MNTech]

Open means no connection. Most likely there is a bad solder joint on one end of R8. If you measure resistance from ground to R7 you should get either 120 ohms or 300 ohms, depending on which end you chose.
You can check the operation of the switch by measuring from U2 pin 10 to ground. It should be 600, 300 ,120 ohms as you go from X1 to X5

[vinnieb]

Ah interesting! I match those values for measuring ground to R7 ends.

U2 pin 10, as you look down on the board, 3rd up from the right hand side? Was best I could tell from chip diagrams, couldn't see an obvious end marker on the surface mount version here. If that is the one, I get a consistent 120 ohms no matter on switch position there.

Going to re-check tomorrow, maybe I've messed up the soldering on that switch by the looks of it?

[MNTech]

Yes, you have the right position for pin 10.
The pin 1 side of U2 is marked by a bevel running the whole length of the IC package. It's not hard to see when you know what you are looking for.

I am having trouble understanding how all the resistors measure correctly and R6+R7+R8 is correct yet the resistance at pin 10 is 120 ohms.

V1 test point to the nearby DGND should measure 600 ohms.

[vinnieb]

Got it!!

So, first I have to admit I was accidentally focussing on the SEN1 switch rather than the SEN2 which is what I now understand you meant. It was the SEN1 movement from 0.1V either side that didn't seem to have the expected effect on the test signal measurement, so maybe why I thought that was the one. Anyway, I went back over your steps, and indeed as you suggested, the resistance at pin10 moved through the ohms you gave. I checked the mechanical operation of all the switches too and all seemed fine on that side.

So I tried to repeat your approach with SEN1, and from the diagram the first port of call seemed R3, and checking from ground to there came back with nothing. A quick resolder, got the expected 200k (well about 190k in my case) value, and bingo, correct test wave shows up!

Thanks for the help and giving me a new approach to track down problems!

[MNTech]

Well done!
This kind of kit is intended to be a learning experience, maybe you got more than you bargained for.
Now you can think about upgrading the firmware, there is discussion of why and how in earlier posts.

[JardCrocker]

Hi...i am a new user here. As i can observe here the resistors R6, R7 and R8 divide the signal by 5 when the switches are set for 5 volts per division.If R8 is open the signal would not be divided and would appear be 5 times higher as we are seeing.

pcb assembly