Distorted trace. Who can help?

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Kirill
Posts: 2
Joined: Mon Dec 18, 2017 9:54 am

Distorted trace. Who can help?

Post by Kirill »

Hello,
I completed assembly of my DSO138. It works fine, but with one exception. When I supply a sine wave to the input, and its amplitude is larger than 4 divisions at the x1 amplification (or 2 divisions at the x2 amplification, or ~1 division at the x5 amplification), the trace shape is distorted as snown in the attached screenshot, Any ideas what could go wrong?
Thank you so much.
Attachments
Note distortions in red circles
Note distortions in red circles
MNTech
Posts: 59
Joined: Wed Sep 13, 2017 2:23 pm

Re: Distorted trace. Who can help?

Post by MNTech »

The distortion in the photo is two divisions from the center. If I understand your post correctly that changes with the SEN2 switch setting.
In that case it's unlikely the DSO138 is the problem. Check a signal from a different source.
Since the frequency appears to 1000 Hz I assume you are using some type of signal generator.
Kirill
Posts: 2
Joined: Mon Dec 18, 2017 9:54 am

Re: Distorted trace. Who can help?

Post by Kirill »

MNTech wrote:In that case it's unlikely the DSO138 is the problem. Check a signal from a different source.
Thank you for the response. I checked the signal with another oscilloscope connected in parallel. The signal is fine. Also, I tried different signals and the problem persists.
The distortion is 2 divisions from the center when the SEN2 is in the "x2" position. If I increase the amplitude from the signal generator smoothly, the height of the trace increases but the distortion remains at the same level - exactly at -2 divisions.
When the SEN2 is in the "x1" position, the distortion is at 4 divisions from the center (in this case I need to shift the trace upwards to see it).
If I switch the SEN1 to a different position (for example, 1 V instead of 0.1 V), then the distortion is observed when the amplitude of the input signal is ten times higher.
MNTech
Posts: 59
Joined: Wed Sep 13, 2017 2:23 pm

Re: Distorted trace. Who can help?

Post by MNTech »

Having another oscilloscope will make troubleshooting much easier. From your description of the symptoms the problem is in U2B.
You should be able to see the distortion at test point V1 to confirm this.

U2, a TL084 is in a 14-SOIC (0.154", 3.90mm Width) package.
https://www.digikey.com/product-detail/ ... -ND/404933

ChipQwik will help you remove U2 without damaging the board or the part.
https://www.youtube.com/watch?v=FTQqjggeklo

The SMD1 removal kit cost nearly as much as the DSO138 but I'm sure you find other uses for it.
https://www.digikey.com/product-detail/ ... -ND/304148
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