Sorry... calibration problem (again)

[Jules]

I'm sorry to bother this forum with a calibration related problem.
I had finished my DSO Shell, checked all the values and everything it's ok.
Now, when I try to do the 0,1 V calibration via C3 with input set to AC, I see a trace that goes out of screen

As for C5 the trace seems good, except for the overshot (red circle in photo), but C5 does not modify it at all. I suppose the the issue is the one described in the FAQ.

Any suggestion to fix C3?
Thanks and best regards,

[jye1]

For the 0.1V scale issue did you try lower the sensitivity so as the waveform can stay within the screen? Looks the gain of analog channel has been doubled. Please check R6 to see if it is open.

[Jules]

Thanks for your suggestion, I'll check it later today.
Thanks,

[Jules]

Hi again,
I did the test trying to lower the sensitivity, but have no results (see photo)

As for R6 I checked the solder and it's ok. How do I check if it's open?
Thanks and best regards,

[jye1]

I replied yesterday but the post was lost due to attack to the forum. I feel the gain of the analog channel could be wrong and suggest you to verify the resistors R1 - R15 for correct values.

[Jules]

Hi,
Do you mean R1 and R15 or all resistor from R1 to R15? The values are correct as I measured every resistor before mount.
Best regards,

[jye1]

I mean all resistors. You can verify again with them in circuit. Just measure their resistance with an ohm-meter. The readings are usually smaller than their marked values due to some other components are in parallel. The numbers give you a rough idea. If you want to be accurate disconnect one end to do the measurement.

From the photos the 3.3V test signal appear more than 6V on 1V/div scale. This shows the gain of analog channel was wrong. The gain was set by the resistors so I asked you to check.

Other than the resistors poor connections of SENSEL0 - SENSEL3 can also cause wrong gain. So please also check the related pins at the pin-headers.

[Jules]

Hi,
The resistors were correct, but there was a poor connection at SENSEL0 - SENSEL3.
It looks like that everything is working correctly now.

Thank you very much!

[Bercebus]

Excuse me, what are SENSEL0 - SENSEL3 in the DSO SHELL? I think I have the same problem that Jules

[jye1]

These signals were labelled on the schematics. They run from U1 on the main board through J4 on mainboard and J2 (or J3 on the analog board) and to U3 and U2 on the analog board.