DSO-138 Calibration not possible

Post Reply
Posts: 1
Joined: Fri Sep 08, 2017 8:53 pm

DSO-138 Calibration not possible

Post by Ottmar »

I have assembled the DSO-138 from YVE-Tech. Very good building instruction! All functions of the DSO138 are available and are working well.

Calibration is not possible. On the test signal there is a strong spike of the rising edge and also on the descending edge. Refer to Probe Calibration Figure B "Too much".

This can not be fixed by turning the Trimmer C4 and C6! Any turning on C6 / C4 is without noticeable influence on the overshoot.

The circuit for the calibration SEN1 = 0,1V, SEN2 = X5, CPL = AC / DC, with the resistors and capacitors has been tested several times by me and is fault-free.

Is it possible to get some advice to solve this problem?

In the circuit diagram the value = 220pF is given for C2, while the kit C2 contains the value = 330pF. This is in contradiction to the user manual 7. Ceramic Capacitors and the circuit diagram "CPL" C2 = 220pf. What is right?

Greetings Ottmar
Posts: 3
Joined: Wed Mar 14, 2018 9:41 am

Re: DSO-138 Calibration not possible

Post by sinewave »

Is there an answer to this question? as I am experiencing the same exact issue.
Posts: 59
Joined: Wed Sep 13, 2017 2:23 pm

Re: DSO-138 Calibration not possible

Post by MNTech »

The input of the scope uses basic resistor voltage dividers to set the range. However, like any real world, they are affected by stray capacitance. Extra capacitance is added in parallel to both the upper and lower resistors to compensate. The ratio of the impedances of the total value of stray and added capacitance must match the resistor ratio for flat frequency response.
So what can go wrong? Either a resistor value is wrong or there is a bad capacitor.

If the input resistors are all correct the resistance measured at the BNC connector will be 1 megohm. Also the ratio can be checked by inputting a small voltage and seeing that the displayed trace moves the correct distance. With SEN1 = 0.1 and SEN2 = X5 a 1.5 volt battery will move the trace 3 divisions.

The overshoot you describe says the capacitance of the lower section (C4) is too low. If the resistors are correct it's time to find a replacement for the variable capacitors. If you happen to have a fixed capacitor around 20 to 30 pF you can tack that across C4 to see the effect on the overshoot.

If I remember correctly the adjustment for the 1 volt range makes a much smaller change in the over/undershoot than the 0.1 volt range.
Post Reply