High gain branch not working correctly

[AABatteries]

Hi everyone,

I have a genuine DSO150 bought from Banggood (serial numbers matching, see attachments) and it works very well with one issue - when I'm probing a circuit everything is workig well when I'm at >0.5V/div, but when I go down to 0.2V resolution or lower, the signal just 'flattens' and it's not reading correctly - please see attached picture below where you can see the change from a sine wave to a flat reading after I increased V/div to 0.2V.

Resolution change from 0.5V to 0.2V and resulting loss of proper reading

So my scope only works correctly on resolutions 0.5V/div and higher. From the FAQ it seems like the high gain branch is not working correctly - and it is dependent on R1, R2 and U1A. I've testes the resistors before assembly and R2 was ~512 kOhm, but R1 was 5.7 MOhm instead of 5.1 MOhm as per assembly manual. Could this be it, or is U1A faulty and needs to be replaced?

I have verified the "Levels vs sensitivity settings" chart from the FAQ, and both 0.5V and 0.2V settings have correct values on SENSEL pins. Also AV+ and AV- have a correct voltage (5.0V and -5.0V respectively)

What should I test next? Should I apply for a refund or something?

[jye1]

You can do checking by

1 ) apply a DC voltage at input and measure V1. V1 should be about 90% of input. Otherwise U1D might be bad.
2 ) Make sure higher gain channel is selected. Check V3. V3 should be twice of V1. If this is not the case the multiplexer U2 might be defective.

Before doing this it is suggested to carefully check soldering including those pre-soldered parts.

[AABatteries]

Hi,

I checked the solder joints - they seem ok, none of them are suspicious. I took extra care when soldering and the joints look excellent. I have no issues with pre-soldered parts, they look equally fine.

I checked the voltages - after applying 8.5V at input I tested V1 and it's giving me zero volts ( 0V ). Same goes for V3, zero volts reading. Continuity checker shows that AGND is dead short to both V1 and V3.

Should I replacing U1 get it to work? What is the part number exactly that I should look for?

I will apply to banggood for a refund and ask for resending of the analog board if possible.

Thank you