Calibration problems

[Squatch]

I am having some issues with VT DSO-2820R. I have been monitoring signals on channel A from +/-200mv to +-1 volt. I am seeing some discrepancies when I change the voltage setting from +/-2V to +/-5V and the readings are way different. I have a signal generator connected now and the output is 1KHZ@.696 VRMS (double checked with Fluke87III true RMS meter) and channel A set at +/-5V it reads 0.696VRMS but when set to +/-2V setting it reads 25mvRMS and all other settings down to +/-50mv it reads 25mvRMS. The visual signal on the screen is smaller as well.
On channel B I have the opposite problem All low level settings are fine but when I set it to +/-5V or greater it reads 20X higher than it should.
Anybody got any suggestions to try?
Thanks!

[VirtinsTech]

Thank you for your questions.

The ADC Voltage Range selection boxes are located in the middle of the second toolbar from the top. They are different from the Display Ranges selection boxes which are located at the bottom of the screen. When you switch between a voltage range >= +/-5V and a voltage range <= +/- 2V, a relay click sound should be heard. Also, please check the oscilloscope probe switch position (x1, x10).

If the problem still exists, could you please provide the following screenshots for further diagnosis:

1.[Setting]>[Calibration]
2.[Setting]>[Calibration]>"Advanced">"A: +/- 2V"
3.[Setting]>[Calibration]>"Advanced">"B: +/- 5V"
4. Oscilloscope and Spectrum Analyzer windows showing the problem

[Squatch]

I ran the test you ask for. I checked the 10X probe setting and they were in the 1X position for both probes.
When I change from 2Volts to 5Volts I don't hear the relay chatter on either channel.
Thanks for your assistance!
I suppose it needs a new relay or 2?
I had one more screen shot but I see 3 is the limit.

[VirtinsTech]

The chances for two relays to fail at the same time is very low, unless somehow the power from the USB port is not enough. You can try it on another computer or use another USB cable. A relay click sound should also be heard when switching from a sampling rate of 200MHz to a sampling rate of below 200MHz.

Also, under Gain(dB) column of the screenshots, "32767" is displayed for both cases. This is also a little strange as the number does not seems to be the ones set by the factory. Did you try to re-calibrate it by yourself?

[Squatch]

Using the example on page 21 of the VTDSO-2820R manual rev1.1.
Re-calibration of channel A through all the voltage ranges was successful.
When calibrating channel B erratic readings started and the scope trace was all over the screen when the screen froze with an error. “Transfer error!”
After doing more testing it was found that the USB connector may be at the root of the problem. Some masking tape was applied to the cable connector at the input and lashed it under the DSO chassis and tension tightly. The unit is stable over time and calibrations can be done. A second cable was tried and we had to tape it too. What would you suggest we do?
Thank you for your assistance!

[VirtinsTech]

If the instability is caused by the USB cable, then you can change it. The new USB cable should be of good quality and KEPT SHORT.

If the instability is caused by the USB socket on the USB DSO side, then the USB DSO hardware may need to be replaced. Contact us through email if you need to replace the hardware after the warranty period.

If the instability is caused by the USB socket on the computer side, then connect to some other USB port of the computer or change the computer.

[Squatch]

I am still troubleshooting the unit. I have found something new.
With the cable taped as I said before I am now seeing the data slow to update to the screen after 18 - 20 minutes ( at 100kHz and 20,000 record length) . At a 20,000 record length per sweep it starts really fast the slows to a 1 second screen data update. After about twenty minutes it finally freezes completely.
The software is fine, I can run/stop and close it out completely. Unplug the USB cable and re-plug the USB cable(Hardware reset) and it works again but for a much shorter time now.
How BAD is it???
Thanks again for your assistance!

[VirtinsTech]

Looks like a USB port, USB socket or USB cable issue.

[Squatch]

Thanks so much for your expert help. With your help we found the problem and I repaired my VT DSO-2820R.
It seems that I have caused the problem with a ground loop and damaged the USb soft start IC (MIC2025-1YMM).
I purchased a new chip from Mouser for less than a buck and now I am back using my Digital scope with no problems.
Thanks Virtins!