Strange problem with DSO 138 - trace rises to infinity

[kisiel]

Hello,
I have quite a strange problem with my DSO 138 oscilloscope - a few seconds after turning on my oscilloscope (even without probes) the traces immediately rises over the visible screen, no matter what settings I'm using. Measurements of voltage seem to continuously increase up to over 1000V which is of course not happening in reality! What is also interesting, is that it worked before and suddenly broke down. In attachment I add a picture. Can you please help me?

[johnsam]

I can’t help you because my unit did the same thing after working correctly after assembly and sitting around for a few months. When I powered it up it displayed the calibration signal for a couple of seconds before the trace flatlined at the top of the screen. I did find an intermittent electrolytic capacitor in the -5 volt power supply circuit. Replacing it did not correct the trace problem. I checked the op amp circuit path and determined that it functioned properly. I checked all DC voltages. I soldered all through hole components on the component side of the board. No luck.

I’m guessing that the ‘scope is a goner because I can’t replace the microprocessor and don’t want to go to the trouble of programming it. It was an interesting project. It will go into my spare parts box.

[Ish]

Get your DSO 138 out of the spare parts box. I was having the exact same problem and I fixed it by turning it on, waiting for the trace to go high, then doing a default settings reset by holding down both the + and - buttons for about 2 seconds. The trace returned to the normal zero volt position and has worked fine since.