K8031 calibration

I have just built a K8031 Digital PC Scope.

I have adjusted RV1, CV1 and RV2 as per the calibration instructions. However when I select 0.3V/div and connect to test point J3 I am unable to get the signal trace flat by adjusting CV2. There is a spike on the leading edge of the square wave which I can only reduce to 0.15V.

The only way I can remove the spike it to add a 110pF capacitor in parallel with C10 effectively increasing C10 from 220pF to 330pF. This then gives enough adjust ment on CV2 to get the top of the signal flat. I suspect 270pF would also be enough but I haven’t got an apprpriate capacitor available to try that.

I note that C10 is shown as 150pF in the schematic but is 220pF in the parts list.

I have changed scope lead and used an external signal source but the spike remains on the 0.3V/div and 0.1V/div ranges.

Is changing C10 to 330pF or 270pF a sensible solution to this problem or am I missing something? Other than this, the scope is working well.

In the manual there is not mentioned the time/Div setting for this adjustment: Should be 0.1ms/Div range.

If you haven’t, please check the adjustment of CV1 and CV2 on that time/div range.

It is OK to add the capacitor in parallel with the C10 if there is not enough adjustment range on CV2.

Thank you. Yes I have tried calibrating CV1 and CV2 using 0.1ms/Div. CV1 is okay but I still have the spike on CV2. I will change C10 so that I can get enough adjustment on CV2.