K8031 Transient adjustment and calibration problem

Instruments PC Oscilloscopes and Recorders
1

Dear Velleman staff & community members,

Recently i bought a k8031 oscilloscope from a local distributor. I assembled the device and tried to calibrate it. The offset adjustment works, however when i try to do the transient calibration, the scope fails.

Description of the problem:
I select 1V/div in pcLab and connect my probe to J4. The square wave that should be displayed does not fit on the screen, so I can’t adjust CV1. Even when I set RV2 to 0 ohm, I cannot get the waves to fit in the screen to calculate the rms value.

Checks done:

  • Checked RV2 resistance. It varies from 0 to 500 ohms as expected.
  • Checked signal ATT0-3, they all change when switching from 3V/div down to 10mV/div
  • Measured all the resistors in the analog part of the pcb to check for the correct value
  • Checked all the diodes for correct operation
  • Measured the voltage between probe ground and power ground -> 2.5V
  • Checked if RY1 to 5 open and close properly

Steps made to further diagone/resolve problem:

  • Removed RY5 from pcb and reconnected the inner two connections with a 2k resistor, this caused the wave to be displayed at 3V/div and 1V/div, however the minimum RMS at 1V/div is still 3.68V and at 3V/div 11.04V
  • I found that the resistance between the outer holes where RY5 resided, is only 2.1k, is this correct?

At this moment I really don’t know where to look for the error anymore. So I hope one of you has experience with this problem and know a way to solve it.

Thank you in advance!

2

Most probably there is something wrong with the input section.
Please check (once again):
R10 = 910k
R8 = 91k
R7 = 10k

RV5 seems to work OK.
It is closed on 1V/div range to increase the gain.
Now you removed it and the gain is equal on 3V/div and on 1V/div ranges.
The gain is too high due to some problem in the input section around R10, R8 and R7.

[quote]- I found that the resistance between the outer holes where RY5 resided, is only 2.1k, is this correct?[/quote]This is OK. There is R17 = 2k2 between the outer pins 1 and 7 of RY5.

3

Thank you VEL255, that did the trick,

It was R10, I removed it from the pcb and it measured 90k, although the resistor color code says it is 910k.
Until now i have never had such an error. The resistor colors are: white, brown, black, orange, brown. That should be 910k. Attached is a picture of the ‘anomaly’

I replaced it with an 910k resistor and the oscilloscope is working perfectly now. Thank you so much!

Greetings Sven

4

OK. I’m glad you found the problem and got the K8031 working!