EDU 08 Offset no function

I cannot adjust the offset (RV") to 0v
when i adjust the poti there are no mofification in the display.
i have alwas - 18v an the signal is to bottom.

i have 3v at the RV2.
is the ic def?


Please check the voltage on all pins of IC2.
The voltages should be about:
1: 1.6V
2: 0V
3: 0V
4: -5.2V
5: 0V
6: 0V
7: -1.6V
8: 6V

my check

1 0,6V
2 0V
3 0V
4 0,6V
5 0V
6 1,4V
7 0,6V
8 6V

It seems the negative supply voltage charge pump is not working.
Please check the transistor T4 is BC327 and T1 is BC337.
You may use this post as a reference to check the voltages:
The circuit diagram is for EDU09 but it is quite similar to EDU08.
There are different reference designators.

ok i check it…
thanks for the good support!

my results

T4 T1 ok

A = 1.67 V
B = 0.44 V
C = 5.32V
D = 3.09 V
E = 6.1 V
F = -0 V

All other values except E and F are ok.
The voltage on pin 4 of IC2 (0.6V) should be equal to voltage on point F (0V).

Also the voltage 6.1V at point E is strange, should be -2.6V.
The point F is the negative pin of electrolytic capacitor C27.
The point E is the negative pin of electrolytic capacitor C26.
Please check these voltages F and E on C27 and C26.

27 0,7 V
C26 6,22 V

Is the 6.22V same as the battery voltage on SW1 when turned on?

SW1 is 6.02 V

OK, no short to battery.
Very strange problem.
Before sending the unit to repair please check the diodes and other components of the charge pump section.
Especially check the ZD1 is not mixed with other diodes.
If everything seems to be OK but problem persists, you can return the EDU08 for inspection/repair to your local Velleman dealer or direct to:

Velleman Projects Tech. Dept.
Legen Heirweg 33
9890 Gavere

It seems that if D1 disconnected (or defective) the results are equal to your results.
Please check.

ok i will check it D1…

Please check also D2.
Same result if this diode is disconnected or defective.
Same result if either of the diodes D1 or D2 has wrong polarity.

D1 D2 and all Diodes ok
but no results

polarity also ok

Maybe there is trace break on the PCB.
Please check the continuity between C26 negative pole and D1 anode.
If OK, check continuity between D1 cathode and D2 anode and between D2 cathode and GND.

ok i will check soon

Hi VEL 255!

Great Job…the OSCI work now great!

The D2 cathode would have no GND while C26 was a broken pad soldering (see Photo)
after reSoldering of C26 is the Pad broken.

i have make a bridge at the other side of the board.

All Voltage are ok and i am happy!!!

Anyway very Good Job Vel 255
Thanks and have a nice Day

No problem.
Glad to be of assistance.
Enjoy your EDU08 :slight_smile: