EDU08 Display Problems


#1

Hi,

I may have a similar problem like described in previous posts. I’ve assembled the EDU08 and when switching it on, the screen backlight works, but nothing is displayed on the LCD itself.
In my case the screen backlight is unstable and flickers sometimes, but not always.

There are also very strange Voltage readings at SK1.
I measured with the unit switched on, and the ground test lead attached to one of the holes for the screws, which is ground if I’m not mistaken.

I got this:

top row. left to right:

3.3 3.1 3.2 3.2 3.2 1.1 1.2 0.0 0.5

bottom row, left to right:

0 3.3 3.3 3.2 3.2 2.0 0.7 0.8 1.7

I would really like to understand what I did wrong, so I would appreciate if you could guide me to troubleshoot the unit.

Thanks & best regards

A video to show what happens when switching it on:
http://www.callistix.net/vellmann_edu08/MVI_0038.MOV

Some photos of the assembled board, and the way I measured at SK1:





#2

It seems the RES (reset) signal on the display is low.
This disables the display operation.
This RES signal is on the second last pin on the bottom row of SK1.
You got 0.8V but there should be 3.3V.
Please remove the top board and check the voltage on this pin on the bottom board connector SK7.


#3

[quote=“VEL255”]It seems the RES (reset) signal on the display is low.
This disables the display operation.
This RES signal is on the second last pin on the bottom row of SK1.
You got 0.8V but there should be 3.3V.
Please remove the top board and check the voltage on this pin on the bottom board connector SK7.[/quote]
Hi team,

Measuring the pin on the lower board with detached top board I get a stable 2.6V.
Measuring again on the upper board while it’s attached to the lower board I get somewhere between 0.8V and 2V, fluctuating (which might explain why the LED backlight flickers?).

EDIT

I also noticed that the Ampere needle of my power supply jiggles, as if the circuit would draw an unstable current.
I use a lab power supply instead of batteries, which is set to exactly 6V DC - but I also tried batteries before with the same problem.


#4

[quote]Measuring the pin on the lower board with detached top board I get a stable 2.6V.[/quote]There should be 3.3V.
Please check that the battery voltage on SK6/SK5 is 6V.
If yes, then check the voltage on both ends of R20 (and D8).
There should be 3.3V on both ends of R20 and so also on the second last pin on the bottom row of SK7.
Please check the circuit diagram to see how this pin (#15 of SK7) is connected to R20.

[quote]Measuring again on the upper board while it’s attached to the lower board I get somewhere between 0.8V and 2V, fluctuating (which might explain why the LED backlight flickers?).[/quote]This may indicate a bad connection or loose solder joint somewhere…

[quote]I also noticed that the Ampere needle of my power supply jiggles, as if the circuit would draw an unstable current.[/quote]This may also indicate a possible solder problem (solder bridge or loose connection) somewhere.

Does this occur also with detached top board?

This may also indicate that the board is drawing excessive current. This may overload the 3.3V regulator and the voltage drops randomly.


#5

top board attached

SK1 (top left to bottom right):

3.3 0 0 0 0 0 3.3 0 0.5 0 3.3 3.3 0 0 0 0 1.7 0.1

SK5/6: 6V

R20:
3.3V (pin facing away from SK7)
1.7V (pin facing SK7)

D8:
3.3V (pin facing away from SK7)
1.7V (pin facing SK7)

top board detached

SK7 (top left to bottom right):

3.3 3.3 3.3 3.3 3.3 0 3.3 0 0 0 3.3 3.3 3.3 3.3 3.3 0 3.3 1.2

SK5/6: 6V
R20: 3.3V (both pins)
D8: 3.3V (both pins)

Many thanks for your help so far :slight_smile:
Best regards.


#6

Seems my account gets blocked again and again, could you please fix this?


#7

The previous voltages are taken again after removing excess flux from the board, which apparently interfered with the measurements.
All Voltages, Ampere readings and LCD backlights are stable now, no fluctuations!

But the low voltage on SK1 is still there.

I’ve also noticed that they are entirely different depending if the top board is attached or not, so I measured both.

Thanks for your help so far :slight_smile:
Best regards.


#8

[quote]SK1 (top left to bottom right):

3.3 0 0 0 0 0 3.3 0 0.5 0 3.3 3.3 0 0 0 0 1.7 0.1[/quote]
All the voltages are OK except on pin 15 (1.7V), where should be 3.3V and on pin 17 (0.1V), where should be 0.0V.

[quote]The previous voltages are taken again after removing excess flux from the board, which apparently interfered with the measurements.[/quote]If this was the cause of the previous problem, there may still be some remaining flux under the SK1 connector causing some conductivity and pulling the voltage of the pin 15 too low and cause also voltage rise on pin 17, due to possible leakage current.

You may “gently” try to force the pin 15 to higher voltage by following way:

  • Solder (about) 10k resistor in parallel with diode D8 on the bottom board.
  • Attach the top board and check the voltage on pin 15 of SK1.
    If the voltage increases, this may indicate there is still some excess flux on the board.

Also the voltage values when top board detached are all OK except the last one (1.2V), should be about 0.3V.

[quote]SK7 (top left to bottom right):

3.3 3.3 3.3 3.3 3.3 0 3.3 0 0 0 3.3 3.3 3.3 3.3 3.3 0 3.3 1.2[/quote]

Maybe also on this board there is still some flux, but the main cause of the problem may be on the top board.


#9

I’ve desoldered SK1 (which was super difficult to do, but I finally managed), cleaned everything and resoldered it.
Unfortunately it makes no difference.

[quote=“VEL255”]You may “gently” try to force the pin 15 to higher voltage by following way:

  • Solder (about) 10k resistor in parallel with diode D8 on the bottom board.
  • Attach the top board and check the voltage on pin 15 of SK1.
    If the voltage increases, this may indicate there is still some excess flux on the board.[/quote]

I started with a 10k resistor, which slightly increased the voltage, but I had to go all the way down to a 180 Ohm resistor to get 3.2V on pin 17.
Unfortunately there’s still nothing on the LCD, although I’ve noticed one or two horizontal lines with some black pixels in one occasion.
They have disappeared though, and the LCD shows nothing again.

I’ve cleaned everything very carefully, except underneath the IC sockets, the relay and the SK7 connector.
I’m afraid to damage these parts trying to desolder them for cleaning, so I’ve measured the resistance between adjanced pins with a multimeter instead to find short-cutting pins, without finding anything.


#10

I am very sorry for the inconvenience.
It seems the problem is not the flux.
There seems to be other problem.
You can return the kit for inspection/repair.
If you are a US resident, please call (817) 284 77 85 for RMA#
If not, please return the kit to your local Velleman dealer or direct to:

Velleman Projects Tech. Dept.
Legen Heirweg 33
9890 Gavere
Belgium