K8031 problem

Hello. I have a problem with my K8031.
I connected input to J4 and all is OK
clan-nt.narod.ru/8031/k8031ok.JPG

I scroll Volts/DIV for 1 point and got that:
clan-nt.narod.ru/8031/k8031_3v.JPG
Vrms now 3V insted of 2.5V

I scroll Volts/DIV some more points down and all ok again:
clan-nt.narod.ru/8031/k8031okagain.JPG

I scroll Volts/DIV some more points down and got trash:
clan-nt.narod.ru/8031/k8031_loves.JPG

Now I scroll Volts/DIV up, some points above middle of scroll bar I got that:
clan-nt.narod.ru/8031/k8031_2v.JPG
Vrms now 2V insted of 2.5V

I scroll some points up and got trash:
clan-nt.narod.ru/8031/k8031_trash.JPG

I scroll some points up and all is OK once again.
clan-nt.narod.ru/8031/k8031_okagain2.JPG

I used PCLAB from CD and 3.06 from this site,no difference.
What is wrong? It is PCLAB’s bug? Or problem is in my K8031?

Please help with my problem.

ps. sorry for my terrible english. you can answer in russian too.

It seems that there is same problem as discussed in the post “K8031: High level spikes on the trace at high TB speed”:
forum.velleman.be/viewtopic.php? … 93c006caa4

You may try the same solution.

this solution not helped
any other ideas?

It seems that the AD converter is defective. You have to change the TDA8703.
Please read the instructions how to proceed:
velleman.be/be/en/support/warranty/

New screenshots
I connected external Sine generator to k8031 and got this - when volts\DIV is in upper part of scroll bar - all is OK

when it is in middle i got trash in 1 point

when it is lower then middle i got trash in 2 points

at last when i connect probe terminals to each other i also got trash in several points of volts\DIV scroll bar

[quote]* removed the TDA8703

  • bended up pin 19 so that it will not make contact in the socket any more
  • soldered (on the IC pins near the IC body) a resistor of 22E between pins 18 and 19
  • soldered (on the IC pins near the IC body) a capacitor of 100 nF between pins 19 and 20 with as short as possible leads
  • placed back (the modified) TDA8703[/quote]

this solution do not helped me… any ideas?

I connected external analog scope to TDA8703’s pin #8 and got there signal without trash. Is my TDA8703 bad?

i bought k8031 in another town and it will be very difficult to return it. can i buy tda8703 from you directly?

Before replacing the TDA8703 please check:

There may be soldering bridge between two pins of the AD converter. Or there is bad contact (soldering) in one of the output data signals from the AD converter.

Check all the data signals (soldering, possible soldering bridges) from the AD converter to the RAM IC5 and to IC6.

OK thanks for help, i will try it

I just checked all soldering points and connections betvin ic 4 ic5 ic6 - all just fine… I can-not understand whats wrong

It seems that everything else is OK but the AD converter…
Closer look to the screenshot images, shows quite clearly that the AD converter TDA8703 is defective.
It seems to be working OK only when the trace is at the upper section of the screen.

Hi,
A TDA8703 does not cost very much (9 Euro’s) and is (at least here in Ostend) easy to obtain in an ordinary “electronic components” shop. :smiley:
So, it should be easy to test if the TDA8703 is defective or not (if yes: your problem is solved, if no: you have a spare TDA).

i changed tda8703 and all is OK now! thanksl for help.