Problem : Noise synchronised with LCD changes

MIDIbox FM
1

Hello,

After some time turning the knobs i have some noise synchronized with the parameter changes or simply with the cursor flashes.

It often geting louder and louder and start being real sound… often drums…

Sometime the opl3 seems to become instable and a very loud sound starts which does not stop untill i turn off the box.

Then i turn on and every thing is fine…

Does anyone has that kind of problem ?

Did i solder something wrong or is it a known problem because of the pins which are shared between the LCD and the opl3 module.

Thanks for your help,

Xavier

2

Are you running the YMF262 and YAC512 from a different regulator than the PIC? (You should if you didn’t)

Else, I think you’ll have to play with 74??? logic buffers to prevent the signals from going to the YMF262 when the LCD is receiving data…

3

What’s the reasoning for using a separate regulator?  Wouldn’t they be sharing a ground line anyways and thus get the same noise?

4

The ground noise won’t be shared if they are grounded properly, two separate regulators eliminates the noise the PIC and LCD are sending to the power rails. (You could put a 2 ferrite beads and 2 filter capacitor, one of each for the PIC, one of each for the OPL3 and that should work.) Als share grounds in one common point, don’t use a wire in series between multiple points.

5

Thanks for the reply but i don’t really get what you’re talking about.

regulator ? you mean the 7805 ?

I followed the typical Midibox schema…

I’d just like to know wether i’m the only one to have this problem… in this case i will try to investigate a wiring problem.

If it’s a known problem, i will be OK with it.

Regards,

Xavier

6

regulator ? you mean the 7805 ?

Yup!

Have you followed the grounding scheme?

Maybe you got a noisier LCD than other people.