[sdiy] TB-303 smps interference

[cheater00 .]

Hi guys,
So I've tried Robin's idea to bypass ground from the volume knob to
the headphone jack. Well, "tried" - I remembered later it's been
installed all this time. But still, the signal is unhappy and I get a
lot of buzzing. I've tried bypassing the headphone amp chip to no
effect. For details see my reply to Robin.

I've decided to try and investigate a little bit differently. I
normally check for the buzzing through a fuzz/compressor box, with my
headphones hooked up. It'll take tiny signals and make them very
audible.

I've hooked up a scope probe to the input of the fuzz. The clip was on
bat -ve, and the probe (in 10x mode) I've used as a radio "wand",
scanning around the PCB for signals that sound similar to my buzz.
Some places radiated quite well I must admit. On the keyboard there's
this jumper right at the end of the Bb key. There was a lot of that
buzz in that area, and then going up to the LED which was blinking (on
button 1 aka the C key). The left of the two wide flat cables was also
radiating a lot of that, and then the area between the uPD650C and its
memories. To investigate, I've hooked up directly to the voltage rail,
right before the diode that goes to the uPD650C. That buzz is right
there. So perhaps I should add better bypass to this chip?

The LEDs get their current from this chip, through the transistors on
the button board.

Perhaps I should try and (better) decouple those transistors too?

Should I watch out for anything specific when working with the
uPD650C? Is it particularly sensitive to ESD?

I've been thinking of adding some series resistance to its Vcc rail,
would this be a good idea? Say ~500 Ohm in series with the diode,
before the diode. Together with a good bypass this should lower its
influence on the other components...

Cheers,
D.