[sdiy] TB-303 humming noise problem

[Oren Leavitt]

I would definitely suspect the switching DC-DC converter in the power 
supply section as a start point.

- Oren

On 2/10/2013 2:35 PM, cheater cheater wrote:
> Hi guys,
> the TB-303 which I have here has a fairly annoying issue. I was
> wondering if someone could help me figure out what to do about it.
>
> Short description: there's buzzing associated with the pattern
> selection LEDs blinking. Below I give a long description of the
> symptoms, then what my measurements showed, and then what I tried.
>
>
> The setup is, I have a nearly completely vanilla 303 (changed the
> output capacitors for larger, and changed the cutoff adjust resistor
> for a trimpot), and it is operating from batteries.
>
> When I  turn it on, the pattern selection LED is blinking. As it is
> blinking, when it's on there's a faint buzzing noise, around 1 kHz,
> audible through the headphone output. It's not audible if the
> headphone is connected to the line out; it can be heard if I use a
> headphone amplifier. It is actually an alternating sound: when the LED
> is on, it's a more buzzing, lower tone, and when the LED is on, it's a
> much fainter, higher tone (maybe around 8 kHz?). The faint high tone
> can be barely heard via the headphone output.
>
> If you e.g. press the pitch mode button and hold it, the selected
> pattern's LED stays on. The buzzing is constant at that point. Bear in
> mind that all the while the normal mode LED is on (constantly, not
> blinking).
>
> It does not change when I change any of the 6 small synthesis
> parameter knobs. Naturally when I change the tempo knob the LED starts
> blinking faster, alternating the sound more rapidly. The volume of the
> buzzing does not change at all when the volume knob is turned. The
> buzzing does, however, change in sound when I select another track,
> and when I select the pattern section (A/B). It does so in groups:
>
> section A:
> patterns 1, 2, 4: alter between high-pitched faint tone (led is off)
> and a sort of 5th tone, low (led is on)
> patterns 5, 6, 8: alter between same high-pitched faint tone (led is
> off) and the base tone of the 5th interval from above (led is on)
> patterns 3, 7: only the high-pitched tone (led is off and on)
>
> section B:
> patterns 1, 2, 4: alter between the 5th low tone (this time when the
> led is OFF) and a tone maybe half an octave higher, just as loud (led
> is on)
> patterns 5, 6, 8: there is a nearly constant pitch somewhere between
> the tones of patterns 1, 2, 4, however there's still some modulation
> of the tone when the led changes state
> patterns 3, 7: only the 5th low tone (both when the led is off and on)
>
> Notice that in both sections, the LEDs for patterns 3 and 7 are
> darker. Is this normal? Also the section A led is darker.
>
> This is what I've measured:
> 1. the BU terminal is completely clean, as is the +12V rail.
> 2. The 4.5 rail and the 6V rail and the 9V rail all have this buzzing
> easily visible on the scope. It turns on and off with the LED, and so
> on. According to the gain setting, the ripple should be about 5mV each
> for the "5th interval low tone" buzz, and only barely visible
> otherwise.
> 3. Testing across C65, the buzz is clearly visible. It is also clearly
> visible testing across C1.
> 4. The buzzing cannot be seen on either the line or headphone output,
> even on the highest magnification setting.
>
>
> Here are things I have tried:
>
> 1. new batteries, of course.
> 2. using a power supply. I tried several.
> 3. checking if the gnd cable from the switch board is attached
> correctly. I also tried crocodile leads either going between the same
> junctions, or going to the battery pack's negative terminal.
> 4. testing capacitors in the power supply. They all tested low ESR -
> between just under 1 Ohm up to 2 Ohm. Similar new elco's measured in
> the same range.
> 5. checking for cold solder joints. Tried redoing all I could
> identify, and can't find any more. I'm left with simply redoing every
> single solder joint.
> 6. placing a 10nF bypass capacitor across C65 (it's the one that the
> arrow points to on the PSU schematic, the "5" is unreadable). I
> believe it was an MKT.
>
> I would be thankful for any suggestions. This buzzing is ruining an
> otherwise perfect synthesizer, and I hope I can improve its operation
> somehow.
>
> Thanks,
> D.
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