[sdiy] TB-303 humming noise problem

cheater cheater cheater00 at gmail.com
Mon Feb 11 11:56:22 CET 2013

I think I may have found something. I'll check out that exact fix.
Also one of the posts above mentions the mix-in jack normalization
terminal being dirty, so I'll check that out as well..


This noise is a common issue, it can be heard on some 303s more than others.

Usually you can structure your gain staging to reduce the noise to
tolerable levels, such as setting the 303 to maximum volume and
attenuating your mixer/audio interface.

I have seen the problem go away with a replacement Volume
potentiometer or you can try the solution that Robin Whittle has
documented for the Devilfish modifications:

"[January 2012. Starting with SN 260.] Some, many or all TB-303s have
a low-level buzz in the background, irrespective of the Volume pot
While this is well below ordinary signal levels, it might be annoying.
The buzz from LED activity – especially with four LEDs on at once,
such as when selecting patterns 1, 2, 3 and 4 – is apparent when
running from batteries. A higher frequency buzz, from the Interrupt
oscillator, may be audible when running from an external power
The lower frequency buzz problem seems to be most apparent with the
original LEDs or perhaps with any Red LEDs. Blue LEDs have a higher
threshold voltage and draw less current.
The cause is ground loop problems within the machine. The fix is a
wire between the Output socket ground terminal and the ACW (ground)
terminal of the Volume pot."


On Mon, Feb 11, 2013 at 7:44 AM, cheater cheater <cheater00 at gmail.com> wrote:
> Hi Oren,
> I thought so too, but the frequency at which the flyback works is very
> high, whereas this is in mid acoustic range.. I was wondering if the
> logic chip, which finally turns the LEDs on and off, took up so much
> current?
> Cheers,
> D.
> On Sun, Feb 10, 2013 at 10:12 PM, Oren Leavitt <obl64 at ix.netcom.com> wrote:
>> 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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