[sdiy] TB-303 humming noise problem

[Paul Anderson]

My suspicion is that the LED is being driven using pulse width modulation, and that's what you are hearing. Can you post a link to the schematic? There's probably a capacitor nearby that is in a bad mood. 

--------
Paul Anderson -- VE3HOP

On 2013-02-11, at 1:44 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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