[sdiy] TB-303 humming noise problem

Mon Feb 11 09:00:10 CET 2013

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Csaba 

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On 11/02/2013, at 08:45, Paul Anderson <wackyvorlon at me.com> wrote:

> 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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