[sdiy] TB-303 humming noise problem

[cheater cheater]

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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