[sdiy] TR-909 schematic/PCB error

[matt holland]

Agreed with the thanks and fwiw understood that redundant really meant
"unused".

mh

On Sat, Nov 12, 2016 at 2:34 PM, <rsdio at audiobanshee.com> wrote:

> Great work! Thanks for sharing your notes, particularly for taking the
> time to make those images and describe things in detail.
>
>
> On Nov 12, 2016, at 6:21 AM, rburnett at richieburnett.co.uk wrote:
> > Just a quick Email to let you guys know about an inconsistency/error I
> discovered in Roland's TR-909 drum machine schematic vs the PCB layout.
> The error concerns the circuit for synthesis of the rim shot voice and
> doesn't affect any of the other sounds.
> >
> > The schematic shows the final output from the voicing circuitry being
> taken from the output of a high-pass filter consisting of IC50b and it's
> surrounding components.  This is marked with a green arrow in the service
> notes schematic...
> >
> > http://www.richieburnett.co.uk/temp/909rs_schem.gif
> >
> > However this is not the case.  The output from the rim shot voicing
> circuitry in the TR-909 is actually being taken from the output of IC50a
> (marked with a red arrow on the service notes schematic.)  This means that
> the rim shot sound that you hear from the individual instrument output, or
> the stereo mix outputs, hasn't passed through the high-pass filter
> circuitry.  In fact these 4 resistors, 2 capacitors and op-amp in the
> yellow shaded area are totally redundant in the TR-909.
>
> Redundant? I think "unused" would be the most accurate. The engineering
> sense of redundant includes things that function when something else fails,
> but in this example there is no way that this particular high pass is ever
> used.
>
>
> > I had suspected this for some time because my first digital model for
> the rim shot based entirely on the schematic did not produce the correct
> wave-shape for this instrument.  However, when I left out the routine for
> the final high-pass filter stage, it matched my TR-909 perfectly.
> >
> > I sold the original drum machine many years ago, but this discrepancy is
> easy to see in the PCB artwork of the service notes.
> >
> > http://www.richieburnett.co.uk/temp/909rs_pcb.gif
> >
> > The RS sound leaves the voicing board at the top edge on pin 38 of
> connector W5 highlighted with a red arrow.  The track meanders down the PCB
> to the RS level pot (red arrow) and then goes leftwards and back up the
> board to where it's connected to the output of op-amp IC50a (red arrow.)
> As if any more proof is required, the high-pass filtered output from op-amp
> IC50b (blue arrow,) is connected to just R421, but is going nowhere else on
> the board!
> >
> > I've never heard this error mentioned before, so it’s likely the
> designers of hardware clones just copied the schematics from the service
> notes without checking them.  For instance, Trevor Page's TR-9090 schematic
> shows the output being taken from the high-pass filter output, which
> doesn't match the real drum machine.  A couple of VST software clones I
> checked also both appeared to have included the unused HPF in their models!
> >
> > Please feel free to link this post or forward to any forums discussing
> the TR-909 if you think they would be interested.
> >
> > -Richie Burnett
>
>
>
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