[sdiy] TR-909 schematic/PCB error

rsdio at audiobanshee.com rsdio at audiobanshee.com
Sun Nov 13 07:23:59 CET 2016


Yes, in all senses other than engineering, redundant means unused or superfluous. However, I'm such an engineer that I can't help but think of the engineering sense of the word, especially when reading about an engineering topic.

In engineering, a redundant component is one that is not strictly necessary to functioning but included in case of failure in another component. Backup systems are redundant, and while we hope that they remain unused, we definitely depend on them being connected and capable of functioning when needed.

Brian


On Nov 12, 2016, at 10:06 PM, matt holland <matt at mattholland.org> wrote:
> 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.





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