[sdiy] TR-909 schematic/PCB error

rsdio at audiobanshee.com rsdio at audiobanshee.com
Sat Nov 12 23:34:36 CET 2016


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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