[sdiy] TR-909 schematic/PCB error

[rburnett at richieburnett.co.uk]

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.

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,