Someone  reminded me  recently that  this circuit  should be  included here.  I
almost totally forgot about this one.

This circuit modification provided what I  call a  "NOT" output  on the  Yamaha
TX802. In essence it's a mono effects send which is controllable via the  patch
settings  or  via  MIDI.  However  it's done  in such  a way  that no  software
modifications  are  required.  My  TX802  is teamed  up with  a Yamaha  SPX50D.
Traditionally a Guitar FX processor which makes it ideal for  processing an  FM
box. They work together as single unit.

The problem:
The Yamaha TX802 provides as standard, a pair  of stereo  outputs labelled  "I"
and "II". It also provides a separate output for each of the voice pairs. 8  in
all of course. In software the  voice output  assignments are  as follows.  [I]
only, [I+II], [II] and [OFF]. In other words Left,  both, right  and none.  The
problem is that there is no control over the 8 individual outputs what so ever.
So that even if you mixed them all together using a simple mixer,  there is  no
way of assigning them to an effects  unit. They  are always  on and  delivering
their respective voices to the output. Even when the voice is switched off from
either of the two stereo outs. What's really required is a single FX send which
all 8 voices can be assigned to as needed.

This  modification  provides  this  by  inverting  the  logical function  which
assignes to either of the two stereo ports.  Basically when  it's "NOT"  coming
out of left or right, it comes out of the "NOT-OUT". Thus when it reads on  the
TX that a voice channel is turned off, it's actually assigned to the  "not-out"
It can be programmed into a patch and requires no modification to the  software
and only some relatively simple hardware.  I've used  one of  the 8  individual
output jacks as an actual port but if  desired you  can build  an entirely  new
one.

NOTES ON CONSTRUCTION:
The TX802 uses a matrix of analogue switches. part number  NJU7301D Similar  to
CMOS bilateral switches such as the 4016 and 4066 but capable of running from a
+/-15 volt supply. (30 volts total) These have far superior properties to  that
of the cheap CMOS devices but have a price tag to match. Further you won't find
them at any parts dealers. Well none this side of Japan anyway. However, as  it
turns out, they are totally identical to the Harris Semiconductors DG201  which
has been around for years and as common as mud cakes. They're still not exactly
cheap but you can get them anywhere. Well perhaps not at RAT SHACK but anywhere
else.

These switches live  on the  output of  the samplehold/alias  filters for  each
voice channel. They assign these outputs to one or  both of  the stereo  output
ports on the back. The control logic is derived from one  of two  octal D  flip
Flops. IC 64 and 65. TC40HC374. (74HC374) One of these flip flops controls  the
assignments for the [I] output  and the  other assigns  to the  [II] out.  This
matrix  of switches  is then  followed by  a simple  mixer/inverter which  then
drives the respective outputs.

The NOT-OUT duplicates one bank of 8  switches (2  DG201s) and  a single  mixer
channel as mentioned above. The  circuitry is  almost identical  to that  which
already lives in the 802. All except that it's control logic  passes through  a
bank of NAND gates. The switches are on when the logic is low and off when  the
logic level is high. Thus to switch both [I] and [II] outputs off, all  latches
must be high. The nand gates invert this function but only of the corresponding
outputs from both flip-flops are high. Then the nand gate will go  low and  the
not-out switch will turn on.

I built mine on  a piece  of my  custom matrix/doughnut  board that  I make  in
house. You could use vero board or what ever you can get. The  hardest part  is
wiring it to  the TX802's  circuit board.  Normal CMOS  static precautions  are
required so that you don't blow up your 802 in the process. There are a lot  of
wires to connect so I used rainbow ribbon cable. I soldered to the underside of
the 802's circuit board. I did this so as not to have to wire  directly to  the
ICs themselves. However this requires the removal of said circuit board. I then
tacked the wires in  place with  a dob  of supa-glue.  Not too  much now.  This
simply insures that if you do have to replace the ICs for any reason, you don't
also have to rewire all that ribbon cable. AT the other end of the ribbon cable
I crimped  on a  standard IDC  connector and  soldered it's  reciprocal to  the
doughnut board. Wiring it into the new stuff from  there. This  means that  you
can remove the board at any time without having to unsolder the Ribbon cable. I
used 32 conductor cable so that it could also carry the powersupply rails  (+5v
+/-15v and GND) as well as the audio. I didn't wanna be havin' to unsolder that
sucker every time or during debugging.

I disconnected output number 8 from it's  separate output  and used  it as  the
not-out port. This saved me drilling a hole in the back of  the 802  for a  new
one. I also used it's associated de-thumping transistor. This is simply  pulled
to ground on boot-up or power down such that you don't hear a huge thump as the
power comes on.

As it  happens my  wiring of  the cable  with IDC  etc was  justified. I  later
decided to also return the SPX50D's stereo outputs back to the 802. Where it is
mixed with the [I] and [II] outputs. Such  that the  whole thing  comes out  as
one,  pre-mixed  stereo  block  complete  with  FX  as  required.  This   extra
modification  isn't  shown.  It  was  just done  as required  and probably  not
necessary for most people performing this MOD. However if you need to do  this,
it's fairly simple. I used an extra mixing Op-amp  for one  reason or  another.
You can use a simple 10K resistor to pin 2 of ICs 58 and 59 respectively. IC 58
is the mixer for the [I] output and IC59 is the mixer for the [II] output.  You
may or may not  also wish  to pass  this through  a non-polarized  electrolytic
capacitor of about 10uF for DC blocking. Once again I used output ports 7 and 6
as the returns for  the SPX50D  since I  was making  the mod  so as  it not  be
necessary to use any of the individual outputs.

Of course no responsibility will be taken if you damage or destroy your 802  in
the process of this Mod. I expect that  you would  have a  reasonable level  of
competence in electronics before you'd even attempt  such a  thing. Mine  works
just wonderfully. Your's should too.

View or Download Schematic in GIF file format