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.
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