S-Triggers from TR-707 Drum Machine

I have a Roland TR-707 drum machine and I have a couple of pre-MIDI Korg synths (a Mono/Poly and a Polysix).  Since there's nothing more fun than sweet acid-like basslines combined with propulsive trance-inducing drum beats, it's crucial that my old synths be able to synchronize to the drum machine.  This post is about modifying my 707 to finally make it control my synths the way that they should be controlled -- my 707 now outputs S-Trigs!  First, here's a demo...

Previous Attempts

TR-707 Trigger Out:  The 707 does have an output ("Trigger Out") whose purpose is to synchronize to external gear, such as the arpeggiators in these old synths.  As shown in a previous post, however, the built-in Trigger Out doesn't work for driving the Mono/Poly.  It does work with the Polysix, but it is late by 40 msec due to the V-Trig vs S-Trig issue (as discussed later).  So, in the end, the built-in Trigger Out doesn't work for my old synths.

Arduino MIDI-to-Trigger Converter:  Since the stock 707 Trig Out doesn't work, I hacked together my own MIDI-to-Trigger converter. This Arduino-based device listens to the MIDI clock output by any device (such as from the TR-707) and generates properly synchronized trigger pulses to drive my synths (two arpeggiators at once is fun!).  The problem, though, is that I don't like all the clutter of the extra wiring associated with this setup.  It's not ideal.

Modifying the 707 for S-Trig

V-Trig vs S-Trig:  Why doesn't the 707's built-in Trig Out work?  The problem is that 707 outputs "V-Trig", whereas my old Korgs expect "S-Trig".  What?  A V-Trig is a signal that is normally low (0V), which then pulses high (5V) for each trigger.  Unfortunately, my two old Korg synths expect an "S-Trig", which is the opposite -- a normally high signal that then shorts to ground for each trigger.  This difference between V-Trig and S-Trig means that my 707 doesn't play well with my Korg synths.

The 707 is Actually S-Trig Internally:  After looking through all the schematics, I noticed that inside  the 707, the circuitry actually utilizes S-Trig to trigger its internal drum sounds.  The only V-Trig signal generated by the 707 is the Trig Out provided to the user.  As shown in the schematic excerpt below, even this signal actually starts as an S-Trig that is then inverted into V-Trig by Q13.  So, given that all of these S-Trig signals are already inside the 707, I decided to modify the 707 so that it could output its S-Trig signals.

The TR-707 Trigger Out is "V-Trig", but internally it is "S-Trig"

The Simplest Plan:  Looking at the schematic above, one easy way to output S-Trig would have been to simply take the S-Trig from CPU pin 21 and wire it directly to the output jack.  But, this would have had two problems: (1) these old CPU chips are generally too fragile to be in direct electrical contact with the outside world, and (2) I would have lost the existing V-Trig output, which I might want to use with other gear in the future.  I need a better plan.

A Better Plan:  My better plan is to find a more robust source for S-Trig signals (ie, anything but the CPU) and to output the signal using other jacks on the back of the 707 (so that I keep the built-in Trig Out).  Which jacks?  Well, I never use the cassette tape interface, so I'll re-use those two jacks to give myself *two* S-Trig outputs.  Exciting!

Here's the back of the TR-707.  I'm not going to replace the built-in V-Trig.
Instead, I'll mod the 707 to output S-Trig on these two jacks that I never use.

Re-Wiring the Jacks

Where to Connect:  To use these two jacks, I need to disconnect the jacks from the existing circuit.  Looking at the schematic (excerpt below), the easiest solution would be to remove the resistor R51 for one of the jacks and to remove C30 for the other jack.  While this would work, it's usually a good idea to always have a series resistor in line with an output jack in case someone (ie, me) plugs in something stupid by accident.  The in-line resistor helps limit any currents that could damage the circuitry.  So, to leave a resistor in-line, I chose to pull the right leg of R51 and to pull the right leg of R43.   That disconnects the other circuitry while leaving the resistors in-place for me to re-use later.  It also leaves in-place the capacitor (C30), which I don't want, but which I can deal with later.

I can disconnect the two jacks from the existing circuit by de-soldering the legs of these two resistors.

Smell the Solder:  To disconnect the targeted leg of each resistor, I applied my soldering iron to the back side of the circuit board (PCB) and used small-nosed pliers to wiggle/pull on the resistor leg until it came free.  You can see each resistor's free leg in the photos below.  Later, I'm going to solder a new wire to the free leg, so make sure that the legs are still accessible.

De-soldering one leg of each resistor.  Make sure you de-solder the correct leg!

Getting the S-Trig Signals

Find a Source:  After preparing the output jacks, my next step was to find a good source for S-Trig signals.  Looking at the 707's schematic (see excerpt below), I see that the internal drum sounds are triggered by IC28 (a TC40H174P, which is a basic CMOS flip-flop).  This looks like a great place to get our S-Trig signals because, if IC28 were to get damaged as a result of this mod, this chip is much easier to replace than the CPU itself.  [Note that TC40H174P is no longer available, but a 74LS174 seems to do the same thing].  Let's get our S-Trig signals here.

Get the S-Trig from IC28, because that 40H174 is easy to replace if I damage it.

Soldering in the Wire:  To actually get this signal out, I could solder a wire directly to IC28, but my poor soldering skills mean that I risk damaging the IC by overheating it.  Instead, I'm going to solder my wire to R83, because resistors are very hard to damage.  You can see my soldered result below -- the red wire goes to R83 to get the trigger for the "Rim/Cowbell" sound.  For my second S-Trig, I chose to grab the trigger signal for the "High Tom" (the yellow wire, connecting via R87), but you can choose whichever sound you'd like.

Soldering wires to get the S-Trig signals that I want.

Connecting Back to the Jacks

The Connection Plan:  Now that I have the S-Trig signals on the red and yellow wires, I soldered those wires to the free legs of R51 and R43 (see schematic below).  These are the two resistors that I partially de-soldered from the circuit board to disconnect the jacks from the rest of the circuit.  By soldering to the free leg of each resistor, I've now connected the jacks back to the S-Trig signals produced by IC28.

Connecting the output jacks to the S-Trig signals from R83 and R87.

Jumper C30:  The last modification is to remove the effect of C30 (see schematic above) because it will interfere with the S-Trig signal that I'm trying to output.  The easiest way to "remove" it is to simply solder a jumper wire across its two terminals.  You can see the little jumper that I added in the picture below.  Easy.

I added a jumper wire across C30 to remove it from the circuit.  Otherwise,
it could interfere with outputting the S-Trig signal.

Tuck in the Wires:  The picture below shows my final wiring.  The red wire is connecting the "Rim/Cowbell" trigger from its source on the bottom-right up to the output jack on the top-left.  The yellow wire connects the "High Tom" trigger from the bottom-right up to the top-left.  I tucked in the wires between the components to ease my re-assembly of the drum machine.

The final wiring of the "Rim" and "High Tom" triggers from their source on
bottom-right up to the output jacks on the top-left.

Put It Back Together

With the wiring complete, I re-assembled the drum machine and tested it out.  I confirmed that the built-in "Trig Out" still works (using my Micro Brute, which accepts V-Trig) and I confirmed that my two new S-Trig outputs work (using my Mono/Poly).  You can see my test of the S-Trig outputs in the video at the top.  Hacking success!

I re-used the "Tape Out" and "Tape In" jacks as outputs for S-Trig signals.
The left jack is activated on the "Rim/Cow" triggers.
The right jack is activated on the "High Tom" triggers.