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Roland Rhythm TR-77 - how it works.
 
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March 2010

The "Roland Rhythm" TR-77 was Roland's very first product, from 1972. It makes its distinctive drum voices from what are called "tuned resonant" circuits, which consist of a capacitor, an inductor and a resistor - very basic!. The patterns are preset rock, jazz and latin styles of 1 or 2 bars. There are no user-programmable patterns, because there is no "memory" - the patterns are hard-wired. That is, they are literally made from a matrix of wires between the transistor flip-flop counters, connected by diodes, to the inputs of the voice circuits, depending on which buttons on the front panel are pushed in (Pushing one in causes the others to disengage, but if you're careful you can manage to get more than one to stay in, giving you a mix of patterns.)

Here's a demo video: first I start the metronome, then touch the start plate to kick the rhythm off, then hit the fade button for auto fade out... back to the metronome and through a few combination of rhythms. Notice it takes a certain pressure to keep more than one button latched down!

 

My main motivation for investigating the gubbins of this old beast was to see if there was a way to synchronize it to a source of external clock. I also wanted to know if there were any good voice mods possible. But another reason was that I wanted to understand how this basic type of pattern sequencing worked, because my amateur electronic explorations hadn't really covered this so far.

77

Above: Photo from the rear with the heavy wooden top/end cheek/music stand assembly flipped up to the front. At the bottom right corner I've taped a cardboard box lid over the transformer and mains terminals (WARNING: LETHAL VOLTAGES HERE) for safety while working on the machine.

Drum Voices

There are thirteen voices: Bass drum, low conga, low bongo, high bongo, rim shot, cowbell, claves, tamborine, cymbal, high-hat, maracas, snare drum and guiro. The first seven are based on tuned LC circuits or "tanks", that "ring" when struck with a brief negative pulse of about 5 to 12 volts. Of these, the cowbell uses two detuned tanks. The tamborine, cymbal, high-hat and maracas are triggered pulses of filtered white noise (supplied by a common transistor), with different envelope times and filters. The snare is a mixture of both a noise filter and an LC tank.

 

Above: video demonstrating the voices (except guiro), triggered from the points along the front edge of the voice board. The other end of the wire in my hand is clipped to matrix pattern line no. 21 on the logic board. At 00:52 I start a latin pattern, and "solo" on some of the voices with the wire!

Above: Schematic of the voice circuits (copyright Roland Corp) from the service manual. For (only slightly!) more detail, download the PDF from http://www.burnkit2600.com/tr-77/ The left column shows the LC tanks, the middle the triggered noise circuits with the guiro at the bottom, the right shows the noise source, pre-amp and the single VCA chip.

The guiro is completely different. It is itself a two-transistor multivibrator, i.e. an oscillator, who's output is summed with some white noise and fed to a bandpass filter. It has two control inputs that, unlike all the others, require a positive voltage of about 5 volts. One input triggers the sound itself, the other raises it's pitch periodically in the pattern cycle. These positive CVs are provided by pattern numbers 6 and 12 respectively on the logic board.

 

Above: guiro oscillator being triggered by positive CVs. First by a wire connected to matrix line 6 on the logic board. Then by a square wave from a signal generator at 1 hz, then 10 hz. You can hear slight pitch changes throughout. This is because the pitch CV input is hardwired to matrix line 12 (red wire joining the board near the pale blue cap) causing a permanent cyclical kick upwards in pitch (see chart below).

Most of the trim pots on the voice board simply change the output volume of the voices, which is important because there are only four separate voice volume sliders on the front panel (- bass, snare, guiro and a combined maracas-high hat-cymbal volume -) but not very exciting from a sound modification point of view. The snare pot increases the mix of tuned sound to white noise, the rim pot increases the aggressiveness of the sound and also seems to add a slight echo (!). The guiro pitch trim doesn't do much. Much more dramatic effects can be had by bridging the caps in some areas. The guiro can become quite gnarly by increasing the resonance of the BP filter (cap C371). I found by bridging the cap across the combined output of the "noise" instruments (cap C356, to 470pf I added a 220nf) there was a great improvement in the quality and clarity of their top end, so I kept this in.
Also, some sounds have a slight "accent" response to different amplitudes of trigger voltage, namely the bass, snare and rim shot.

Something to keep in mind with any machine like this that uses lots of inductors in it's sound generating circuits - it's a fantastic amplifier of HUM! Make sure you keep other units or wall-warts well away, especially from the rear right corner where the voice board is.

One last audio feature worth mentioning is the single IC to be found here. Because the TR-77 is ALWAYS running when the power is on, Roland used a VCA chip to gate the audio output on and off. A transistor switch triggered from the start bar causes a change from the OFF voltage (about +7.5 v) to the ON voltage (about +3.0v). Even better, by slowly changing this control voltage by charging a capacitor, it allowed the wonderfully wacky "fade-out" feature! Those seventies cocktail bars would've been gobsmacked - "it sounds just like a record!"

Above: the TR-77's lonely VCA chip.

Above: cap 356 bridged. To the right is the snare trim pot. The black disc is one of the many inductors on the voice board that make up the tuned "tanks".

Sync and Timing

The timing information in the TR77 originates with the master oscillator, a two-transistor multivibrator, which generates a square wave (about 5 volts or more) who's frequency is controlled by the tempo slider on the front. This frequency is four pulses per quarter note (i.e. sixteenth note division), and is fed to the first of five flip-flop stages (also made of two transistors each), which each act as a divide-by-two counter. So the first counter, output Y, counts two sixteenths and so completes a cycle in an eight note time division. This is fed to the next counter, output X, which counts two eighth notes and completes a cycle in a quarter note. This chaining continues so that output W cycles per half note, output V cycles per whole note (1 bar) cycle, and finally output U cycles every 2 bars. These are all square waves of about 10 volts p-p. Touching the start bar or the restart footswitch causes the master oscillator and all the flip-flop's voltages to be slammed briefly to ground, causing the count to start over (see the video below). One other thing to note - each counter's square wave output has it's inverse waveform also output (e.g. for Y, there is Y' .... possibly pronounced "Y dash" from my vague maths recollection).

 

Above: short video of the scope showing the effect of the reset circuit on the master oscillator (top trace) and the "V" counter (bottom trace - cycles one square wave per bar or 16 sixteenths). You can hear me hit the touch bar quickly starting and stopping the pattern a few times before letting it run.

Now here's the clever bit: it's by the ingenious combination of two or more of these various counter outputs that the pulse patterns are derived to trigger the voices. This is the "matrix"! (It's shown on page 10 of the service manual). If you're tired of sloppy timing in your drum machine patterns, then maybe you need to start hard-wiring them like this!! The connections are via capacitors, diodes and resistors - very simple (see the edge-detector circuit drawn below). These patterns are presented at the numbered terminals along the front edge of the logic board, demonstrated in the video below..

Above: Some of the matrix patterns available on the logic board. The other end of the wire I'm holding is clipped to the cowbell trigger point on the voice board. No other instruments except a four/four bass drum pattern are sounding.

Page 11 of the service manual (below) shows the graphical representation of the pulses present at each matrix line over a two bar period (32 sixteenth notes).
Note line number 6 and line number 12 are specifically for the guiro as mentioned above. They provide positive 5 volt gates, not negative pulses.

Above: TR-77 logic timing chart (copyright Roland Corp)

External Sync Mod

Initially I figured I'd need a divide-by-six circuit to get the 24 ppqn of standard (Roland) din-sync down to the required 4 ppqn that the TR77 expects to see. I breadboarded up a circuit using CMOS chips, a 4027 and a 4013, but I didn't have a great deal of luck - sometimes it worked, sometimes it didn't. I suspect the chips didn't like the quality of the power I was feeding them from the TR77. There are NO voltage regulators in there! Just some basic filtering after the transformer. Also, there was the problem of how to set the start voltage accurately each time with the first clock pulse.

Above: Resistor R4 (clipped off the board) where I accessed the master clock input.

I got round this by using the output from one of my Kenton units' ARP CLOCK, which can be set to give a beautiful symmetrical square wave in sixteenths time that is rock-solid and starts bang on with the RUN/STOP voltage of din-sync every time (Kenton to the rescue again for me!).

(update April 2010: I'm now using the trigger out from the Innerclock Sync Lock, which runs at 4 ppqn and has the added benefit of adjustable swing!)

I converted one of the switched mono phone jacks at the rear of the unit into a clock input, and the restart jack as a run/stop input. The clock wire was taken to the logic board at resistor R4 (the other side of which is connected to the base of Q1 of the master oscillator). The run/stop was connected as shown below in (a)...

Above: the RUN/STOP input with flying transistor!

At (b) is a negative-edge detector circuit, found repeatedly throughout the matrix to provide the negative pulses for the voice circuits.


Without the benefit of a programmable clock pulse from a midi-to-CV converter such as a Kenton, a CMOS counter circuit with it's own power supply and buffered output would probably be the best bet.

Possible Creative Mods

Join in the discussion over at http://www.burnkit2600.com/tr-77/ if you have any suggestions. At least one thing is clear from having a close look at these old machines - even if you don't have access to one, it shouldn't be too hard to make the circuits from scratch yourself!

One popular idea has been to bring out the trigger inputs to the individual voices for external access. However, that means you need a source of multiple negative pulse triggers controllable from your sequencer. Which leads to the next idea, of also bringing out the matrix lines as well, giving you cross-patchable patterns for each voice. I'm sure you could even just twist a couple of wires together to combine the rhythm lines for any voice, as they all are diode protected.

Serious sound mods may require switched capacitors and/or inductors, not simply pots. Inductors like these could be hard to find - although I've heard they're still obtainable from suppliers of parts for old Thomas organs and the like. Mods like these could start to take up a bit of space. But by removing the wood cover and replacing it with perspex or sheet metal, one then gains a huge area of real estate for pots, switches, and even a patchbay or pin matrix, with room for sound-mangling circuits on the underside!

 

 

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