TR-77 midi interface and rackunit casing

TR-77 in bad condition
TR-77 in bad condition

I happened to get a Roland Rhythm 77 in quite bad condition from an online auction. I dont want to know what happened to this poor device, i was only interested in the voiceboard. This was the perfect candidate for the first real world interfacing of my Midi-Trigger-Pulse interface.

To get more common information about the TR-77 you can read my article from 2014 ‘Viscand 18’ – Cloning a Roland® CR-78. Very much info about the TR-77 about modding and triggering is also offered by Burnkit2600 and Circuitbenders

Power supply

Circuitbenders offers many interesting facts about the TR-77’s power supply. The voiceboard really needs at least 18 V for a clean sound. It also works with 12 V but the sounds are slightly distorted then and have a peak level. To make things easier i got a 23 V wallwart. The midi interface also gets it’s powersupply through 23 V. There is a 5 V regulator onboard for the microcontroller. The interface needs very little current, the regulator still keeps cool although it has to convert 18 V into heat.

Glueing bolts into the case
Glueing bolts into the case

19″ casing

The original TR-77 case was quite large and there seemed to live some tiny animals in there already. I decided to throw it away and got a 19″ case. PSU jack on the backside, all others to the frontpanel: Midi input, illuminated ‘Learn’ button (for Midichannel and firmware updates), 5 level pots for Volume, BD, SD, HH/CYM and GUIRO. Snare and HH/CYM also got some normalized single output jacks. The faceplate is a simple inkjet printout glued onto the aluminium front with glue from the spray can and a finish with transparent adhesive tape. For the metal bolts to attach the PCBs i used 2 component glue. No overlaying screw heads on the outside of the case.

TR-77 Midi Interface
TR-77 Midi Interface

Midi Interface & GUIRO

The circuit for the trigger converting (+5V to 12V negative trigger) is identical for all instruments (see Circuitbenders) except Guiro. Instead of making a short drumsound, Guiro provides a permanent chirping sound in 2 different frequencies. The PCB offers 2 pins: one is labeled “GUI” and switches Guiro on (+7 Volt) and Off (0 Volt). The other pin is labeled “5” and determines if Guiro is played with low (0 Volt) or high frequency (+9 Volt). Besides the 12 trigger outputs, the interface also supplies 2 special outputs which will be set HI (+5V) or LO (0 V) by incoming midinote. Using a transistor and some resistors the signal of the interface can be connected to the Guiro pins. As long as C5 is held, Guiro will be played. If C#5 is played in addition, the Guiro frequency gets high.

Level control and single outputs

TR-77 finished
TR-77 finished

The TR-77 got some sliders for setting volume of the complete mix and individually for Bassdrum, Snaredrum, Maracas/Cymbal/HiHat and Guiro. The 4 sounds are leaving the voiceboard through pins 1 (BD), 2 (HH/CYM), 3 (SD) and 4 (GUIRO). The signals are running to 10k sliders (or pots) and back to the voiceboard to the input pins 6 (HH/CYM), 7 (SD), 8 (GUIRO) and 9 (BD). You can as well connect the appropriate in/out pins directly if you like. Snare and HiHats got their own normalized single output jacks. If something is plugged in, the sound disappears from the mix.

VCA input and cap to desolder
VCA input and cap to desolder

Velocity & Accent

The circuit of the TR-77’s drumsounds is not – like in the Drumbrains of Pearl, Simmons or Tama – layed out to be played dynamically over midi velocity. But there still is a small range between “Sound does not trigger” and “Sound triggers clean” where the sound reacts to different trigger lengths. Luckily the HiHats work quite well here.

Now the soundboard also got a CA3080 VCA for the output signal. Normally it will be muted by feeding 17V into pin “FC” as long as the machine is stopped to prevent noises and zippering reaching the output. Pulling FC to GND opens the VCA. As there are no clock generators, diode matrixes or bad power supplies any more, the VCA could be misused to build a simple accent unit. Unfortunately i dont have a schematic of the voiceboard-version with CA3080, my scheme only shows some MFC6040. Nevertheless i noticed a cap connected to the VCA control input for stabilizing or ‘softening’ the control signal. It seemed to be responsible for the accent to sound weak and wrong. After getting it out, the accent worked much better (see photo for position of the cap).

Guiro and Accent helper circuits
Guiro and Accent helper circuits

To generate the accent signal i used the RUN/STOP pin of the interface. As soon as midi velocity exceeds 90, the interfacepin provides a short pulse. A little helper circuit lifts the +5 up to +12. It’s nearly the same as for Guiro except there is an additional 10µ elco for longer decay on the falling edge. The output of the accent circuit is HIGH per default and will be pulled to GND when accent happens (opening the VCA). As we want to manually control how much accent we need, we must have an Accent-pot: the first pin is on GND, the accent signal goes to the 3rd pint, the middle pin is connected to “FC” (VCA control input). Fully CCW “FC” is pulled to GND completely opening the VCA, the default state. Now the more the pot is turned clockwize, the more the VCA will get closed, opening shortly on accents. In fact it is not areal accent, but more kind of a leveldrop when no accent is present.

This works quite well although the accent is fully there half the way of the pot. With accent fully tweaked, sounds without accent completely disappear. This is a cool feature as it supplies blending between a simple pattern (only accented sounds) and a more crowded/dense version of the pattern (all sounds). Other feature: when setting all sounds to full velo, the accent pot allows shortening the decay of the voices as the sound is cut off by the VCA. The only downside is, the idea with the accent VCA came to me after the layout was built. So the accent pot is besides Guiro.

Demo

MP-3 with some programmed patterns

MP-3 showing the Accent feature. At the beginning Accent is fully ON, will be turned down up to 0:30 and back to full in the end. You also can listen here how bad the TR-77 voiceboard sounds when using 12 Volt instead of 23 V