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Modifications for the TR-808

Sound mods, 4 Level Accent mods and 32 Banks of Memory.

© Robin Whittle, Real World Interfaces  Melbourne Australia
rw@firstpr.com.au  7 October 2012

prices-au/ Prices for Australian customers.
prices-int/ Prices for customers outside Australia.

To the Real World Interfaces page, including links to TB-303 and TR-606 modifications.
To to the main First Principles  site.   ABN 29 836 876 922.
 

sounds/  Sound samples.  MP3, WAVs and via SoundCloud.

For technicians: notes on repairing or replacing TR-808 switches and pots, especially the Start / Stop and Tap switches: TR-808-switches/ .



Sound Mods, 4 Level Accent Mods and 32 Banks or Memory

I have been modifying TR-808s since the early 1980s.  In September 2012 I made these modifications available again, with some improvements.  Please see sounds/  for some demo sound recordings.

I am working on a MIDI In and Out system, for MIDI Notes and Sync.  This should be available early in 2013.


The 32 Bank Memory System is identical in principle to that for the Devil Fish:

Devil-Fish-32-Bank-Mem-Manual.pdf 

There are 5 toggle switches, moving side-to-side, and a pushbutton switch.  These are located on the front left side panel.  This provides 32 complete sets of the TR-808's original memory.  The main purpose is not so much to provide more pattern and track memory, but to allow the switching of memory banks while the machine is playing patterns.  The CPU reads from memory for each beat, so this enables switching to another pattern in the middle of a pattern rather than waiting for the current pattern to end.  This means fresh notes can be dropped in with the flick of one or more switches. 

The 32 banks are selected by the 32 combinations of the 5 toggleswitches.  The pushbutton reverses the bit produced by the bottom toggleswitch.

(In the 1980s I installed 11 and 21 bank memory systems, with bank selection via a thumbwheel switch or pushbuttons and a 7-segment LED display.  Information about these mods is at the end of this page.)

The 4 Level Accent Mods enable normal operation of the Accent system (a 5 volt trigger pulse on non-accented notes with a 5 to 14 volt trigger pulse for accented notes) and a new mode of operation.  In the new mode, the trigger voltage for triggering all the activated drum sounds on the current beat comes from one of four pots.  One of these will be selected on each beat depending on whether the Accent and the Cowbell is programmed.  Each such selection is signaled by a flash of the LED below the knob. 

White
 ACC & CB
Blue
 ACC
Yellow
 CB
Red
  

While playing a pattern, one of these LEDs flashes for every 1/16th note, and the setting of the corresponding pot determines the trigger voltage for all drums which are programmed on that beat.  The range is between about 2 volts (fully anti-clockwise, about 8 o'clock) and 14 volts (fully clockwise, about 4 o'clock).  The ~2 volt level is adjusted so it just triggers the drum sounds.  (This mod also changes some internal circuitry to make the Cymbal and HiHat circuits have the same threshold of about 2 volts.  Normally, they require about 4.5 volts before they will make a sound.)

While the internal sequencer is playing a pattern (or being switched between patterns with the 32 Bank Memory System), the dynamics of the pattern can be completely altered by turning the four knobs. 

The toggleswitch selects between normal and 4 Level Accent mode.  The blue and yellow pushbuttons over-ride the Instrument Select switch so the buttons and LEDs work with the Accent and Cowbell respectively, rather than whatever drum channel the Instrument Select switch selects.  This makes it easier to program the desired Accent-Cowbell combination on any of the 16 beats.

These 4 Level Accent mods are identical to what I installed in the early 1980s, except that I am now using slightly smaller knobs with metal-shaft pots.   The earlier pots were not as robust as I would like.

Please see sounds/  for some sound recordings in which I demonstrate the operation of the Sound Mods and the Four Level Accent system.
The Sound Modifications are an extension of what I first installed in the early 1980s, with new pots and knobs, three small pots for the Tom/Conga Decay function (instead of toggle switches and trimpots) and three additional functions marked * below.

 The features are:
  • * The Bass Drum decay range is extended to include self-oscillation, so any decay time from the normal minimum to infinity can be achieved with the Decay knob. 

  • * The Bass Drum Tuning knob enables higher and lower tuning.  (Top white knob.)

  • The Snare sound is changed in a number of respects:
    • Sharper attack for the Snappy noise pulse.
    • Reduced cutoff frequency in the Snappy noise pulse high pass filter, to make it gutsier in the lower mid-range.
    • The two tonal components of the sound are boosted in level.

  • The Snare sound is composed of the Snappy noise pulse and the ringing of two Twin-T resonators, with the Tone pot controlling the mix of the outputs of the two resonators.  There are two Snare Tuning pots, one for the upper resonator (blue knob) and one for the lower (yellow knob).   Tuning is at the normal pitch when these pots are anticlockwise (ACW) and is reduced by around an octave when they are clockwise (CW).

  • The Cymbal Soft Attack switch (above the three small pots) gives more of a ride cymbal sound, compared to the normal crash sound.  The attack is soft and the decay is somewhat extended.

  • The Toms have a pulse of filtered noise, which is barely perceptible, but gives them something of a rumbling sound.  These pulses are shortened somewhat.  (In Conga mode, there is no such pulse.)

  • The Tom/Conga circuits each have a Decay pot, where ACW is the normal decay time and CW is a much shorter decay time.  These are the three small pots with silver indicator lines.

  • The Hand Clap sound is composed of a series of close-spaced pulses of filtered and distorted noise plus a softer pseudo-reverb exponential decay pulse of softly filtered noise. The Clap Reverb Disable switch (below the three small pots) turns off this pulse, leaving the dry, stark, and potentially extremely loud main pulse-cluster of the Clap circuit.

  • The Hand Clap Density pot (red knob) controls the density of this cluster of pulses, from being very light (ACW) through normal, to each pulse being bigger, to all pulses being much louder and joined together (CW).

  • The Noise Level pot (white knob) controls the output level of the internal White Noise Generator.  ACW creates a somewhat lower than normal level of noise.  12 o-clock creates a somewhat higher level.  3 o-clock creates a much higher level.  Fully CW produces an extreme level of noise such that the Tom "rumble" circuits are occasionally overloaded, even when not being triggered, so there will be sporadic crackles through each of whichever of the  Tom/Conga channels are in Tom mode.

    The internal White Noise Generator runs from a noisy transistor and is genuinely random broadband noise.  It is used for:

    • The Snappy pulse of the Snare Drum.  Increased noise levels make this a very strong pulse indeed.

    • The Rumble pulse of the Toms.  High noise levels lead to more rumble to the point of completely unphysical rumbles and muffled crackles.

    • The Maracas sound is based on this signal, but it is hardly affected by the noise level.

    • The Hand Clap sound, both the pulse-cluster and the pseudo-reverb, is based on this noise source.  Higher noise levels lead to gutsier and potentially very loud hand-clap sounds.  It is generally best to disable the Hand Clap Reverb when high noise levels are used.  The resulting clap sounds, especially with the Density control turned somewhat to the ACW and with the Noise Level pot set to about 2 o'clock, are a unique and gusty discrete sound.  Since these are generated from real analogue noise, no two Hand Clap sounds are the same.

  • * The Accent Button forces Accent on even if the internal sequencer's pattern has no Accent on the current beat.  This Accent state also drives the 4 Level Accent system.

  • There are three audio signal inputs.  (6.5mm mono sockets on right side.)  When an audio signal is plugged into one of these, that signal takes over the role of an internal noise signal.  These are not to trigger drum sounds - they are signals from which drum sounds are made.

    • Cymbal / Hi Hat Alternative Noise Input.  Normally the Cymbal and Hi Hat sounds are created from a mix of 6 square-wave oscillators.  This signals is filtered, distorted in a gating circuit (which controls the volume) and filtered again.   By plugging an audio signal into this socket, the 6 square-wave oscillators are no longer used and the Cymbal and Hi Hat sounds are made from the input signal instead.  Bright, high, chord sounds are an obvious choice of signal - they cause the Cymbal and Hi Hat sounds to be metallic shimmering (low level) or mashed and splattered chord-related sounds.  Playing a single pitch into this signal results in single-pitch square-wave Cymbal and Hi Hat sounds. 

      Playing a continual cymbal sound (such as from a large cymbal hammered softly with a mallet) results in realistic sounding Cymbal and Hi Hat sounds, depending on the level.  (I did this in the 1980s.  If someone can send me a suitable long, constant level, recording of a ride cymbal being hammered reasonably gently, I will use it in this way in another sound demo.)

      The Cymbal and Hi Hat circuitry is beautiful, inspired and completely unique to the TR-808 and TR-606.  This Alternative Input enables the creation of many bright, delicate and potentially wild sounds.  It is my favourite part of the TR-808.

      Each TR-808 has a unique tonality to its Cymbal and Hi Hat sounds, due to each machine's six square wave oscillators having a unique set of frequencies.  As demonstrated in one of the sound recordings sounds/ it is possible to use this socket as an output for this mix of six square waves - by gently inserting a plug until it touches the tip contact of the switch, but not hard enough to lift the contact from the normally closed contact.  Then, it is possible to record a few minutes or whatever of this mix of square waves.  This can be played back into a second  TR-808 via this Input, and the second machine will have the tonality of the first machine, assuming the playback level is adjusted to match the required signal level.  Of course, this recorded signal could be subject to manipulation by changing its playback pitch, but distorting it, by adding reverb, random vibrato or whatever via audio editing software, so the original tonality could be used in the same machine, with various modifications.

    • Hand Clap and Maracas Alternative Noise Input.  Whatever signal is input here replaces the internal white noise generator in the Hand Clap and Maracas circuits.  Random mid-range to high frequency noise, chords, distorted and/or reverberated chords or various other signals result in staccato Hand Clap sounds which are unique to the Modified TR-808 and would never result from conventional synthesis techniques.  These can be musically dramatic - please check out the recordings in the sounds/ directory.

      This socket (and the next) can be used as an output for the internal noise source, with the same technique as described above for the CY/HH Alternative Noise Input.  It is (in audio terms) "white" noise - real noise from a reverse-biased transistor junction going into avalanche breakdown.  There are many other sources of noise in the modern world, but this is a source and it might be handy.  The volume and to some extent the character of the noise will vary with the setting of the Noise Level pot.  There's no significant difference in the audible quality of the noise between one TR-808 and another.

    • Snare Drum Alternative Noise Input.  This signal replaces the internal white noise generator for the Snare Snappy noise pulse circuit.






Switch replacement, lithium battery and various repairs

TR-808s are now 30 years old.  They typically require some cleaning and fixing as part of doing any modifications.

TR-808s use ALPS tact switches which are open to dust.  I replace them with sealed Omron tact switches which have a similar feel.  Since these are sealed against dust and liquids I expect they will last a very long time, such as decades, rather than the few years it typically  takes for the unsealed ALPS switches to become erratic.

I fix Volume pot noise problems by dismantling it and cleaning it with isopropyl alcohol.  The 22 small pots (without knobs) typically become noisy.  I clean them in-situ with isopropyl alcohol and compressed air.  Likewise the slider and toggleswitches.   I usually do the same for the Tempo pots.

The output sockets, rotary switches and the small red pushbutton switch have not, in my experience, given any trouble.  The keyswitches for Run/Stop and Tap are prone to failure.  I have a fairly involved method of repairing them, but ideally there would be replacements for these which were less prone to failure.

There is a fault in all TR-808s which corrupts the first pattern and the first track.  It is caused by capacitive coupling of turn-off inductive transients from the power transformer into the write-enable signal for the memory system.  I fix this with a simple low pass filter at the memory and by tying the line inactive-high when the Mode Selector is in the 1st Part or 2nd Part positions.

The memory backup battery contacts can become corroded.  I think the best approach is to install a lithium battery instead.  This fits in the same compartment but has soldered connections.  The lithium battery will last for at least ten years and probably several decades.

Some TR-808s have corrosion on their right-hand circuit-board (Cowbell, Cymbal and Hi Hat).  This appears to arise from contamination in the factory, and can develop over decades into potentially severe corrosion of parts of the circuit board and its components.  This is not disastrous, but a bad case can take several hours to fix.

I can install back-to-back high-current diodes in the ground lead so that there is a 0.5 volt float between the ground pin of the mains cable (assuming it is a three pin power plug) and the chassis and ground of the TR-808.  This will eliminate various ground noise problems which might otherwise be caused in some circumstances.

Some people in 220-240 volt countries such as Australia have a TR-808 which was built for the Japanese market (2 pin power cord and 100 volt power transformer) of for the North American market, with a 115 volt power transformer.  I can install a toroidal power transformer and make some changes to the internal power supply so that the machine runs on 240 volts.  If anyone from a 100V to 120V country has a 220V to 240V TR-808, I may be able to transplant a power transformer or install a new toroidal power transformer to make it operate from the lower mains voltage.

Prices

Please see:
prices-au/ Prices for Australian customers.
prices-int/ Prices for customers outside Australia.

Turnaround time is likely to be several weeks.  We have some good boxes to pack the TR-808 in for the return trip to customers outside Melbourne.   I am working on a PDF manual for these modifications and will supply comb-bound versions of these.

I have not included a price for installing new LEDs - the A, B, 1st Part and 2nd Part LEDs and the 16 LEDs in the Step buttons.  I can do it, but it is inordinately difficult to install new LEDs in the Step buttons, even when I am removing the buttons to replace the tact switches.  The original Red LEDs are fine and I recommend leaving them as they are.

Documentation for older modifications
From about 1982 onwards, I worked on a few dozen TR-808s, which can now sometimes be found for sale on eBay.  All these were for Australian customers, but some may now be in other countries.  The mods are:
Please see this file for documentation of the 1980s versions of the Sound Controls, 4 Level Accent system and the 7 segment LED-based 11 or 21 bank memory system:

808-old-mods-2006.pdf

I used to install a MIDI In system, with full velocity control of each drum channel, an extra 0 to 12 volt velocity sensitive trigger output and Sync reception.   Documentation is in:

midi-808-rwi.pdf