Real World Interfaces logo

Modifications for the TR-808

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

© Robin Whittle, Real World Interfaces  Daylesford Victoria Australia
rw@firstpr.com.au  20 October 2015

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/ .



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 was planning a MIDI In and Out system, but am no longer working on this.  I can however install a system to receive MIDI and generate Roland/DIN Sync.  This receives MIDI In via the Sync socket, drives the TR-808's internal sequencer, and can drive up to three external devices, such as TB-303s, TR-606s or another TR-808 via the Sync Lead:  ../dfish/sync-lead/ .


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

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 toggle-switches.  The pushbutton reverses the bit produced by the bottom toggleswitch.

This memory system is now available with a two or four channel Dynamic Bank Switching (DBS) system,  which is not shown in these photos.  
This enables externally applied audio or CV signals, via 3.5mm sockets on the left side of the machine, to alter which of the 32 memory banks is selected.  This means that the internal sequencer will play different patterns and one or more individual beats from different patterns in response to these signals. 

Please see the Devil Fish 32 Bank Memory system with DBS manual, which also covers DBS for the TR-606 and TR-808: http://www.firstpr.com.au/rwi/dfish/DF-32-Bank-Mem-DBS-Manual.pdf


The 32 Bank Memory System is available with two or four channels of Dynamic Channel Switching: either two or four 3.5mm Audio/CV input sockets (each with a detector circuit and LED, with a threshold of about +1.15 volts) which, when activated, inverts the address bit produced by two or four of the four toggle-switches which supply four of the five address bits to the 32 Bank system.  It is possible to add to this a centre-off Enable switch for these inputs and so their address inversion functions.  Left and Right positions enable the inputs, with Centre disabling them.  The Right position is momentary: it is spring return to centre.

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






The TR-808's drum circuits are triggered by pulses which start at 0 volts, rise to a certain voltage for 1 millisecond, and then fall to 0 volts again. 

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.

The User Manual contains the same text as above: TR-808-Four-Level-Accent-Mods.pdf

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 User Manual TR-808-Sound-Mods.pdf contains the following description of features:
  • * 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 "Bridged T-Network" (as the Roland service manual describes them) 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. 

    In mid-2014 I introduced an optional Accent Input socket.  This 3.5mm input socket, mounted on the right of the machine, enables external audio or CV input signals to turn on the Accent, if it was not already turned on by the Internal Sequencer or the Accent Button.  The detector circuit and accompanying LED, next to the socket, uses a +1.1 volt detector circuit similar to that of the Dynamic Bank Switching system mentioned above.

  • 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 high-pass filtered square-wave Cymbal and Hi Hat sounds. 

      Miking up a continual cymbal sound (such as from a large cymbal hammered softly with a mallet) and feeding this signal into this input 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 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 inserting a plug gently until it touches the tip contact of the switch, but not hard enough to lift the tip 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, other than the effects of TM4 inside the machine (for a standard TR-808) or the Noise Level pot (on a modified TR-808) which affects the white noise level sent to the Snare, Toms and the Hand Clap / Maracas circuit.

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






Switch replacement and various repairs

TR-808s are now about 32+ 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) sometimes become noisy.  I clean them in-situ with isopropyl alcohol and compressed air.  Likewise the prescale switch and the toggle-switches and the Tempo pots.

The output sockets, rotary switches and the small red pushbutton switch have not, in my experience, given any trouble.  The key-switches 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.  I have used a set of replacement switches with matching 3D printed buttons from Social Entropy.com, with great success.  These kits are not yet officially available but I have some here.  I am developing self-adhesive laminated replacement orange labels for the START / STOP and TAP buttons to replace those which may have been lost, including if the top plastic cover is missing.

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.  (Technicians: the details of this mod are described in the TR-808-Memory-Backup.pdf file linked to below.)

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 (230 volts) 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 230 or 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.



New Battery Arrangements for Retaining Memory Data

(These modifications will not apply to TR-808s with the forthcoming Quicksilver 808 system installed, since that has its own internal non-volatile memory.  The TR-808's memory chips will be removed, and there will be no need for battery power to retain the contents of the Quicksilver 808's memory.)

This is a separate set of modifications for the TR-808, whether or not the machine has a 32 Bank Memory system installed.  (With the TB-303, the new memory backup battery arrangements are part of the Devil Fish mods.)  The purpose is to provide a more robust and convenient backup power system for the memory system than is possible with the original arrangements, which involve three AA batteries in the battery compartment in the bottom panel.

Prior to May 2015, I offered an optional memory powering system involving a permanently installed (soldered connections) large capacity, "1/2 AA" cylindrical lithium battery.  In 2015, due to restrictions on air-freight carriage of items including lithium batteries, I developed some alternatives. 

For customers outside Australia, we normally ship the modified machine without the lithium battery in the internal battery holder, using Australia Post's Express Courier International (EMS) service.  The customer can install their own 1/2AA cylindrical cell.  We can also ship the modified machine overseas with its lithium battery installed, with FedEx.

For the full details of the original arrangement (option A, a permanently installed lithium battery) and the two new Options B (for air-freight shipping to overseas customers) and C (for road freight shipping to customers in Australia, or via FedEx to customers overseas) please see the PDF manual:  TR-808-Memory-Backup.pdf

For both Options B and C, the three AA cells (it is best to install only alkaline cells) can still be used to provide backup power to the memory system.  A large capacitor enables the machine to retain memory contents for a longer period than usual without these three AA batteries and without the machine being run from an external power supply.  There is an internal holder for a cylindrical "1/2 AA" lithium battery, which can be accessed by removing the right part of the case.  This is the same type of battery we have been using in Devil Fishes, TR-808s and TR-606s since 1996, except that it is the version without leads, so it clips directly into a suitable battery holder.

Overseas customers can install a 1/2 AA lithium battery in this holder, after which there is no need for AA alkaline batteries.  The lithium battery should last for ten or more years.

This modification includes a recessed switch and two LEDs which test the charge and connectivity of both the three AA alkaline batteries (if any are installed) and the 1/2 AA lithium battery (if this is installed):



This modification is not essential, since the three AA alkaline battery arrangements are generally reliable, provided fresh alkaline batteries are installed every few years, and there is no corrosion or other problems with the battery holder for these.  It is a desirable modification for anyone who wants a reliable memory, since the AA cells and their six battery holder contacts are less than 100% reliable.  (If you use AA cells, be sure to insert adequate packing between them and the battery compartment door, since the light foam there - if it still exists - is typically too soft, to hold the batteries in their holder.)

The purpose of the modification is to provide an easy way of testing the voltage and connectivity of the three AA batteries, if any are installed, and the internal lithium battery, if one is installed - and to enable overseas customers to install their own lithium battery, while avoiding the problem of air-freighting the machine with a lithium battery installed.  For overseas customers we will ship the machine with three fresh AA batteries installed.

For Australian customers, we will ship the machine with a lithium battery installed, but we do not install new AA alkaline batteries, since these are not needed once the lithium battery is installed.
 


Prices

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

Turnaround time is likely to be a month or more.  We have some good boxes to pack the TR-808 in for the return trip to customers. 

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