The Amdek Rhythm Machine is a versatile unit which can be assembled and modified with the minimum of technical difficulty.

Amdek's RMK-100 provides the musician with a versatile DIY unit capable of storing 16 programmable rhythms, each 16 steps in length. Rhythms are arranged as two banks of 8; A and B, which can be used together to create fills and chains.

These patterns trigger up to four instrument voices; Bass drum, Snare, Open Hi-hat and Closed Hi-hat with programmable Accent and auto Hi-hat features available.

Extensive 'user' options have also been provided allowing custom modifications to be made.

The Kit

All the parts required to complete the unit are supplied including case, components, connecting wire, solder and an Amdek spanner.

The only other tools you require are: a 15-30W soldering iron, cutters, pliers and both crosshead and slot-head screwdrivers.

When all of the components have been identified and marked off on the list assembly can commence.

The first stage is to fit the four pots and two push button switches into the subchassis. Twelve leads can then be cut, stripped and tinned before being connected to the pots. LED leads can be cut and tinned next, along with those of the battery snap.

Four more leads are then cut, tinned and soldered to the output and DC supply jacks (Steps 1-6).

The PCB supplied separates into two parts: The logic board, containing the control circuitry and the source board which houses the voicing circuitry. This action should be carried out with care along a straight edge such as a table or desk.

Once this has been done, three threaded spacers (or long nuts as Amdek call them) are screwed to the logic board, and the battery snap threaded through the hole provided. A knot should be made in its lead to prevent any connections being broken when the batteries are changed.

Leads can now be soldered to the logic board after the usual cutting, stripping and tinning procedure. Once the LEDs have been pushed into the subchassis their leads, along with those of the pots and power jack, can also be connected.

The logic board is then offered up to the subchassis and secured using the hex nuts of the rotary switch and pots, along with the slide switch screws. All the loose wires still to be connected should be kept outside the assembly to ensure that they are not trapped.

A jumper lead should be prepared and connected to the pushbutton switches along with 3 leads from the logic board.

The remaining loose wires can now be connected to their respective points on the source board to complete the wiring (Steps 7-14).

Masks are then fitted over the slide switches and a piece of sponge pushed into the battery box. When this is complete the finished subchassis can be slid into the top half of the case and secured using 4 screws - making sure that the LEDs and pots are aligned correctly.

All that remains now is to attach the jacks to the rear of the case, secure the source board to the logic board via the threaded spacers, screw on the bottom of the case and add the cosmetic touches; rubber feet, knobs and switch keytops (Steps 15-20).

Four HP7 batteries are fitted into the battery holder which slides into the battery box and is secured using the cover plate.

The Circuit

The complete circuit diagram for the RMK-100 is shown in Figure 1, the circuitry being divided in two parts: logic and source.

The logic side is built round IC3 which is a 256x4 bit CMOS RAM, data outputs being connected to the respective instruments on the source circuit.

A clock is generated using two schmitt triggers from IC5, the rest of the package being used to buffer and shape the 'Write' and Start/Stop controls. The clock output is fed to a buffer, Q7, and then via SW5 to one of the two counters contained in IC4. The four outputs from this counter provide the address for the 16 locations in memory which represent the rhythm. Rhythm selection is provided by SW6 which is encoded into binary format by the diodes D20-27 to produce the lower 3 bits on the address lines. Fill control is provided by the second counter in IC4, selecting bank B on the 2nd, 4th, 8th or 16th pattern.

Chaining options 1-4, 5-8 and 1-8 are also provided by this counter which overrides the 1-8 selection on SW6.

Voices are generated using analogue circuitry. Transistor Q15 and its surrounding circuitry produces a damped oscillation when triggered via D31 to simulate a Bass drum.

Noise is produced by the circuitry around Q12/13. When triggered it is gated by Q10, at the same time as a damped oscillation is produced by Q14 and associated circuitry thus allowing a Snare to be simulated.

Six oscillators, based around IC6, are mixed and filtered by half of IC7 to provide the metallic quality of the Hi-hat, which is gated using long and short envelopes to represent open and closed modes.

All of the voices are mixed and fed through the 'Accent' stage which normally attenuates the signal unless triggered via D30 which lets more signal through.

Operation

Programming can be accomplished easily in beat/rest fashion. The rhythm (1-8), bank (A or B) and instrument (Bass drum, Snare, Open Hi-hat or Closed Hi-hat) are selected and the 'Write' mode entered.

The orange, or start, button is pressed for a beat and white, or stop, for a rest. Each depression causes the unit to step to the next location. Rhythms are built up in this fashion for each instrument.

During playing, fills can be introduced which bring in the 'B' selection on the 2nd, 4th, 8th or 16th 'A' pattern. Complete chains can also be constructed of up to 8 rhythms played sequentially.

Accents are programmed as Closed Hi-hat and can be replayed with or without auto Hi-hat.

If you have any problems with construction or operation contact the Amdek 'Hotline' at Roland UK ((Contact Details)).

Modifications

Amdek have thoughtfully provided the user with an extra socket which can be used for several different mods:

1) Foot Switch: Connect the socket to pad 23. A footswitch control can be connected to provide remote start/stop operation.

2) External Clock: Connect the socket to pad 24. A 0-5V trigger pulse can be connected and used to drive the internal circuitry. This can be useful to 'sync' off a click track.

3) Clock Out: Connect the socket to pad 26. The internal clock can be accessed and used to trigger external circuitry or to produce a click track when connected to a suitable interface.

4) Step Length: By connecting a switch across the two pads on the logic board (J1) the step length can be altered from 16 to 12.

5) External Voicing: All four of the instrument triggers (pads 12-15) can be connected via extra switched jack sockets (drill holes in rear panel). Therefore internal voicing is triggered until a jack plug is inserted. Circuits such as the E&MM Syntom and Synbal can then be connected turning the Rhythm Machine into an even more creative music-making instrument.

E&MM's special offer price for the Amdek Rhythm Machine is £74.00 inc. VAT and P&P. Please order as: Amdek RMK-100 Kit.