Sequencer 256

ne of the rarest pieces of equipment from EMS, the massive Sequencer 256 is an important part of electronic music history, is probably the first commercially available digital sequencer, unrivalled for a decade or more. Tested sequencer, functions basically work but control voltages are unstable. Traced to bad trimmer resistors. Replaced all trimmers on C card, +2.5V reference out of range of adjuster, traced to faulty 741 op­amp, replaced. +/­2.5V now calibrated correctly. Replaced trimmers on B card, recalibrated internal clock. Slew rate processor calibrated – note slew rate CV in is hard­wired to Channel B dynamic out. Set Channel B vernier to 5 if you want to disable this facility. Replaced trimmers on A card, keyboard output voltage still unstable. Traced to noisy resistors in output circuit and FET’s loose in sockets. Soldered FET’s in place, added 10k multi­turn trimmer in place of existing 1k pot and 4k7 padding resistors, octaves now stable and calibrated. Adjusted so that middle ‘F’ is reference note at zero volts. Adjusted trimmer so that Zero Store = full scale on meter when indicating free memory. Some keyboard contacts intermittent, cleaned Calibrated verniers so that 5 = zero volts out. Scaling adjustable on verniers to +/­1.2V/Octave. Layer 1 key circuit has been previously modified for +/­10V output, layers 2 – 4 are standard at +/­5V, but can be modified on request. Clock rate vernier sometimes slips, replaced vernier (retained original knob and cap) and calibrated for zero volt on meter and 4Hz clock frequency when vernier set to 5. Cleaned panels, tolex top and keyboard. EMS Keyboard lead included, tested and cleaned pins. Mains lead missing, manufactured new one. Sourced manual. Final test of all functions with Synthi A and SH­09. Cosmetically the end cheeks are not great - they have lost some veneer on the corners and edges; but the important bits - the control panel, etc - are very good indeed.