AM1027 Clocked Sequential Control

ARP 1027 Module

Overview This is a replica of the 1027 Analog Sequencer from the ARP 2500 Analog Modular Synthesizer, which has 10x step rows and 3x control voltage on vertical columns. The ARP module is double width and has a voltage controlled analog clock which drives the 3x rows of control voltages on the right. The left hand side of the panel has the clock controls; stop, start, rest, step and frequency. There is also a useful pulse width control.

The 1027 is a basic sequencer in today’s terms, but probably the first sequencer to use a logic chip and the only one with 10 steps! Don’t be worried about having to always use 10 steps in your sequence, their are position gate outputs for each step, which can be patched into the reset input for shorter sequence lengths.

BIP2611-1 Chip

My AM1027 is installed with two AM1045 Synthesizer Voice Modules into an Oak 6U Cabinet made in France, with the panels taking the top 5U. The bottom 1U is the 15V power supply with gate, trigger, CV sockets along with a Roland 100M 8-pin keyboard connector. I use a Roland 181 keyboard to control the AM2500 racks, although most of the time the 1027 is driving the 1045’s in this particular rack, and downstream 1026 modules.

AM1027 Module My design uses the original ARP schematics and the core is laid out over two PCB’s; a main PCB which is 150 x 150 mm and mounted onto a metal sub assembly, and a smaller 150 x 100 mm PCB for the analog clock, mounted on top. Metal film resistors and modern capacitors have been used, with no attempt to use legacy carbon composition resistors. The voltage controlled clock uses an AM 4001 Expo Generator which contains a matched Linear Systems transistor pair and 100 ohm SMD Tempco.

High quality (and expensive) 100k Spectrol potentiometers with 1/8″ shafts have been used for the control voltages, they are PCB mounted which means the 3x potentiometer PCB’s are mounted straight to the panel. The original Switchcraft square push button switches with incandescent 12V bulbs have been retained, along with the correct round Switchcraft “red” push buttons and black slide switches. The clock and pulse width potentiometers are TT precision pots with a very precise feel.

ARP 1027 RS232

The correct green coloured row of indicator lens have been used with 5mm diffused LED’s. I experimented with 12V incandescent bulbs but they went dim at higher clock rates. I had to use ARP like silver control knobs, as I don’t have enough coloured knobs left. The Frequency and Pulse Width control knobs are similar to the original, I bought them in quantity in the 2000’s as part of my parts sourcing for this project. The large amount of hand wiring in the original 1027 has been largely replaced with ribbon cables, but the left hand panel remains hand wired.

The ARP 1027 has an internal 15-pin RS232 connector for driving the sequence pattern to a 1050 or 1026 module. I have replicated this using a 10-pin DIL socket, and a 10-way ribbon cable that can be used to connect to downstream modules. The control voltages are from 0V to +10V.

BIP2611-1 The core of the original sequencer was a very early decade counter the Burroughs BIP2611-1 chip. This was launched in the late 1960’s for providing the decimal counting signals for Nixie tubes, although the 2611 version could not drive them directly. It can work at 300 kHz, which is rather too high a rate for a sequencer! Understandably these are nearly impossible to find and are rare collector items. I have used a modern CMOS CD4017 decade counter, which has the same functionality but inverts the logic state and therefore requires some changes.

Controls The module has the following front panel controls:

  • Clock Rate potentiometer
  • Clock Pulse Width potentiometer
  • Clock ON and OFF Illuminated Push Buttons
  • Clock Rate & Pulse Width RED LED
  • Clock STEP and RESET Push Buttons
  • Clock Frequency HIGH/LOW switch
  • Clock Source INT/EXT switch
  • Clock Control GATE/TRIGGER switch
  • 10x potentiometers for Control Voltage A steps
  • 10x potentiometers for Control Voltage B steps
  • 10x potentiometers for Control Voltage C steps
  • 10x green LED lamps, one for each step

Connections There are 20x 3.5mm jack sockets mounted on the panel:

  • GATE1 to GATE10, position step outputs on front panel
  • STEP and RESET inputs
  • ON and OFF trigger inputs
  • CLOCK OUT output
  • OUTA, OUTB, OUTC – the control voltages
  • OUT1 to OUT10 – the position gates

Module Build & Outcome I have a set of original ARP 1027 blueprints that I located in 2004 but it wasn’t until the Spring of 2017 that I laid the circuits out onto 4 PCB’s; a MAIN PCB for the analog clock and output drivers, three POT PCB’s to hold the control voltage potentiometers, and a slim LED PCB. I ordered a test set of PCB’s and populated them later in 2017 but the project was paused in 2018 whilst I moved into a new workshop.

I restarted the build of the AM1027 in early March 2019, and used a test AM4001 expo generator with non-matched 2N390X transistors to start up the prototype. After correcting a couple of my schematic errors (which took a few hours to locate) I had the clock oscillating nicely with a pulse waveform, along with push button start/stop control and indicators, as well as the clock LED.

I spent another 20 minutes working out what the three curiously unmarked trimmers were for, and how to adjust the clock to its factory settings.

  • R9 Low Frequency – trim to set lower clock speed
  • R12 High Frequency – trim to set upper clock speed of 400Hz
  • R19 Pulse Width – trim to set Pulse Width at 10%
  • R22 Offset – trim to 0 mV

The clock has two ranges;

  • LOW – 20 pulse per minute to 30Hz
  • HIGH – 30Hz to 400Hz

With the clock working just like the ARP factory, I could now put the CD4017 counter chip into its socket and see what happens. There are a number of changes needed to the ARP circuit to switch from the BIP2611 chip to the modern 4017. The voltage reduction circuit around the BIP-2611 chip can be removed, the pull up resistors go to ground rather than +15V, and they are larger values than the 470R resistors in the original.

The CD4017 can drive circuits directly without buffers, unlike the BIP-2611 which can not, and the 4017 has positive logic rather than the inverted outputs of the BIP-2611. The PNP transistors need to be swapped out for NPN with the Emitter and Collector pins swapped. I used 2N3904’s and removed a set of 10k pull up resistors.

Stepping The STEP button did not work with the new 4017 chip, as the circuit produced a pulse voltage less than the CMOS high logic level of 11V, it was down at 9.5V. By changing R30 from 3k9 to 10k the pulse voltage became 11.5V and correctly triggered the counter chip. I also changed the pull up resistor on the STEP button to 330R from 1k to get the right sized voltage trigger into the CD4017. The external step and reset inputs come straight into the CD4017, so I may buffer these with Schmidt triggers to tidy up the waveforms, however they work reliably at the moment.

The position gate outputs and the CV mixed outputs needed adjusting to +10V with another set of resistor changes. Once the circuit was working well I designed a front panel in April 2019.

2019 Update During October I started to commissioned the 1027 with a test fitting on plastic mounting card, so I could then order the 2mm thick aluminium front panel. I added a LED PCB for the sequence position LED’s with a 16 pin flat cable to the MAIN PCB to reduce the amount of hand wiring. I also put both the analog and digital circuits onto one single PCB (REV02).

The MAIN PCB is mounted to a sheet of 2mm aluminium (150 x 200 mm) using plastic PCB standoffs, this is fastened to the front panel using an aluminium L bracket and M3 screws with metal standoffs. The 3x POT and LED PCB’s are attached via 16-pin ribbon cables, along with the position gate outputs. There is a vast amount of wiring in a 1027!

I used the original  ARP 1027 Service Manual to commission the module, its has a setup process of 14 steps! The AM1027 was fully commissioned on 23 November 2019.

2024 Update I sold my 5U 1027 in 2022 as I downsized my studio space, to make way for the AMSynths workshop expansion. In 2019 I designed the Behringer version of the 1027 in Euorack using the same CD4017 chip. The B2500 range has proved very sucessful.

In 2024 AMSynths launched a collection of Eurorack modules to complement the B1027 and utilise its position gate “BUS” which lives on a rear mounted 12-pin IDC socket. The AM1028 Salve Module, AM1029 Gated Ratchet, AM 1037 VC Envelope Genartor all connect to the B1027 using this “BUS”. Here are the “BUS” connection details for anyone wishing to build modules that connect to the B1027:

  • Pins 1 -8 of the 12-way IDC are position gates 1 – 8
  • The output voltage is 0 to +5V (+4.86V in practice)

You can find more information about the AMSynths range of ARP Euorack modules here.





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