Buchla 227 System Interface Module

I completed and verified this Buchla 227 System Interface module for someone else.  This module was partially built so I completed the build and tested the module.  I do not know the details for sourcing the specialized parts, panel, knobs, or PCBs.  This is quite a versatile module and is fairly complex but is not well documented.  I did find some documentation in a Buchla Synthesizer User Guide written by Daniel J. Scheidt dated November 16, 1981.

Buchla 227 Description


The right top corner has a 44 pin connector that mates with an expansion card.  The expansion card has output jacks for program A-D and monitor A-D.  There are input jacks Tape 1 A-D and Tape 2 A-D which connect to the Tape Output jacks and are normalled to the Tape Input jacks.  The Aux 1 A-D and Aux 2 A-D jacks connect to the Auxiliary Output jacks on the panel.  There are jacks for reverb Send, reverb Out (from tank), and Reverb return A-D.  The four DIP switches connect the reverb Out to the Reverb return A-D so need to be in the right position to enable reverb.

The reverb return controls mix the mono reverb signal to the front channels B & D or the rear channels A & D.

There are four independent mono input sections each with a separate level, bass, treble, reverb send and assign control.  Assign pans the channel to program A, B, C, or D.  Program A and D are the rear channels and B and C are the front channels.  There are two CV inputs, X and Y, which will pan the signal from front to rear (A>B) or left to right (A>D).

The tape section is comprised of Tape Output jacks which are directly connected to the expansion card Tape Input jacks and normalled to the panel Tape Inputs jacks.  Inserting a plug into the panel Tape Input jacks on the panel breaks the connection to the Tape Output jacks and the expansion card Tape Input jacks which are still connected.  The sliders attenuate the Tape Input signal.

The Multiples are groups of four jacks wired in parallel.  The Auxiliary Outputs are wired to the Aux Inputs on the expansion card.

The monitor select switches patch the selected input to the monitor bus.  The monitor bypasses the input levels, assign and tone controls on the mono input sections and the input attenuators on the tape inputs.  The Sys switch monitors the external MTS signal on the interface cable.  With no MTS signal this effectively mutes the monitor bus.  The program bus is monitored if all switches are off.




The build thread is very long so you have to go through it all to pick up the modifications and build tips.  I've summarized the build thread modifications as of 6/1/2014 as well as my own modifications.

227 System Interface build thread modifications

  1. R206, R210, R212 and R218 change from 680R to 15K to improve the meter calibration.  Even with these values the meters are touchy to trim.  I modified both the trimmer and the resistors.  See #8 below.

  2. The potentiometer CW and CCW connections for all four Bass controls are swapped.  These need to be cut and swapped with wires on PCB1.  Input 1 runs are according to this image but inputs 2, 3, & 4, while similar, are on the PCB front.

  3. The Tape 1 and Tape 2 LEDs are swapped.  These need to be cut and swapped with wires on PCB1.

  4. The legends for R146 and R147 are swapped.  Per the legends on the PCB, R146 is 3K3 and R147 is 4M7.

  5. There was some discussion on the forum about changing the value of the four Assign potentiometers from 100K to 25K.  The 100K controls worked well so I did not make this change.  However, I did modify R4.  See #10 below.

  6. There was another discussion on the forum about the four PCB2 D3 silk screens being reversed but they were correct on this PCB.


My modifications

  1. The headphone channels C and D are swapped.  You can verify this by listening to the front and panning a channel from A to D.  You will hear the signal on B and D.  Likewise you can listen to the front and pan and hear the signal on A and C.  Simply cross resistors R234 and R235 on PCB1 as shown.

  2. I reduced the sensitivity of the meter trimmers by changing TR14, Tr15, TR16, and TR17 to 100K and resistors R206, R210, R212 and R218 to 150K.  That provides a nice range of adjustment for the meters.

  3. The meters read lower in program than monitor so I changed R261, R262, R263, and R264 from 39K to 33K.  Now at unity gain output the meters read the same in program as monitor.

  4. I found that changing R4 from 68K to 47K improved the Assign control linearity just a bit so I changed all four resistors.

  5. Tape In channel C had slightly less gain than the other channels so I changed R114 from 39K to 42K2.

  6. The reverb return on channel B was much stronger than A, C or D so I decreased the level to match the other channels by changing R128 from 6K8 to 2K2.

Additional modifications (updated 5-18-15)

  1. I repaired a different 227 and IC3C, part of the assign circuit, oscillated rail to rail  mid-rotation of the Assign control.  Strange, the assign circuit seemed to work reasonably well but I tamed the oscillations with a 100K resistor betweens pins 8 and 9.  It didn't seem to change the panning but the circuit no longer oscillated wildly.




PCB1 contains the controls and related passive components.  You can see the 10 mod wires on PCB1 and the crossed resistors.


The LED and bass control modifications require ten cuts to runs.  Eight cuts are on the front of PCB1 and two are on the rear.  I decided to add all the mod wires to the rear so they would be more easily accessible.  This image shows the two LED mod wires.


This image shows the eight bass control wires.



PCB2 mounts on the rear of PCB1 and contains all the active circuitry.


I mounted the reverb tank parallel to PCB2 using four standoffs, two on the outer meter studs and two on the nearby PCB screws.  The meter standoffs are 2" 4-40 aluminum standoffs (Mouser 534-2207) and the PCB standoffs are 25mm M3 aluminum standoffs (Mouser 534-24448).  I made some M3 studs and glued them into the standoffs to make them male-female.  I used short and long solder lugs on the standoffs for the spring attachment points.


Initially I had the springs facing away from the PCB but there is sufficient room to flip the tank so the springs face towards the PCB.  This protects the springs and shortens the depth of the module.  In this photo the module is face down so the springs are sagging towards the meter lugs.


PCB3 is the expansion card with all of the external jacks and the four switches.  Switches 1 - 4 (A - D) must be to the right to patch the reverb output to the reverb return bus.





There are 17 trimmers on this PCB consisting of a single MTS level trimmer and four groups of four trimmers: assign, meters, program bus signals, and monitor bus signals.

  1. The four trimmers on the right adjust the linearity of the Assign control for each channel.  This adjustment needs to be done first.  Adjust the trimmer until the LEDs light up in the correct positions for each channel.  Connect a variable 0 to 10V CV to both X and Y and set the Assign control to A.  Adjust the CV from 0 to 10 volts and fine tune the trimmer for a smooth pan that lights up B and D evenly with a fade to C.

  2. Buchla blue knob 200 series audio levels are +4 dBu which is 1.228 V RMS or 3.47 V pk-pk for a sine wave so I calibrated 0dB to this level.  All four meters displays the voltage at the input when that channel is set to monitor which provides a direct path to the meter from the input.  Connect a 3.47V pk-pk sine wave into one of the input jacks and monitor that channel.  Trim all the meters to 0 dB.  I found the meter adjustments way too touchy even with the resistor change to 15K so changed the trimmers to 100K and the feedback resistors to 150K.  This sets the minimum gain at 2.5X and the meters calibrated quite nicely.

  3. The four program trimmers should be set so there is equal gain between channels.  Set the Bass and Treble controls to mid position and all the trimmers at maximum: A and C fully CCW and B and D fully CW.  Use the Assign knob and monitor each output on the XS2 connector.  Find the channel with the lowest output and trim the other channels to match.  I found that the gain on program D was quite low so changed R159 to 10K to increase it.  When I set the program output to a 3.47V pk-pk sine wave the meters read low at -3dB so I changed R261, R262, R263, and R264 so the meters would read 0 dB.

  4. The four monitor trimmers should be set so there is equal gain between channels.  Set all the trimmers at maximum: A, B, C and D fully CCW.  Use the Assign knob and monitor each output on the XS2 connector.  Find the channel with the lowest output and trim the other channels to match.

  5. TR4 adjusts the level of the MTS signal in the interface cable (MTS is monitored when the SYS switch is on).   Connect a 3.47V pk-pk sine wave to MTS and monitor SYS.  Trim the meters to 0 dB.  This adjustment needs to be done last.

227 trimmer PDF image



The 227 requires ~ 375 mA of +15 and ~188 mA of -15V with the internal regulator supplying the +12V, all the meters operational, reverb on, and the monitor at full volume.

I noticed the meter levels are quite hot (they will peg if an input is applied and the levels are turned up) immediately after power up and drop shortly thereafter.  The meters also drop just a little after panning to a channel.  The level change seems to occur in vactrols V3-V6.   A shot of cold spray on the selected vactrol increases the signal level for a short time so it is possible the vactrol warms and changes the operating point although I did not investigate further.   Once the module warms up for a minute or two it operates reasonably well.

The meters are a relative indication as they do not read accurately in dB.  I calibrated the meters both with the trimmers and resistor changes so that the input channels 1 - 4 read 0dB when in program or monitor with a 3.47V pk-pk sine wave.  I made a few measurements of meter readings vs. output voltage.

V rms V pk-pk Meter dB Actual dB
1.23 3.47 0 0
1.05 2.96 -3 -1.4
0.85 2.40 -7 -3.2
0.75 2.12 -10 -4.3
0.55 1.56 -20 -7.0

The signals on the expansion card were intermittent.  The fingers were soldered too deep into the  connector so there was not enough depth to make reliable connection with the mating connector.  I rebuilt the extension connector with the same alignment as as I used on the 208 where the top of the PCB aligns with the bottom of the hole in the connector tail. Be careful to not get solder on the edge connector pads.




The 227 V2 PCBs were redesigned to eliminate the vactrols using ATTYINY84 microcontrollers. These controllers come unprogrammed so you have to program both the hex code and the low fuse.  I made an adapter to program them using the 6 pin cable from my STK-500.  While I don't want to be in the programming business, contact me if you need help programming these.

There is a bug in the B227-rev1.0.hex code.  There is a dead spot in the panning knob right at 12:00.  There is no dead spot when using the CV input.

One side effect of the vactrol change is the LEDs do not completely extinguish when off. I helped a customer via email to correct this issue.  There is a three resistor level shifting network from each LED (except Sys) to the DG202 analog switch control input.  This resistor network supplies enough current when off that the LEDs do not completely extinguish.  The control input on the DG202 has a high level of >2.4V.  The absolute maximum rating on the control input is the power supply +2V or 20 mA.  The LEDs are switched from +11.6V through a series resistor to ground so we can simply use the cathode voltage of the LED as the control input.  There is a clamping diode internal to the DG202 between the control input and +15V so I added a series 10K resistor to limit current in case the +12V supply is up when the +15V supply is down.

The modifications are to remove the three resistors for seven of the LEDs and add a 10K resistor between the LED cathode and the DG202 control input.  Both of these signals can be found on nearby resistor pads.  The Program LED modification is simply removing two resistors and changing the value of the third.  All modifications are made on the motherboard.

Channel 1 LED
Remove R11, R12, R13. Add a 10K resistor between LED cathode on R14 ( left pad) to IC5 pin 1 (left pad of R11).

Channel 2 LED
Remove R37, R38, R39. Add a 10K resistor between LED cathode on R40 (right pad) to IC5 pin 8 (left pad of R37)

Channel 3 LED
Remove R63, R64, R65. Add a 10K resistor between LED cathode on R66 (left pad) to IC5 pin 16 (left pad of R63)

Channel 4 LED
Remove R89, R90, R91. Add a 10K resistor between LED cathode on R92 (left pad) to IC5 pin 9 right pad of R89)

Tape 1 LED
Remove R105, R108, R109. Add a 10K resistor between LED cathode on R114 (left pad) to IC6 pin (top pad of R108)

Tape 2 LED
Remove R106, R110, R111. Add a 10K resistor between LED cathode on R115 (bottom pad) to IC7 pin 1 (top pad of R106)

Reverb LED
Remove R107, R112, R113. Add a 10K resistor between LED cathode on R116 (right pad) to IC8 pin 1 (top pad of R107)

Program LED
Remove R154 and R158. Replace R156 with a 10K.