Buchla 285 Frequency Shifter V2

I got the opportunity to evaluate and calibrate the V2 285 Frequency Shifter.  This module is very similar to the original with the exception of an additional PCB to mount the front panel controls.

 

This is a three PCB stacked module with a lot of trimmers, but the calibration procedure is pretty straightforward..

 

The front PCB1 is the panel controls, reference oscillator, and associated trimmers.  This PCB is a prototype so there are a few wires that are not on the final version.

 

The middle PCB is the two 90 degree phase filters and associated trimmers.

 

The rear PCB3 is the multipliers for the frequency shifter, the balanced modulator, and associated trimmers.

 

 

Operation

This scope waveform shows the Sum Out (cyan) and Diff out (magenta) with a 100 Hz Signal (yellow) and the Reference oscillator adjusted to ~50 Hz.

 

This scope image shows the Variable Out (magenta) with the Amplitude control set to 50% and the Ring Out (green)

 

Here is a much higher signal frequency with a triangle reference that shows the envelopes more clearly.

 

 

Calibration

Calibration is not difficult but does require a two channel oscilloscope.  Note anywhere where you need a single sine wave input you can jumper the sine TP1 to the input, as for the frequency shifter or balanced modulator multiplier calibration.

285R Frequency Shifter Calibration Instructions  (This document is the instructions below)

Reference Oscillator Calibration

  1. These adjustments are all on PCB1.  Connect your scope to TP1.  Set the Reference control to some reasonable frequency (100 - 250Hz) and adjust TR1 marked SINE for a nicely shaped sine waveform.  At the extreme trimmer setting I still had a bit of a double hump so increased R16 until I was able to adjust a nice sine waveshape.  This is most likely due to tolerances in the Q1 JFET.

  1. Set TR2 marked FREQUENCY fully CCW.  Reduce the Reference control to full CCW.  Adjust TR2 for 1 - 2 Hz.  Note that the reference oscillator will go through zero (stops)  and restarts.  Make sure you don't go through zero.

  2. Increase the Reference control to 10 - 20 Hz.  Adjust TR3 marked SYMMETRY for best symmetry.  Reduce the Reference control to full CCW.  Check the waveshape and increase the frequency with TR2 if necessary to achieve a nice reference sine waveform.  I had to increase the frequency to about 1.5 Hz to have good symmetry.

Filter Calibration

There are two sets of filters that need to be calibrated.

  1. Plug an external sine oscillator into Signal with a +/2V amplitude.  Set Reference to Ext (up).  Connect a dual channel scope probe 1 toTP3 on the rear PCB.  Connect probe 2 to the TP5.  Make these adjustments on the upper set of trimmers marked SIG.

  2. Set the external oscillator to 11 KHz and adjust TR8 marked 11KHz for a 90 degree phase shift between the two sine waves.  This image shows the two waveforms.  I used the cursor as a visual aid to adjust the cyan waveform zero crossing to the magenta waveform peak.

 

An alternate setup and more useful for the lower frequencies is to display the two signals in XY mode. A 90 degree phase shift will form a circle if the two signals are equal (left image). They likely differ in amplitude so will display an ellipse but adjust until the ellipse is symmetrical about the X and Y axis (right image).

 

  1. Set the external oscillator to 1100 Hz and adjust TR7 marked 1100 Hz for a 90 degree phase shift between the two sine waves.

  2. Set the external oscillator to 110 Hz and adjust TR6 marked 110 Hz for a 90 degree phase shift between the two sine waves.

  3. Set the external oscillator to 11 Hz and adjust TR5 marked 11 Hz for a 90 degree phase shift between the two sine waves.

  4. Set the external oscillator to 1.1 Hz and adjust TR4 marked 1.1 Hz for a 90 degree phase shift between the two sine waves.

  5. Plug the external sine oscillator into Ref.  Connect probe 1 to TP6 on the rear PCB.  Connect probe 2 to TP2.  Make these adjustments on the lower set of trimmers marked REF.

  6. Set the external oscillator to 11 KHz and adjust TR13 marked 11 KHz for a 90 degree phase shift between the two sine waves.

  7. Set the external oscillator to 1100 Hz and adjust TR12 marked 1100 Hz for a 90 degree phase shift between the two sine waves.

  8. Set the external oscillator to 110 Hz and adjust TR11 marked 110 Hz for a 90 degree phase shift between the two sine waves.

  9. Set the external oscillator to 11 Hz and adjust TR10 marked 11 Hz for a 90 degree phase shift between the two sine waves.

  10. Set the external oscillator to 1.1 Hz and adjust TR9 marked 1.1 Hz for a 90 degree phase shift between the two sine waves.

Frequency Shifter Multiplier Calibration

Make these adjustments on the rear PCB.  

Plug a 1 KHz external sine oscillator into Signal with a +/2V amplitude.  Set Reference to Ext (up).  Connect a dual channel scope probe 1 to TP4 and probe 2 to TP7.  Set these channels to AC coupled and a low setting in the 10mv - 50mV range.  This scope image shows the initial waveforms on TP4 and TP7.

  1. Adjust TR20 for a minimum waveform on TP4.

  1.  Then adjust TR16 to further minimize it.

  1. Adjust TR22 for a minimum waveform on TP7.  Once this is set to minimum, then adjust TR19 to further minimize it.

  1. Remove the external sine oscillator and plug it into Ref.

  2. Adjust TR17 for a minimum waveform on TP4.  Once this is set to minimum, then adjust TR15 to further minimize it.

  3. Adjust TR23 for a minimum waveform on TP7.  Once this is set to minimum, then adjust TR21 to further minimize it.

Sum & Diff Null Option 1:

  1. The Sum Null TR14 and the Diff Null TR18 set the balance between the two multipliers.  Make these adjustments on the rear PCB.  Plug an external sine oscillator into Signal with a +/2V amplitude and set Reference to Int.  Adjust the display until you get a stable Sum (cyan) and Difference (magenta) output.  I used my cursors to highlight the peaks differ in amplitude.  Adjust the Sum Null TR14 first until you get the peaks of the Sum Out even.

  1. Adjust the Diff Null TR18 second until you get the peaks of the Difference Out even.  There is interaction in the trimmers so you may have to go back and iterate to get them even.

Sum & Diff Null Option 2:

  1. The Sum Null TR14 and the Diff Null TR18 set the balance between the two multipliers.  Make these adjustments on the rear PCB.  Connect the sine TP1 to both the Signal and Reference input and set Reference to Ext.  The sum output will be 2X the reference frequency and the difference output will be 0.  Adjust the Sum Null TR14 first until you get the peaks of the Sum Out even.

  2. Adjust the Diff Null TR18  to minimize the Diff Out.

Balanced Modulator Multiplier Calibration

The four lower trimmers are not marked on the prototype PCB but I assume they will be on the final version.

  1. Plug an external sine oscillator into Signal with a +/2V amplitude. Connect a scope probe to Ring Out. Set the channel to AC coupled and a low setting in the 10mv - 50mV range.  Make these adjustments on the rear PCB.  Adjust TR27 for a minimum waveform.  Once this is set to minimum, then adjust TR25 to further minimize it.

  2. Remove the external sine oscillator and plug it into Ref.

  3. Adjust TR26 for a minimum waveform.  Once this is set to minimum, then adjust TR24 to further minimize it.

Congratulations!  This completes your calibration of the 285 Frequency Shifter.

 

 

Troubleshooting Tip 1

A customer contacted me with a 285 he built that had very little signal at TP2, TP3, TP5, and TP6.  I tried to help him by email but we made no significant progress.  Parts were installed correctly and the right value.  He finally sent me the module for repair.  I measured the voltage down the filter chain and it decreased from +/-6.2V down to +/-0.5V; there was basically no headroom at the end of the filter.  I pulled one of the J201s and put it on the curve tracer.  The Vgs(off) parameter was out of spec.  I pulled the other 5 in the filter chain and every one was out of spec.  These J201s were purchased on eBay.  I bought a quantity of J201s from Small Bear Electronics and didn't even bother to measure them.  I simply installed them and powered on.  This scope image shows the input signal (yellow) the output of the filter chain with the Small Bear Electronics JFETs (cyan) and the eBay JFETs (magenta).  Note there is a 2:1 difference in the scale between cyan and magenta.   Always buy quality parts.

 

 

Troubleshooting Tip 2

A customer sent me his 285 that would not oscillate lower than about 5 Hz. The triangle integrator would just go to the negative rail.  I looked at IC1 and IC2 for near 0V performance and found both oscillated.  I added 510 pF capacitors across R20 and R21 to tame the oscillations. This does lower the frequency response of the CV input but does not affect normal operation.

 

I had assumed this would fix the issue but it did not.  The issue was at 0V into the integrator the output would drift to one of the rails. The symmetry control would adjust which rail it would drift to.  The comparators  won't affect this since they change the sign of the input, but at 0V there is no integration.  I eventually determined that the thru zero detector IC6 had too much range. The hysteresis is set for +/-1.4 mV. I lowered R38 from 2K2 down to 100R which drops the hysteresis to +/- 120 uV, not much. However, I could then adjust the symmetry control to hold the output when the input was 0V.  This is probably a better adjustment procedure for the symmetry trimmer. With this setting I could adjust the frequency down to 0.034 Hz although the symmetry degraded significantly.  This image shows correct thru zero operation.

 

When troubleshooting it earlier by email we had changed C7 from 15 nF to 33 nF in an attempt to determine if the issue was frequency or current related. Leaving C7 at 33 nF I adjusted the low frequency to 1.1 Hz and that gives an upper frequency of 926 Hz. I also adjusted R13 from 6K8 to 1K5 to improve the sine shape. At 1.1 Hz the symmetry isn't perfect but still reasonable.  Adjusting the symmetry at this low frequency changes it for higher frequencies and causes the integrator to drift to the rails at 0V.

 

At the upper frequency of 926 Hz the symmetry is quite good. I did adjust R8 from 330K to 270K to increase the maximum frequency to 1.1 KHz. Since there is no exponential converter, this is a V/Hz oscillator, and the larger capacitor lowers the frequency range for a given delta CV. I adjusted R11 from 680K to 560K so the CV input would provide the same range. I did not adjust the FM CV input.

 

 

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