Thanks for your response. Only after writing that message did I actively search for the calibration document on the Doepfer page.  I followed its instructions of using the Quint setting on the A-156 and the sweepable output from the A-176, removing the A-150 from the signal path.  I've managed to fix the P4 setting so I get octave jumps to within 0.01V over 5 octaves while sweeping the A-176 CV1 coarse control. I'm still working on this.  But I don't think replacing the trimmer is necessary, merely proper research and following of instructions.  However, I don't understand why jumping from 0 to 5V through the A-150 would cause me to be unable to adjust P4 to the correct inverval. 

On Sunday, August 21, 2016, Jacco Ville jaccoville@... 1 < Doepfer_a100@yahoogroups.com > wrote:

 

  The A-155's Row 1 Pre Out was monitored using the DCV setting on hand-held DMM.  Step 1 was held to output the maximum value, fully CW. With range set to 1V, I adjusted P9 until 1.000V on DMM.  However, switching to 2V and 4V, the scale was still not wide enough.  I chose to tune 4V to 4.000V which left 2V at 2.003V and 1V at 1.002 or so. A calibrated quantizer would correct this if necessary. 

*OK

  My method to test the A-156s was to use an A-176 CV Source to send 0.000V and +5.000V signals to an A-150 Dual VCS.  The A-150 was toggled manually using a manual gate.  The output of the A-150 was verified using the DMM and sent to the CV Input of the A-156.  The output of the A-156 upper or lower unit was now monitored by the DMM.  Results varied.  

  Most units produced 0.000-0.0010V with the 0V at input and 4.972-4.983V with the 5V at input.  Adjusting P4 only resulted in reaching 5.000V in one unit and that was at the maximum rotation of the trimmer. Most units only were adjustable to around 4.985V, and some of those started with a 0.002-0.010 offset.

*Maybe an aging problem Can’t tell you.

  I attempted to adjust P1 but saw no change at the output.  I must not understand the procedure. 

*The trimmers P1 and P2 don’t change the output, only to determine the threshold of the quantizer.
The outputs of the DAC are fixed at 0.08333 steps (1.000V divided by 12, 1 octave has 12 notes) but you can adjust wether the quantization takes place from which point. Just before or spot on the quantization step for example.

Do either of you see a flaw in my test procedure   Can I install a different size trimmer for P4 or a 1% resistor in place of R21 and improve the performance  

*I am not able to give you an answer on this.

Perhaps this should be moved to another thread, but one of the upper units has been unresponsive for ages and this calibration quest has prompted me to address it.  The output is stuck at around 1.169 or 1.194V, it was different after turning power off and on.  I thought perhaps it would be an issue with the CV In or Out socket itself, but prior to today wasn't sure of the offset or held voltage.  Could the problem be Q1 or BU3   Is a voltage held indefinitely in the PIC or is it reset after power off  

*Sent it in for repair, but I’m shure Dieter may give a better suggestion

Cheers and good luck

Jacco Ville

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Van: Nicholas Keller maq163x2@... 1

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