JLH-831 Voltage switch, alternate voltmeter ONLY check out and calibration procedure Rev 0 - January 30, 2001 Congratulations! You have completed construction of your JLH-831 voltage switch and you are now ready for check out and calibration. If you don't have two 1-volt-per-octave VCOs, you can complete the calibrations with a decent digital voltmeter. However, if your meter is only 3 1/2 digits (like most handhelds), your calibration may not be perfect. But, it may still be better than the by-ear method. First, connect your 831 to a proper +/- 15 VDC power supply. Set all the selector switches to the "no shift" or 12 o'clock positions. Turn on the power to the 831 and make certain nothing is smoking or overheating. With nothing connected to the input, connect a voltmeter to the "A" output. The voltage should read somewhere near zero. Do not be concerned if you see a small voltage quantity in the tenths of millivolts. There is some DC offset inherent to the circuit. This offset does not have any negative effect as you will find later. Rotate the "A" shift selector switch: * to the "1" position. Your voltmeter should read near +0.250 volts. Do not be concerned that it is not exact as you have not calibrated the 831 yet. However, it should be close. * to the "2" position. Your voltmeter should read near +0.333 volts. * to the "3" position. Your voltmeter should read near +0.417 volts * to the "4" position. Your voltmeter should read near +0.583 volts. * to the "5" position. Your voltmeter should read near +0.833 volts. * to the "-1" position. Your voltmeter should read near -0.417 volts * to the "-2" position. Your voltmeter should read near -0.583 volts. * to the "-3" position. Your voltmeter should read near -0.667 volts * to the "-4" position. Your voltmeter should read near -1 volts. * to the "-5" position. Your voltmeter should read near -2 volts. Return the "A" shift selector to the "0" position. Repeat the above steps for the "B" and "C" shift selector switches with the voltmeter connected to the "B" OUT and "C" OUT respectively. If the voltages read reasonably close to the above numbers, your 831 is probably functioning correctly and you are ready to complete your check out and begin calibration. Check or place all your 831 selector switches to the no-shift position. Connect your 1-volt-per-octave keyboard CV to your 831 "A" IN. Connect one oscillator to your 831 "A" OUT. The oscillator should track and perform as if the 831 was not connected. Repeat for the "B" channel. Repeat for the "C" channel. If your oscillators appear to track normally, your 831 is functioning properly. Calibration: Set all the selector switches to the "no shift" or 12 o'clock positions. Connect your voltmeter to the "A" output. You will find a small offset voltage of something less than 1 millivolt present. Measure that voltage and write it down. Be sure to note the polarity as negative or positive. Next you will rotate the "A" selector switch and check voltages just like we did to confirm the 831 is working. However, this time you will adjust the trimmers as we go. For these instructions, I am going to assume an offset voltage of negative 0.240 millivolts. Note: All octave and interval tuning MUST be done on the "A" channel. Tuning the "A" SHIFT switch: Rotate the "A" shift selector switch to the "1" position. Your voltmeter should read near +0.250 volts. Calibrate the voltage to the sum of +0.250 and the offset. Using my assumed value, that should be +0.24976. You can see the problems that using a 3 1/2 digit meter will give you. In the case of the 3 1/2 digit meter, you must ignore the offset. For the remainder of these instructions, the 3 1/2 digit adjustment number will follow the 5 1/2 number in parenthesis. Remember, these adjustment numbers use the assumed offset voltage of - 0.240 volts. So, you must fill in your own actual offset and adjust these numbers accordingly. * Adjust trimmer VR5 so your voltmeter reads +0.24976 volts (+0.250) * Rotate the "A" shift selector to the "2" position. Adjust trimmer VR 4 so your voltmeter reads +0.33309 volts (+0.333) * Rotate the "A" shift selector to the "3" position. Adjust trimmer VR 3 so your voltmeter reads +0.41667 volts (+0.417) * Rotate the "A" shift selector to the "4" position. Adjust trimmer VR 2 so your voltmeter reads +0.58309 volts (+0.583) * Rotate the "A" shift selector to the "5" position. Adjust trimmer VR 1 so your voltmeter reads +0.83309 volts (+0.833) * Rotate the "A" shift selector to the "-1" position. Adjust trimmer VR 6 so your voltmeter reads -0.41691 volts (-0.417) * Rotate the "A" shift selector to the "-2" position. Adjust trimmer VR 7 so your voltmeter reads -0.58357 volts (-0.584) * Rotate the "A" shift selector to the "-3" position. Adjust trimmer VR 8 so your voltmeter reads -0.66691 volts (-0.667) * Rotate the "A" shift selector to the "-4" position. Adjust trimmer VR 9 so your voltmeter reads -1.00024 volts (-1.00) * Rotate the "A" shift selector to the "-5" position. Adjust trimmer VR 10 so your voltmeter reads -2.00024 volts (-2.00) Return the "A" shift selector to the "0" position. Your voltage should read very near the original offset voltage you recorded earlier. You have now completed calibration of the shift trimmers using the "A" channel. Only two trimmers remain to match channels B and C to channel "A." You should tune these two trimmers by voltage. However, an alternate "ear" tuning method is provided toward the end of these instructions. Disconnect all cables to from the 831 INs and OUTs. You will NOT be connecting the voltmeter to measure the output of any individual channel. Instead you will Connect your voltmeter to the measure the difference between "A" OUT and "B" OUT or "A" OUT and "C" OUT. We will start with channel "B." One meter lead will be connected to the "A" OUT jack red wire (left lug) and the other lead will be connected to the "B" OUT jack red wire (left lug). Tuning the SHIFT "B" switch: If necessary, set your voltmeter for the lowest setting possible. We will be measuring in the tenths and hundredths of millivolts. A millivolt scale is essential for this setting. If your meter will not measure millivolts, refer to the alternate tuning method at the end of this text. If you can read a millivolt scale, here is the procedure. Check or place all your 831 selector switches to the no-shift position. Connect the meter between the A and B output "hot" leads (as described above) You will read some voltage. That is normal and it indicates the DC offset difference between the A and B channels. This offset is inherent to the op amps. It does not impact your 831 operation because it is always there and taken into account when you tune. Also, tenths of a millivolt are not an issue. Write down this voltage. Place the "A" shift and "B" shift switches to the -5 position. The voltage reading will change a little. Adjust trimmer VR11 until the voltage reads the same as what you wrote down. Remember, we are significantly less than 1 millivolt here, so close is good enough. Turn the "A" and "B" shift switches both to each position of the selector switch (so they are on the same position) and watch the voltage. This "difference" reading should be fairly consistent for all settings. However, it will vary slightly. Return the "A" and "B" shift switches to the no-shift position. The voltage should be the same or very close to what you wrote down before. The "B" channel adjustment is now complete. Tuning the SHIFT "C" switch: If necessary, set your voltmeter for the lowest setting possible. We will be measuring in the tenths and hundredths of millivolts. A millivolt scale is essential for this setting. If your meter will not measure millivolts, refer to the alternate tuning method at the end of this text. If you can read a millivolt scale, here is the procedure. Check or place all your 831 selector switches to the no-shift position. Connect the meter between the A and C output "hot" leads (as described previously) You will read some voltage. That is normal and it indicates the DC offset difference between the A and C channels. This offset is inherent to the op amps. It does not impact your 831 operation because it is always there and taken into account when you tune. Also, tenths of a millivolt are not an issue. Write down this voltage. Place the "A" shift and "C" shift switches to the -5 position. The voltage reading will change a little. Adjust trimmer VR12 until the voltage reads the same as what you wrote down. Remember, we are significantly less than 1 millivolt here, so close is good enough. Turn the "A" and "C" shift switches both to each position of the selector switch (so they are on the same position) and watch the voltage. This "difference" reading should be fairly consistent in all settings. However, it will vary slightly. Return the "A" and "C" shift switches to the no-shift position. The voltage should be the same or very close to what you wrote down before. The "C" channel adjustment is now complete. Alternate tuning method for SHIFT "B" and SHIFT "C" (by ear): If your voltmeter is not capable of measuring millivolts, you can use this method to adjust trimmers VR11 and VR12. Disconnect all cables from the 831 INs and OUTs. Set all three "A", "B" and "C" SHIFT switches to the no-shift position. Connect one cable from your 831 "A" OUT to # 1 oscillator 1-volt-per-octave input. Oscillator # 2 needs no connection for CV. Set the 831 "A" SHIFT switch to the "-5" position. Adjust your oscillators so that they are zero beat to the same frequency in this configuration. Use a high range frequency as it will produce more accurate tuning. Once these two oscillators are zero beat to the same frequency, disconnect the cable from the 831 "A" output and plug it into the 831 "B" output. Set the "A" SHIFT switch to the no-shift position (although not apparent, that is important). Set the "B" SHIFT switch to the "-5" position. Your oscillators will be close to the same frequency, but may not be exact. Adjust trimmer VR11 to achieve zero beating. Set the "B" SHIFT switch to the no-shift position. Disconnect the cable from the 831 "B" output and plug it into the 831 "A" output. Set the "A" SHIFT switch to the "-5" position. Adjust your oscillators so that they are zero beat to the same frequency in this configuration. Use a high range frequency as it will produce more accurate tuning. Once these two oscillators are zero beat to the same frequency, disconnect the cable from the 831 "A" output and plug it into the 831 "C" output. Set the "A" SHIFT switch to the no-shift position (still very important). Set the "C" SHIFT switch to the "-5" position. Your oscillators will be close to the same frequency, but may not be exact. Adjust trimmer VR12 to achieve zero beating. End of alternate tuning method for VR11 and VR12. Congratulations! You are complete with your 831 calibration. Yes, it took a while. But this is a very accurate circuit. It needs to be just right so those oscillators are rock solid on the intervals and octaves. If you have trouble with the tuning, you may want to make certain you have your VCOs tracking properly. Even if the octave shifting is perfect, it will not seem so if your VCO tracking is not properly dialed in.