Skip to content

Home

Jump to bottom
Phil edited this page Dec 31, 2016 · 7 revisions

Welcome to the ARP_Odyssey_VCO wiki!

To get the best v/oct tracking and thermal stability I recommend the following things:

  • use metal film resistors with 1% tolerance or less
  • if possible replace T4 and T5 by a single package NPN/PNP transistor like the BC847BPN. If you can’t find one of them try to get a matched pair of 2N3906/2N3904
  • if possible get the correct TEMPCO resistor (1.87k 3400ppm) for R39
  • try to make a thermal connection between T4, T5 and R39 (see image, red) to minimize temperature drift
  • I’ve added D1 and D2 as a reverse voltage protection (see image, blue). There will be a small voltage drop across the diodes which might influence the tracking behavior. You can replace the diodes by wires if you like but be careful not to burn your VCO by connecting its power in reverse!

General information:

  • I’ve replace the transistors with new ones which should be easily obtainable. However, you can experiment with different ones you like or stick with the original transistors. Keep in mind that they might have a different pinout. Check it before soldering them in. Here is an overview of the transistors that I’ve used.

  • Instead of soldering R39, T4 and T5 directly to the board you can solder in an 8-pin ic socket. This gives you the ability to experiment with different transistor pairings and thermal resistors and their behavior on v/oct tracking.

  • connect the jacks to the board as shown here

Calibrating the VCO:

  • R6 is used for pwm tuning

Make sure to turn all five control pots down and leave everything unconnected. Connect an oscilloscope to the square wave output. Now adjust R6 using a screwdriver until you reach a duty cycle of 50% which means that the square waves on and off time need to be equal to each other. Check if the duty cycle stays at around 50% over the whole range by turning up the frequency pot. Do further adjustments if needed.

  • R19 is used for frequency tuning

Turn all control pots down. Now bring the fine tuning pot up to 50% (middle position). Connect the keyboard input to ground. With a scope connected to the square wave output you can adjust R19 until your signal frequency equals 20hz. Let the system run for a few minutes and check again as there might be a small temperature drift.

  • R22 is used for scale tuning

Turn all control pots down. Connect the keyboard input to ground. With a scope connected to the square wave output turn the frequency pot up to exactly 100hz. Now you need to apply exactly 3V to the keyboard input (three octaves higher than ground). Adjust R22 until your output frequency is at exactly 800hz. Now repeat these steps until you have the correct values at 0V (100hz) and 3V(800hz).

Clone this wiki locally