Synthesizer VCO/Cable/Voltage Testbox

This project was originally intended to be a small battery-operated VCO volt-per-octave calibration tool, essentially just a precision voltage source that could be switched from 0 to 5 volts in 1 volt steps. However, as the project was developed, a couple of other handy test circuits were included in the design. A patch cord tester was the first addition, capable of measuring for continuity and shield-to-center-conductor short circuits. Fuse checking and other basic continuity tests can be done with this circuit as well. Secondly, a +/-10 volt bargraph voltmeter was designed, for monitoring control voltages as well as audio signals.

This tester is designed to be used with the SynthCase project, as well as other volt-per-octave modular synthesizers.

Below is the schematic for the Synth Testbox.

The circuitry begins with the power supply on the top left side of the diagram. A standard nine volt battery is connected to a switch to power the Testbox. Next there is a ICL7662 voltage convertor chip that creates a negative nine volt power rail for the op amp circuitry. A comparator circuit using half of a micro-power LT1017 IC turns off the battery LED when the battery voltage falls below just under 8 volts, well before circuit operation is affected by low voltage.

A REF02 voltage reference chip is the heart of the precision VCO calibrator circuit as well as the reference for the battery LED comparator. An adjustable voltage divider creates the rest of the VCO output voltages, which are then buffered with a low-power TL061 op amp.

The patch cord tester turns on a LED by passing the current through the cable being tested, using both the center conductor and shield in coaxial cables. It also uses the voltage drop across an extra resistor and the other half of the LT1017 comparator to sense a shield-to-center-conductor short circuit.

Finally, the bargraph voltmeter is built using a couple of LM3914 chips, in "dot" display mode to conserve power, driving two MV57164 LED modules. A TL061 invertor op amp circuit drives the LM3914 IC that displays the negative voltages. A voltage divider on the input and a TL061 buffer for the positive voltage display completes the circuitry.

The next task was to gather up all of the parts for the project. A small plastic case was chosen to house the Testbox. A locking power switch was selected to prevent accidental battery discharge. Banana and Lemo jacks were chosen to match the connectors on the SynthCase system.

A basic layout was planned out and drawn up.

A 300 DPI image was then created for the front panel. A "metallic" look was chosen for the graphics.

The image was printed out on regular paper and laminated. The area where the bargraph display is located was cut out of the paper before laminating, to create a clear window for the bargraph display to shine through.

Using the laminated graphics as a template, all the holes in the front panel were marked and drilled. It sure is easy to drill through this soft plastic, but it's also a lot easier to make a mistake and end up with a hole that's too big or incorrectly aligned!

The bargraph display modules were soldered to a piece of perfboard and mounting screws were countersunk into the front panel to hold the assembly in place.

Next, the laminated sheet was scoured on the back side with fine steel wool and glued to the front of the case with 3M Super77 spray adhesive. All of the mounting holes were carefully cut out with a sharp knife.

Here is the project with all of the front panel parts installed, looking good! The bargraph display with the laminate window even turned out well.

This is a view of the inside of the case, ready to be wired up.

Next, all of the circuitry was built up on stripboard.

Lastly, a plastic divider and some foam were installed for the battery compartment, all of the point-to-point wiring was completed, and the circuit board was mounted to the case. The completed device was tested and calibrated, and the cover screwed on.

Calibrating the bargraph voltmeter consists of applying +10 volts DC to the meter and adjusting the meter trim so that the last LED is just turning on. Applying -10 volts to the meter is then done to check the negative range and any error can be balanced between the two.

To calibrate the VCO voltage source, a high-quality voltmeter is connected, the output is set to 0 volts, and adjusted to exactly zero on the voltmeter with the offset trim control. Next the output is set to 5 volts and the 5 volt trim is adjusted for exactly 5 volts reading on the meter. Finally, the other four output voltages are each selected and adjusted for the correct value.

Here is the completed project, connected to the SynthCase and monitoring an audio signal. The Synth Testbox is functioning as designed and has exceeded expectations. The voltmeter section works particularly well in displaying different types of signals. DC voltages, LFO voltage swings, audio signals, gates, and even triggers are now easy to see and confirm. The VCO calibrator and patch cord tester are simple to use and very effective in operation.

This handy little box will be kept close at hand whenever working with the SynthCase, making troubleshooting much easier and keeping things well in tune!

Don't forget to check out The Peasant's SynthCase project!