Wednesday, April 13, 2016

Octopus Tester / Component Curve Tracer

The Octopus Tester  is an accessory for an oscilloscope which permits a method of power-off, in-circuit testing (ie; it is used to test damaged equipment without powering it up, and without removing components). Touching the probes to a device will produce a voltage-vs-current characteristic diagram on the scope screen, so a component under test can be quickly compared to a known working component simply by comparison of the image on screen.
One channel of a stereo amplifier is not working. You can use the Octopus Tester to compare suspect transistors on the blown channel to the same transistors on the working channel. Each time you find a damaged device, remove it, and test all components to which it was immediately connected (Resistors should be checked with a multimeter).

The mains transformer produces a 12V 50Hz sine wave with a floating output. This signal sweeps the Device Under Test, and an image is produced on an oscilloscope in X-Y mode. The voltage across the DUT is shown on the X axis, and the current through the DUT (measured as voltage drop across a known resistance) is shown on the Y axis.

Shown here is a diode. Voltage across the diode increases until the diode turns on at 0.6V. Current through the diode is then at maximum.

When testing components in circuit, there will typically be resistive, and reactive elements to the image on screen. You can first familiarise yourself by testing components out of circuit.

After using the device for a short time, you will begin to associate the images on screen with the devices you are testing, and the surrounding circuitry. It then becomes simple to quickly troubleshoot faulty circuits, based on knowledge of the devices you are probing.

The octopus circuit is housed in a small plastic enclosure. BNC connect to scope X and Y inputs, banana sockets are for regular multimeter probes.
Due to the simplicity of the circuit, point to point wiring is the quickest method of construction. Be sure to ground your unit correctly.

Setting up the octupus tester.
Ensure that your scope is set to DC coupling, and that the circuit you are testing has no other path to ground, ie; disconnect all power / audio / data cables to the unit.

It's also necessary to discharge any large caps before testing (You cannot damage anything, but your trace will disappear from the screen).

Some scopes come with a curve tracing function build in. Set-up and use of the component test function of the Hung Chang scope is the same as most other brands I have used.
I have two scopes on my test bench, the Hung Chang pictured is used solely for this function, such is the frequency of it's use.

This is a schematic of the Hung-Chang 3502C scope front end, from which I copied the design of my device. The component testing circuit is highlighted.