Converting a B&K Oscilliscope into a Component Signature Curve Tracer

A signature tracer is a type of curve tracer for testing various electronic components in and out of circuit, and it's a very handy tool for troubleshooting. I built this tracer by converting a 1980's vintage B&K model 1403A oscilloscope. The actual tracer circuit is from Mr. Carlson's Lab on Patreon and the schematic and PCB files are only available to his subscribers. But it only costs a couple of dollars a month to subscribe and the content available is well worth it to me.

This unit is a transistorized general purpose compact 5MHz oscilloscope and B&K produced several similar models including the 1403, 1403A and the 1405. It doesn't have the greatest of specs, but it doesn't need them for the component tracer, and it's compact size is perfect for the job.
For starters, I completely recapped the unit and then removed or repurposed several parts on the front panel to accommodate the curve tracer.
  • The EXT Horizontal input jack was removed and the tracer's oscillator level control was substituted.
  • The external horizontal input switch position was connected to the tracer's Horizontal Out.
  • The AC/DC switch was rewired to serve as the Lo / Hi current selector of the tracer.
  • The Vertical Input jack was rerouted to the tracer probe input and the tracer's Vertical Out line was routed back to the scope's attenuator switch and on to the vertical input.
  • The wire to the Z-Axis input terminal on the rear was disconnected and replaced with the Meter Out line from the tracer. This allows connecting a DVM for a digital readout of the oscillator voltage level.
All modifications to the scope are non-destructive: no new holes drilled or any other changes that can not be completely undone if desired. The only inputs to the scope after the mods are from the tracer, but basic scope functions still work and can be used for verifying proper tracer operation. Building the circuit on protoyping board turned out to be a bit time consuming, but the conversion is fairly straight forward. 

A word of caution concerning transistorized scopes: there is high voltage inside, high enough to be lethal. The unit may be transistorized, but the CRT is still a tube and there are potentials as high as -1400VDC inside this unit. Proceed at your own risk.
Although there are PCB layouts for both the tracer PCB and a power supply on Patreon, this tracer was hand wired on prototyping board because I wasn't setup to make PCBs at the time. The ICs in the picture are opamps and a dual +/- power supply regulator. The board's input power comes from a separate power transformer which was added near the scope's xformer.
Another look at the tracer PCB and power transformer. It is attached using standoffs to existing holes. The power transformer is also mounted using existing holes.
The signature for a diode creates a "knee". This particular diode is a 1N4007. Basically, you're looking for a good, sharp transition.

​Transistor junctions can likewise be checked with the tracer.
A Zener diode signature has two knees  (forward and reverse). The tracer can test Zeners up to about 13 volts. 
Here is an example of using a digital readout with the tracer. The oscillator gain is adjusted to where the zener's reverse breakdown knee is just beginning to turn. The readout shows this is just over 6 volts (the diode is a 6.2 volt zener).
The signature of a capacitor is an ellipse. How open the ellipse is depends on the capacitor value. 
A short displays as a vertical line (zero 0hms). A resistor is a line that is tilted from vertical. The degree of tilt depends on the resistance. This is the signature of a 1Kohm resistor.
This is the signature of a damaged base - collector junction in a 2SC945 transistor from the driver board of a 1970's Luxman R1040 receiver. 
Another example of a less than perfect part. This zener diode's reverse breakdown knee is very rounded, not a sharp breakdown point.
Page created 2/11/2020
Last updated 2/20/2020