Tektronix 575 Curve Tracer Restoration
We reformed the large electrolytic capacitors in the power supplies and inspected the unit for any damaged components. The three vibration isolators for the cooling fan were "perished" so we 3D printed substitute spacers. The fan was difficult to turn, but after some lubrication it rotated freely.
We connected the unit to AC power through a Variac, and slowly increased to AC voltage. Nothing caught on fire so we continued increasing the voltage. At about 65% of the 120VAC line voltage the fan started spinning. We eventually reached 100% of the line voltage and started experimenting with switch and knob settings. We were able to get some traces on the screen, so most of the unit was working.
We connected a PNP transistor, but even though we could adjust the base and emitter voltage settings we could not see any evidence of collector current being displayed on the curves.
We connected a 'scope to the base and emitter and could see the base stairstep. The number of steps could be varied as well as the voltage per step. The collector voltage sweep did not align with the steps per the owner's manual, so with a few adjustments we fixed that.
We need to investigate how the collector current is measured and how it should affect the curves on the display.
A Facebook member suggested connecting a resistor between the Emitter and Collector connections. When the Curve Tracer send current through the resistor the display should show a diagonal line on the display. After a lot of fiddling with the settings we were able see a diagonal line. This means that the Curve Tracer is displaying the Emitter-to-Collector current. That also means that most of the Curve Tracer is functional.
We connected a diode between the Emitter and Collector connections. After more fiddling with the settings we were able to get a voltage/current curve to display. Increasing the Peak Volts on the Collector Sweep caused the curve to go lower on the display. We tried the same experiment with an LED. In the display zero Volts and zero mA are at the top right. Voltage increases towards the left, and Current increases towards the bottom. By varying the Peak Volts on the Collector Sweep we could see the voltage required to turn the LED on was about 1.75V. At that point a small increase in the Voltage makes a large increase in the Current. The image above shows about 2V across the LED and it is dissipating about 100 mA. This is way more power than should be applied to this LED, but it was an interesting experiment. We tried other LEDs, and found one that had much more Voltage increase while the Current increased. We think that this is due to higher resistance in the LED.
Our project for next Saturday will be to continue testing the Curve Tracer and see if we can get transistor curves to display.
After some more fiddling and reading some of the User's Manual we were able to get some transistor curves. The image below is for a 2N404 Germanium transistor that is used in the Wang LOCI-2 calculator that we are working on.
Long Term Tasks:
Calibrate the Curve Tracer
Characterize the transistors and diodes from the Wang LOCI-2 that don't have any available substitute.
Characterize New-Old-Stock transistors and diodes from the DEC field service kit, and compare to the recommended substitutes.