January 5th, 2016 I have a Tek 175, but no Tek 575. I tried to sell the 175 on Ebay, for local pickup in NYC, but nobody wanted it. The 175 is almost a complete curve tracer all by itself. The missing functionality can be provided by modern electronics. Specifically, it needs a step generator (assuming three-terminal devices are going to be measured) and some kind of data acqusition/display. Maybe a microcontroller can do this, with a couple of opamps.

The 175 can be powered up on its own, without a 575. In that configuration, the collector drive is fully functional. The base drive amplifier is unusable without the 575 because it is powered by voltages that the 575 provides to the 175 through the interconnect cable. The collector current sensing in the 175 works fine without the 575. The sensed collector current can be monitored by observing the differential voltage across pins J and H of the interconnect plug on the back of the 175.

I connected an 800-ohm resistor across the collector and emitter banana terminals on the front of the 175. The 175 was in 0-20 collector voltage range. I set the PERCENT OF PEAK VOLTS RANGE variac so that 4 Vrms was being applied to the 800-ohm resistor. The VERTICAL DISPLAY switch was set to 5mA/div, and 5 mArms happens to be the expected current. I observed 100 mVrms across pins J and H of the rear interconnect of the 175. Reducing the PERCENT OF PEAK VOLTS RANGE control for 2 Vrms across the 800-ohm resistor resulted in 50 mVrms across poins J and H. So the 175 works for testing two-terminal devices without being connected to a 575. However, directly probing the pins of the interconnect connector is not convenient or even safe.

The base drive circuits of the 175 can be thought of as two parts: the high-current amplifier, and the feedback amplifiers. The high-current amplifier is solid-state, with one emitter-follower providing the base current for a bank of four transistors, with ballast resistors in series with the emitters. The feedback amplifiers are responsible for providing a current or voltage precisely proportional to the base staircase voltage sent from the 575 to the 175. The high-current amplifier can be used with a low-voltage, low-current (under 5mA) driving signal due to the darlington configuration. The feedback amplifiers require the +300 VDC, +100 VDC, and -150 VDC supplied by the 575. To make use of the 175's original feedback amplifiers without the 575, we would need to find an alternative source of those voltages. Another approach is to use modern opamps instead of the tube-based feedback amplifiers in the 175.

The stairstep produced by the 575 can easily be produced by a microcontroller. The microcontroller can also digitize the collector current signal. So there is a possibilitiy of having a Tek 175-based curve tracer that interfaces with a regular computer (e.g., a laptop or desktop).