Repairing Telequipment CT-71 Curve Tracer - part III

REPLACEMENT KNOBS

The new knob is a hair small, esthetically, but completely functional. I drilled the center hole, installed the new V/div knob and then added the thru rod and inner knob for the Series Resistance. They work great. To wrap this up, I want to add a nut to hold the V/div control a bit more firmly in place, but I can perform any device testing as this sits.

Replacement V/div knob behind the Series Resistance one

The rotating scale for the Series Resistance would align a set of resistance values such that the inner knob, when pointing at the outer .2 V/div scale position, selects the lowest of the eight resistance values that can be picked for the chosen Peak Wattage.

Originally, the Peak Wattage lever would turn the inner scale, but I can just refer to the scale and set up a spreadsheet that lists the settings for each of the four wattage peaks.

Resistance setting guide for .5 Watt peak

This is derived from this rotary scale and the movement imparted to it by the Peak Wattage lever.

Rotary scale and teeth that engage with the Peak Wattage lever

NUT TO HOLD CONTROL ASSEMBLY IN PLACE

The nut on the control assembly was a cylindrical shaft around which the scale rotated It was much too deep to allow my replacement knob to work. I tried to match the threads to bolts at local hardware stores, to determine the thread type for the nut I would buy. It was closest to Metric 10 or 3/8" Imperial, but the threads are finer than those in the stores. M10-1.0 was too coarse, as was 3/8-24.

I decided that it must be M10-0.75 although I couldn't find any of them at the big box stores or local hardware shops. Based on the likelihood that this is the proper size, I ordered some nuts online that were that size; should be here within a week.

Repairing Telequipment CT-71 Curve Tracer - part II

FURTHER DIAGNOSING OF THE HORIZONTAL POSITIONING BEHAVIOR

With the fuse FS1 installed, horizontal and vertical positioning worked properly. I ran through some initial checks of a PNP transistor I had but was still not getting the curves I expected. I had not yet put on the new outer knob and thus had no clear idea what V/div and load resistance values I was using.

Once the knobs and interlocks were working properly,  I set up the tracer on the bench and made use of my oscilloscope and VOM to verify that it was behaving properly. First up was a test of the collector voltage levels and the load resistances. The voltage was easy to look at but to I needed to drive it through my VOM to measure the current in order to properly assess the load resistors and other details.

With that working properly, and the current settings for vertical scale working smoothly, all that was left to test was the step generator that drives the base of the transistor. It was immediately obvious that I had the knob pointer set wrong.

I used the wiring diagrams and back of the control to verify when I was on a particular target setting, then moved the knob to match. Now the behavior of the curve tracer is perfect.

INSTALLATION OF NEW OUTER KNOB FOR VOLTS/DIV SELECTOR

I searched for a suitable knob to use - it had to have dual setscrews to tighten to a 1/4" round shaft and should permit drilling out the front where the inner control shaft will emerge. I picked a larger knob with a distinct pointer on it.

It arrived but it is too large in diameter, blocking the scale on the faceplate for V/div. For the time being I will use it, having drilled the hole and mounted the series resistor knob in front of it. I tried to rotate the controls to their extreme ranges but found that the V/div knob, which should allow for ten positions, would only move through eight of them. I suspect that there is a mechanical interlock from the rotating scale that I removed.

I partially disassembled the controls to check out that possibility. I had to play with this several times before I could get proper operation of the two controls, V/div and Series Resistance. I still need to acquire a smaller outer knob and to add a nut to hold the control assembly better but I am happy with the operation. The new knob is on its way, only needing some drilling once it arrives.

SAMPLE CURVE USING THE DEVICE

Here is a PNP transistor (2N2907A) that I hooked up for my initial test.

2N2907A curve tracing