Sunday, September 1, 2019

Repairing Telequipment CT-71 Curve Tracer - part II


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.


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.


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

2N2907A curve tracing

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