Tuesday, September 3, 2019

Repairing Telequipment CT-71 Curve Tracer - part III


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


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.

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