Monday, August 26, 2019

Repairing Telequipment CT-71 Transistor Curve Tracer - part I

Curve Tracer to be repaired


There were five defects in this unit that I found after inspection and while testing some sample transistors. I will try to repair all of them to bring this into full operability which I can make use of in future repair projects.

One of them is cosmetic - the CRT has a blue filter plate installed under the ruled line cover plate, but that plate slips sideways and doesn't sit covering the entire tube face. I am not certain how this was supposed to be anchored originally, but it definitely needs some adjustment or assistance to sit properly.

Two were clear from a quick visual observation. There is a toggle switch that connects the instrument to the left or right set of transistor sockets, used to make quick A/B comparisons. The handle was broken off. The other issue was a broken rotary selector knob, apparently this is a common problem with Telequipment devices where the plastic decomposes and fragments into powder.

The knob is a complex unit that rotates the clear knob to pick locations on the outer printed scale, with a small concentric plastic knob that independently selects positions on a rotating inner scale that sits inside the broken knob in question. The inner rotating scale will turn based on a nearby lever. Finally, this Rube Goldberg control interlocks positions, so that the outer plastic and inner knobs have constraints on their position based on how the inner scale is positioned.

A set of transistor sockets are built into a plate that plugs into the front of the unit, but that plate is missing entirely from my unit. I have to either secure a replacement or build my own sockets to hook to the unit.

Finally, while testing transistors, I discovered that the horizontal position control is not working properly. For some ranges of motion, the trace jumps wildly or disappears while some ranges seem to work okay. The first diagnosis was a bad control potentiometer, which I treated with contact cleaner. Unfortunately, that made no difference to the faulty behavior.

The potentiometer is hooked across +6V and -6V supply connections, with the wiper feeding into the horizontal defection circuits. This pot can be removed from the circuit and tested by pulling connectors from the main PCB since the pot is fixed to the faceplate and connected by wires.

One complication was that the +6 and -6 wires on this pot were paralleled to the outside lugs of the vertical position potentiometer, thus pulling the wires didn't fully uncouple the pot to be tested. After ensuring that the wipers of both position controls were unhooked from the PCB, as were the +6 and -6 feeds, I could attempt some testing using a VOM.

The two paralleled controls read appropriately, a 22K and a 500K pot yield a combined resistor a bit over 21K. That checked out fine, so I moved on to test the horizontal control wiper individually against the +6 and the -6 ends. I should see a smooth growth in resistance with movement, in reversed directions depending on the end I connected. In fact, I observed exactly that behavior. Observing the vertical pot produced odd results, until I realized that with the 22K resistor across the ends of the 500K device, the wiper would move up and down with a peak at the center, approximately 125K ohms and lower on either end.

The correct operation of the pot told me that I have a more serious flaw in the unit, not caused by the control itself but by the components on the PCB. Drat. This would take more bench time examining the behavior of the horizontal circuit. I quickly discovered that fuse FS1 (250ma fast) was blown. I ordered a new one to replace it as it may have been blown by a test on a bad transistor, but I was be prepared to discover some defect on the board itself that is causing the fuse to blow.


I found a suitable replacement DPDT toggle switch and substituted it for the broken component. I verified proper operation and correct wiring.


I have to wait for the replacement fuse before I can do more diagnostics on the positioning behavior. More on this in part II.


My first approach was to look for another of the knobs from a junker Telequipment machine - while I could use the acquired knob in the short term, the better approach is to use the knob to make a mold where I can cast replacements of more permanent materials.

This knob was used on two different Telequipment models - the CT-71 curve tracer that I am repairing, and the D75 Oscilloscope. I saw two model D75 on eBay but pricing has been prohibitive. One has a Buy It Now price of about $150 and the other soared to $80 (plus shipping) in a bidding war. I will keep watching but this unit is not worth my spending large amounts of money. All told I would feel remiss spending more than about $50 for this repair.

The small concentric pointer knob is one of two electrical controls operated by this assembly, the other is controlled by the outer clear knob itself. The rotating inner scale is driven by the lever which has its own (third) electrical control. I can operate the small pointer and the inner scale lever just fine, but need a method to rotate the outer knob controls while looking for a good replacement.

The two electrical controls coupled to the outer knob and small pointer are interlocked such that they can only form certain combinations of settings. The inner scale simply displays the value associated with the small pointer setting, as this changes based on the lever settings.

Position of rotating scale with right lever at .1 W setting

Position of rotating scale with right lever set at 10 W
Interlocking controls behind the complex knob assembly
The shaft for the control that is turned by the missing outer knob is set back inside the inner scale. The knob had a metal insert with setscrews that would grip the shaft. This constrains the type of temporary knob I might be able to affix to the shaft.

Since the rotating scale has no actual electrical role, I decided to remove it from the control assembly. This will allow me to mount a set of concentric knobs to rotate the two electrical controls. The outer control sets the volts per division of the horizontal sweep while the inner control chooses the collector load resistor size. The side lever which rotates the scale also picks one of four wattage peaks - 0.1, 0.5, 2 and 10.

As the volts per division goes up, so does the collector voltage. Power is the square of the voltage divided by the load resistor, thus one thing the lever does is set the smallest load resistor value possible to implement the wattage limit.

For example, with a 2W limit, the highest setting of 100V/div applies 400V peak to the transistor under test. The load resistor needed to hit 2W at that peak is 80K ohms. Selecting 0.1V/div or 4V peak and a setting of 2W, a 2 ohm resistor is the low value. The absolute smallest resistor, for 10W and 0.1 V/div is .016 ohm and the very largest resistor needed is for 100V/div and 0.1W would be 1.6 Megohm.

The volts/division and resistor controls have interlocks that limit the maximum spread - higher voltages enforce higher minimum resistances, but the inner control can select even higher values to drop the power below the max selected by the side lever. The lever itself shifts the range of resistors being selected by the inner knob. In a sense the interaction of these three cause the inner knob to select 10 linear steps from least to max power.

Based on this, I determined that the rotating scale is not really necessary to my operation. I can use pictures of the position of the scale for each of the four sliding lever power limits and refer to that if I need to know the load resistance being selected by my inner knob position. In most cases, if the power limit on the sliding lever is safe, I can dial any of the 10 power percentages. Occasionally I will want to limit it partway - a 5W transistor with the power lever at 10W would be safe only with the first five (highest) resistances.

I found a knob that I could place on the outer shaft to indicate the V/div setting. The inner knob was installed to show, by the angular position, which of the power values was selected. My four pictures of the scale, one per power limit, let me correlate that with the resistance value.


After examination of the design of all the parts, I could see that this had simply been misinstalled. I rotated it to the proper orientation and put together the stack of filter, gratule and cover on the machine. This is repaired.


Initially I hooked up the transistors to be tested using jumpers. I saw an eBay auction of the plate I need, but the opening price including shipping is fairly high already. As well, the sockets provided fit only a few types of transistors making this of limited utility. If I were a collector of Telequipment testers and wanted to restore this, the original block would make sense, but to use this as a bench tool I thought of a better approach.

I whipped up a design for a small project box that would house several useful transistor sockets plus other means of connection. I will build this and hook it to the CT-71 to perform transistor and diode testing. I have a number of sockets and pins on order to make the adapter box. More on this in part II.

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