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Build a ‘T-40’ Solid State Analyzer

by Terence Thomas

If you have ever put together an electronic circuit with meticulous care, only to find out that it doesn’t work, then you know the frustration of being an electronics experimenter. Unfortunately this marks the end of some budding careers and it presents quite an annoying problem to the professional engineer.

The primary source of problems experienced in the first stages of prototype construction is inconsistencies in the performance parameters of semiconductors. Nearly fifty percent of solid state devices exhibit some significant deviation from the norm. In a perfect world, parts would perform with uniformity but in the real world this is not the case. To make sure you are working with semiconductors that are functioning properly, you need a device that gives you a variety of tests which can assure that the best component is chosen for a given circuit. The T-40 solid state analyzer is just that device.

GO NO-GO

All testers on the market perform a simple go no-go test, which merely confirms that a transistor will function under most conditions. The T-40 Analyzer also has this capability. A PNP-NPN toggle switch enables you to test both types of bipolar transistors. To test, put a transistor in the socket and select the type. If you do not know the type, try both positions. When you press the test button the LED should glow brightly, green for PNP and red for NPN. If the LED is dimly lit before you press the test button, the transistor is leaking and therefore has a narrow dynamic range and is limited to certain applications. A fully lit LED, without pressing the test button, indicates a device that is shorted out and if the LED does not light when the test button is pressed, the device is open and is not usable for any purpose.

The normal-reverse toggle switch reverses the emitter-collector leads and allows you to identify the proper lead arrangement on unmarked transistors. Three test leads enable the user to test devices in circuit or power transistors that will not fit in the test socket.

FETs, SCRs

FET transistors are tested in the same way as bipolar with the gate lead equivalent to the base lead of a bipolar. The drain and source leads are interchangeable. SCR devices also have a gate lead but the anode and cathode leads are equivalent to the collector and emitter of the bipolar transistor respectively. When testing an SCR, the LED should come on when the test button is pressed and stays on after the test button is released.

AUDIO TEST

A feedback oscillator is included in the analyzer, which uses the transistor under test as the drive element. Three parameters will provide information that will enable you to select the best transistor for the job. Pitch is the first determining factor. A rotary switch is provided to switch from go no-go mode to audio mode. With the rotary switch in the audio position and a transistor in the socket, you should hear a pitch. If it is very high, try switching the NOR/REV switch. The lower pitch will give you the proper emitter/collector orientation. Volume is also an important factor and should be louder in the correct position. The third parameter is tonal quality. A raspy tone is less likely to be the proper lead orientation than a pure tone. Transistors with a strong clear signal will work better in circuits requiring a wide dynamic range.

After a time, you should become quite an expert at matching transistors in this manner. It is amazing how much useful information an astute experimenter, willing to devote some time to this mode of operation, can gather.

An A-B test circuit is shown in an illustration that can enable the user to make quick comparisons between two transistors. It can be set up on a circuit designer or you might want to put together a permanent two-socket outboard test unit for matching transistor characteristics.

OSCILLOSCOPE MONITORING

Two test posts are provided at the bottom of the unit that can be used to monitor the audio signal on an oscilloscope. This will facilitate an in depth analysis of any device under test. Wave shape analysis by this method can be a valuable source of information in the fabrication of prototypes.

CONTINUITY TESTING

In the go no-go mode the test posts can be used as a continuity tester for wires and cables. Diodes can be tested using the posts and with the addition of two test leads, circuit continuity tests on circuits can be performed.

A-B TEST

When performing the A-B test, first make sure that both transistors have been checked out for proper collector-emitter polarity. Then place the two transistors in the test circuit and switch from A to B. If the pitch or volume of the two is considerably different, chose the transistor that exhibits the stronger clearest tone and then compare other transistors to it.

CONSTRUCTION

A 6 ¼, by 3 ¾, by 2-inch plastic project box was chosen but the metal cover plate proved to be too flimsy, so a circuit board was cut to size to provide more stability. All switches, socket, micro-mini jack, speaker, test leads and binding posts are mounted to the front panel. As we can see on the backside of the faceplate, there are also two circuit boards. One is mounted over the socket that is connected to the test leads to provide a stable connection point. The socket shows 4 holes but the upper and center socket connections are soldered together to accommodate both in line transistors or triangular lead arrangement. Circuit board two serves as a mounting platform for the audio transformer and capacitor.

Wiring is point to point with a five terminal strip added to accommodate points that are not anchored elsewhere. The two miniature binding posts are actually mounted backwards (Solder side out), and should be isolated from the foil on the back of the faceplate with a piece of tape. Since the analyzer will most likely become the most popular test instrument on your bench, it is important to make the unit mechanically stable.

A 2 ¾ inch square piece of perforated matrix board and a piece of matte board with a hole cut to match the speaker are used to mount the flat Piezo speaker from the front. A Piezo speaker was chosen because its impedance is 1K ohms, making it ideally suited to the circuit requirements of breadboard experimental circuits. Another piece of perf-board is used, like a sandwich, to hold the speaker in place. Just above this board is the 9 volt battery in a holder that is glued to a perf-board. The second board is mounted over the test socket to relieve joint stress due to test lead movement.

MAIN BOARD LAYOUT

As you can see in the illustration, the layout is quite straightforward. The transistor polarity switch is in the upper left corner. Next are the test socket and extension leads. Collector/ emitter reversing switch is on the right side of the socket. Below that is the internal/external switch and micro-mini jack that provides access to the piezo speaker. In the center of the controls is the rotary switch that determines the mode of operation. A dual direction LED is mounted to the left of the rotary switch and functions as an indicator for the go no-go test. Just below that are the test switch and test posts. The piezo speaker is to the right of the controls.

CIRCUIT LAYOUT

In the circuit photo you can see component layout. Starting at the lower left, we see the audio transformer and capacitor mounted on one of the circuit boards. The battery just above is secured with a battery clip glued to the rear support board for the speaker. All toggle and push button switches, socket assembly, test leads, mini jack, LED and rotary switch are mounted in a cluster on the right hand side (From the rear), of the circuit board. Since all parts are mounted on the face plate removal of the circuit for battery replacement is easy.


CONCLUSION

Although the T-40 is a fairly simple, it is remarkably versatile and can provide you with a test facility that has no equal when it comes to the analysis of solid state performance.

PARTS LIST
Resistors
R1-1K
R2-6K8

LED
Bi-directional

Socket
Transistor socket

Clips
Micro hook clips

Terminal strip
5 terminal strip

Capacitor
C1- .047 µF

Jacks
J1-J2 miniature binding posts
J-3 micro-mini jack

Transformer
miniature audio transformer

Batteries
9 volt with holder

Circuit boards

Terence Thomas
336 Warfield Ave.
Venice, FL 34285
(941) 468-2697
e-mail: t1t7t91@yahoo.com

 


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