Lafayette KT86A

8/3/2019 Lafayette KT86A

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LAFAYETTE MODEL KT-86A

TRANSISTOR TESTER (circa 1956)

Lafayette Radio was a large mail order electronics distributor and was one of the first to heavily

promote an interest in the new field of transistors. In fact they devoted a now classic 32 page catalog

supplement to them in 1956 (Catalog T4-56, "Transistors, the Miracle of the Electronics Age"). Amon

the many interesting transistor based kits they offered was one of the first pieces of test equipment

designed to check both transistors and diodes for their operational characteristics. Although simple b

today's standards, it did do the job.

Above is an ad for the first KT-86 tester from Lafayette catalog T4-56. Note the absence of the "A"

suffix. The picture shows what appears to be a prototype product which is clearly not the more

polished production unit that followed. The meter is marked in milliamperes rather than gain and

leakage and the usual Lafayette script logo is missing. Other advertisements for the KT-86 appeared a

early as February, 1956 in Radio-Electronics magazine.





By the fall of 1956 the KT-86 had evolved into the KT-86A. The front panel of the earliest KT-86A

production units has a very nice looking layout. It was housed in a typical 1950's standard 6" x 3-1/2"

bakelite cabinet which was used by many other products of the era such as the Jackson code practice

oscillator (reviewed elsewhere on this site). Construction was simple point to point wiring with just

three switches, three resistors, a pair of test plugs, a meter, and a transistor socket. The unit I acquire

had a nice vintage RCA battery (yes it is positioned correctly) which adds to the originality. TheRETMA code on the back of the meter, 484638, provides a clue as to the date of the unit. Although I

haven't yet been able to identify the manufacturers code of 484, the 638 translates into the 38th week

of 1956 which indicates that this is probably an early production KT-86A.





Above is a look at the cover of the assembly manual and the schematic diagram of the KT-86A.

Another KT-86A is shown above and reveals that a running production change was made by the

manufacturer of the meter. Although the RETMA manufacturers code is still 484, the date code on th

unit is 811 or the 11th week of 1958.



Inside view of

tester. 15v.battery is

changed to 41/2v,

to test surfacebarrier units.

To test a point-

contact or

junctiontransistor, plug it

into the socket

under the meter and find the

leakage first and

then gain.

Diodes are

plugged into the

tester on the left of panel: the

switch is thrown

from P-N-P to N- P-N allowing

meter to indicate

crystal's forward

and back current.



Diodes and rectifiers are tested by reading the amounts of current they pass in each direction. In one

direction the current should be negligible, while in the other it should be about equal to the short circuit

battery current. With the power switch off, test leads from the jacks are connected to the appropriate

terminals on the diode or rectifier. Then with the power on and the selector switches set to P-N-P and

LEAKAGE, the reverse current is indicated. This should never exceed 3 subdivisions on the scale. Then

the selector switch is turned to N-P-N to read the forward current, which should not be less than 2

milliamperes. In the case of rectifiers, which operate at much higher voltages than that in the checker, aunit that which tests OK at this low voltage may not perform satisfactorily at normal working voltage. If

the test is NG, however, the suspect rectifier must be replaced. Mar. 1954.

The premier issue of Electronic Illustrated in May, 1958 contained an article about transistor

checkers. Even though the name Lafayette and model KT-86A were not mentioned, that is

unmistakably what was being described. Each small picture shows the meter reading for a particular

test.

Early transistor based equipment such as radios, amplifiers, oscillators, or test equipment almost

always reveals some new insight into the history and evolution of semi-conductors. The KT-86A and i

various cousins from other manufacturers certainly make a nice addition to any collection devoted to

the dawn of the transistor age.

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Lafayette KT86A