Sencore TF151 - Matching Components; Transistor Substitution Guidance; Checking Diodes and Rectifiers In-Circuit; In-Circuit Diode Test Procedure

the devices used mu st be matched for the best operation of the circu it. If only one
device is to be replaced, the TF 151 ca n be u sed to measure the important parameters
of the good devic e and then measure the r eplaceme nt device to find one that matches.
In bi-polar t ransistor s, the beta and ICBO should be matc h ed for the best operation.
Tllis is especially true in power stages such as an audio out put stage. If the l eak-
age is not mat ched, the circuit can be unbalanc ed and cause troub le agai n in the near
future.
For the best demodulation in the new diode demodulators used in color TV or in a
ring type demodu l ator, the diodes should be match ed for forward and reverse cur-
rents. Tlle rev erse current is often referred to as the le aka ge of the diode. An un-
balance of leakage in a demodulator circuit can cause an upset in the color on the
receiver screen.
Tlle field effect tra ns is tor shou ld be matched for lO SS (zero bi as drain current) as
well as Gm for proper operation in i. ts circuit. In man y cases, the l OSS is extremely
important as it can determ ine the proper operating point as well as the stability of
the circuit.
TR ANSISTOR SUBSTITUTION
Many replacement lin es of transistors are now availab le to the serv i ce industry.
Tlley run from as few as six to as many as 24 different types to replace the different
types used in tel ev ision and radio. Each of these manufacturers print a replacement
gUide for common transistors you may e ncount er. These can be obtained from your
local el ectro ni c parts di stributor.
When the transistor number is not listed in the replaceme nt gU id e, it becomes very
difficult to guess just what type should be put in place of th e defective transistor.
In most cases, it ca n be narrowed down to an IF, Rr, or audio type. To simplify
matters, find a sim il ar number transistor in the set you are work ing on and measure
the parameters with the TF151 and check the rep l acement .list in th e back of the
Sencore Reference Book that comes with the TF151 to find a tran si stor that comes
close. The manufacturers rep l acement gUide should be co n sulted to de t ermine if the
type
is for RF, IF, or audio app li cations. By making a compar i son check with the
TF15 1, mu ch time ca n be saved in waiting for a direct repla ce ment type from the
manufacturer. Tllis may not work in all cases due to the critical nature of some cir -
cuits, but should be tried when you need a fast subst itut e to get thin gs going.
Be sure to check the co ll ector breakdown voltage so that the replaceme nt type wi ll not
break down in the ci rcuit. A good rule to follow is th at the co ll ector br ea kdown of
the replacement transistor should be equal to or greater than the app li ed voltage to
that stage it is r e[3 la ced in.
CHECK I NG DIODES AND RECTIFIERS IN -CIRCUIT
The TF151 w ill check di odes a nd power rectifi e rs in ci rc uit for bo th open or shorts
with a very low imp eda nces across it. To conn ec t the diode for inr c i rcuit checks
see fi gure 8 and use the following steps:
1. Conn ect the bl ack lead to the cathode or pos i tive l ead of the di ode.
2. Connect the red lead to the anode of the diode.
3. Set the Type switch to NPN and the function switch to LO POWER X IO positio n.
If the diod e is a power rect ifi er, set the function sw it ch to HI POWER XIO.
4. Rotate the BETA CAL control fully clockwise. If th e meter reads up sca le
to the
right, the diode is good a nd has diode action. If the meter does not
move, the diode is either open or shorted. However, it may be shunted by a
defective compo nent. Remove the diode from the c ir cuit and rech eck using the
out of circuit check d escr ibed below to determine if it is the diode or some
other component.
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