SENCORE TF46 Portable Super Cricket Transistor FET Tester User Manual

Portable
Super C ricket
Ttansistor/FET Tester
Operation, Application and Maintenance Manual

TF46 Portable Super Cricket Transistor FET Tester

WARNING
PLEASE OBSERVE THESE SAFETY PRECAUTIONS
There is always a danger present w hen testing electronic equipm ent. Unexpected high voltages can be present at unusual locations in defective equipm ent. Every precaution has been taken in the design of your instrum ent to insure that il is as sate as possible.  However, safe operation depends on you, the operator. Become familiar with the equipm ent you are working with, and observe the following safe­ ty precautions:

1. Never exceed the limits of this instrument as given in the specifications section and the additional special warnings in this manual.

2.  A severe shock hazard can result if the chassis of Che equipment being serviced is tied to the “hot” side of the AC line. An isolation transtormer should always be used with hot-chassis equipment. Also, be sure that the top of your workbench and the floor  underneath it are dry and made of non-conduclive materials.

3. Remove the circuit power before making connections to high voltage points. If this cannot be done, be sure to avoid contact with other equipment or metal objects. Place one hand in your pocket and stand on an insulated floor to reduce the possibility of  shock.

4. Discharge filter capacitors (aftcr removing power) betore connecting to any part of the circuit requiring power to be removed.

5. Be sure your equipment is in good order. Broken or frayed test leads can be extremely dangerous and can expose vou toe dangerous voltages.

6. Remove the test lead immediately alter the test has been completed to reduce the possibility of shock,

7. Do not work alone when working on hazardous circuits. Always have another person close by in case of an accident. Remember, even a minor shock can be the cause of a more serious accident, such as falling against the equipment, or coming in contact  with high voltages.

8. Improper Fuse(s) Void Warranty. Fuses are for your protection, so always replace fuse with proper type and current rating. The proper fuse (pe description is marked near the fuse holder and in the manual, Always:
   A. Be sure you are replacing the right fuse. On units with more than one fuse, be sure vou are placing the proper fuse value in the fuse holder.
   B. Have the proper size replacement fuse in stock. With cach new instrament, be sure to update vour fuse inventory with any special value fuses your instrument may require.

9. Explanation of symbols.
   This marking indicates that the operator must refer to an explanation in the operating instructions,
   A terminal at which a voltage with respect to another teroiial or part exists or may be adjusted to 1.000 volts or more.

Portable Super C ricket Transistor/FET Tester
Operation, Application and Maintenance Manual

DESCRIPTION

INTRODUCTION
The transistor is one of the most common active electronic compo­ nents in use today. The num ber of transistors used outnumbers tubes, as well as integrated circuits in most types of electronic design, includ­ ing home entertainm ent, industrial, and m  aintenance equipment. A high-accuracy, easy-to-use transistor tester th a t allows both in- and out-of-circuit transistor testing can make any technician more effi­cient in troubleshooting today’s solid-state circuits.

FEATURES
The TF46 Portable Super Cricket provides the Sencore patented Cricket Gain test, which allows in- or out-of-circuit Good/Bad testing of tran sisto r gain. No basing information is required, as all possible basing combinations are tested with a rotation of the  Perm utator test switch. This test is backed up with the Super Cricket Param eter Test for determ ining th e engineering p aram eters of tra n sisto rs for m atched push-pull circuits, critical gain circuits, and tran sisto r design applications.
All six tran sisto r leakage paths are tested for locating transistors that are ju st startin g to go bad. A special I d s s leakage test is also available for grading FETs. In addition to these tests, the TF46 will provide complete basing inform ation of an unknown tran  sisto r to allow a replacem ent tran sisto r to be chosen when no cross-reference inform a­ tion is available, or to test a transistor in-circuit to be sure it is installed properly. An exclusive Auto-Off circuit saves batteries by tu rn in g the TF46 off autom atically-even  if the power switch is left on.

SPECIFICATIONS

Cricket Good/Bad Gain Test:

Test Method: Detects ability of tran sisto r to invert a square wave (Sencore p atent 3,898,559)
Test Frequency: 2 kHz
VCE = ±4.0 VDC
V gE = 7 VPP on zero reference
Test Currents:
Ic= 12 mA m axim um
2-3 mA average continuous
Ig = 7 mA maxim um
3 mA average continuous

Bi-Polar T ran sistor B eta Test:

Test Method: Dynamic Beta
Test Frequency: 2 kHz
Test Voltage: VCE – ±4.0 VDC
VbE =  VPP on zero reference
Test C urrents:
IC – (Sig Trans) 25 mA m aximum
IC – (O utput Trans) 150 mA m aximum
IB – (Sig Trans) 50 uA m aximum
IB – (O utput Trans) 300 uA m axim um

CONTROLS

1. Handle/Tilt Bail with slip-proof cushion allows unit to be easily car­ ried or used as a tilt stand on a service bench.

2. Good/Bad meter scale used for Cricket Gain test and battery check.

3. Gain meter scales give direct readout of transistor Beta or FET transconductance.

4. Leakage meter scales for all leakage tests, including FET Id ss,

5. Polarity Indicators for determining the polarity of an unknown transistor or FET.

6. Color Indicators for determining the basing of a transistor by relat­ ing the colors of the test leads to the transistor elements.

7. Basing Indicators provide full basing information of an unknown transistor.
   Test Selector Switches:

8. Gain (Lead ID) button used for param eter tests and lead ID testing.

9. Leakage (Diode) test button used for inter-element leakage testing and diode testing.

10. Idss leakage test button for industrial culling of FETs.

11. Perm utator Test Sw itch tests all possible basing configurations when rotated through all positions.

12. Pow er Sw itch controls unit, and resets Auto-Off.

13. Lead Storage Compartment.
    Param eter Gain Test Switches:

14. Enhancem ent FET selector for reading the transconductance of a positive-biased FET.

15. Normal FET selector button for measuring the transconductance of a normally biased FET.

16. Output Transistor selector button for measuring the Beta of a power transistor using the param eter gain test.

17. Signal Transistor selector button for measuring the Beta of a small signal or RF transistor using the param eter gain test.

18. Sliding M eter Cover protects meter during transportation.

19. Battery Test Sw itch (inside lead compartment) reads battery condi­tion on Good/Bad meter scale.

20. Speaker Sw itch allows speaker to be turned off when meter only testing is desired.

21. Battery Compartment (rear panel) for 6 “AA” batteries for portable operation.

22. Meter Zero (rear panel) for setting mechanical meter zero.
    Optional Accessory:

23. 39G85 Touch Test Probe (optional) allows in-circuit transistor test­ ing from foil side of printed circuit board.

OPERATION

INTRODUCTION
The following instructions explain the use of the TF46 in various types of applications. Each section gives details of how a test is to be performed. At the beginning of each test is a simplified front-panel draw ing showing the control setup and which m eter scale  to read. If a control is required for the test, it is m arked on the drawing. If a control is not needed, the label is not shown, and it may be left in any position without changing the test results. Several tests refer to Applications Tips. These tips are located on the last pages of the man­ual. These tips provide information on special testing applications that may be encountered in using th e TF46. Once the operations of the TF46 are understood, the Applications Tips may be used for answ er­ing questions during routine testing.
BATTERY INSTALLATION/REPLACEMENT
The TF46 requires 6 “AA” cells for operation. S tandard alkaline, carbon-zinc, or rechargable Nickel-Cadmium cells m ay be used.

To install or replace batteries:

1. Remove the screw a t the top of the battery com partm ent cover on the rea r of the TF46.
2. Install 6 “AA” cells, observing proper polarity as m arked in the battery com partm ent and as shown in Fig. 2.
3. Replace the battery com partm ent cover and replace the cover screw.

BATTERY TEST
The condition of the batteries may be easily tested by using the test switch in the lead storage compartm ent. The condition of the batter­ies will be read on the “Good/Bad” scale of the meter.

To test battery condition:

1. Turn the TF46 on.
2. Slide the right-hand switch in the lead storage com partm ent to the right. (See Fig. 3.)
3. Read the condition of the batteries on the “Good/Bad” m eter scale.
4. If the b attery test reads in the red “bad” portion of the scale, the batteries should be replaced or recharged.

AUTO-OFF CIRCUIT
The TF46 contains a battery-saving Auto-Off circuit. This circuit will autom atically tu rn the TF46 off after 15 m inutes of operation.
To reset the Auto-Off circuit:

1. Turn the PUSH ON-OFF switch off.
2. Turn the PUSH ON-OFF switch ON again.The TF46 will now oper­ ate for an additional 15 m inutes.

METER ZEROING
Proper calibration of your TF46 requires th a t the m eter movement be properly zeroed. An adjustm ent screw is provided through an access hole in the rear of the unit. To adjust the m echanical m eter zero:

1. Turn the TF46 off.
2. Check th a t the m eter is reading zero.
3. If the m eter is not zeroed, use a narrow blade-type screw-driver to adjust the m eter zero adjustm ent through the hole in the rear of the case. (See Fig. 4.)

LEAD CONNECTIONS
WARNING: Be sure power to the tran sisto r to be tested has been removed and filter capacitors are discharged before con­ necting test leads.

The patented Cricket Gain Test used w ith the TF46 rotary PERMU-TATOR SWITCH allows the three test leads to be connected in any order to the device to be tested. The three E-Z Hook® connectors are color-coded for lead identification testing and may be used w ith the optional 39G85 Touch-Test Probe for in-circuit board testing from the foil side of a printed circuit board.

The small size of the E-Z Hook® connectors allows them to be con­nected to m ost tran sisto r leads, even though only a small am ount of a lead is exposed.
To connect the test leads to the transistor:

1. To open th e connector:
   a. G rasp connector w ith the first and second fingers resting on the plastic collar. (See Fig. 5.)
   b. Place the thum b of the sam e hand at the rear of the ball.
   c. Pull back w ith the two fingers on the collar to expose the contact.

2. Connect each of the E-Z Hooks® to a lead of the tran sisto r to be te st­ed in- or out-of-circuit. In the case of sm all transistors with close leg spacing, use care to prevent shorting of the leads. (See Fig. 6.)

3. Perform the desired tests.

39G85 TOUCH TEST PROBE (Optional)
The optional Sencore 39G85 Touch Test Probe may be used with the TF46 for fast in-circuit testing from the foil side of a circuit board. At the rear of the probe are three recessed term inals which are color coded to the sleeves over the  contact needles (R-Red, G-Green, Y-Yellow).

To use the 39G85:

1. Connect each E-Z Hook® test lead from the TF46 to the m atched color term inal in the 39G85. (See Fig. 7.)
2. Touch each of the contact needles to the tran sisto r term inals in any order. Be sure the sharp needle points penetrate any solder flux or coating on the board for positive electrical contact.
3. Make the in-circuit Cricket Gain test as explained under “Test Procedures.”

TEST PROCEDURES

IN- OR OUT-OF-CIRCUIT CRICKET GAIN TEST

WARNING: Be sure power to the tran sisto r has been removed and filter capacitors discharged before connecting th e test leads.
The Cricket Gain test provides a safe and reliable m ethod of deter­ m ining if a tran sisto r has gain. The Cricket Gain test is a “Go/No-Go” test providing a test tone and m eter indication th a t indicates gain is present. No technical inform  ation is needed for testing for transistor gain. The sam e procedure is used for testing a tran sisto r or FET in- or out-of- circuit.

To test for tran sisto r gain:

1. Connect the three test leads to the three leads of the tran sisto r in any order.
2. Make sure the TF46 is tu rn ed on. Reset the Auto-Off circuit (if nec­essary) by pushing the PUSH ON switch off and then on if the Auto-Off circuit has cycled.
3. Any of th e PARAMETER SELECTOR buttons (to the left of the PERMUTATOR SWITCH) may be depressed, or all may be in the “out” position as they have no effect on the test.
4. Rotate the PERMUTATOR SWITCH one complete tu rn while watching the m eter and/or listening for the test tone.
5. If the tran sisto r has gain, the m eter will read in the “good” portion of the GAIN scale in either one or two PERMUTATOR SWITCH positions. At the same time, a test tone will sound from the TF46 (if the speaker switch in the lead  storage com partm ent is in the “on” position). A tran sisto r may test good in either one or two positions of the PERMUTATOR Switch. (See Applications Tips 11 and 12 for details.)
6. If the tran sisto r tests “bad” in-circuit, check the schematic of the tran sisto r being tested to see if there is a low-impedance sh u n t path around the tran sisto r (between any two elements). Generally, the TF46 will be able to check  in-circuit leakage paths as low as 50 ohms (or lower, depending on the tran sisto r’s gain) or capacitive leakage up to 15uF. It is recommended th a t transistors th a t check “bad” in-circuit be removed from the circuit and tested again  to elim inate the possibility of external loading. If the tran sisto r now checks “good,” perform the out-of-circuit leakage check. In the case where a tran sisto r checks “bad” in-circuit, and “good” out-of-circuit (including  leakage), there is probably som ething wrong in the circuit. Check for:
   1. Shorted PC foils, or
   2. Shorted components in the associated circuit.
   See Applications Tip 10 for additional details.

KEY LEAKAGE TESTS
Leakage tests should be done out-of-circuit, since the associated com­ponents of the circuit may cause false leakage readings.
To test for Key Leakage:

1. Perform the Cricket Gain test as described previously. Note the two positions th a t indicate “good” gain on th is test.

2. Depress the LEAKAGE push button w ith the PERMUTATOR SWITCH in either of the Cricket Gain positions and read the leak­ age on the lower (Icbo/Igss) scale of the meter.

3. Switch the PERMUTATOR SWITCH to the other position th a t gave the Cricket Gain test. Again depress the leakage button and read the leakage on the meter. Refer to Table 1 for typical leakage limits.

4. To determ ine which test is for Icbo, perform the LEAD ID test. The position used for the final step of the LEAD ID test is reading Icbo,
   

See Applications Tip 1 for inform ation on when to use the Key Leakage Test.

EXTENDED LEAKAGE TESTS
Some power output transistors can develop emitter/collector leakage.
The TF46 will check for this leakage w ith the Extended Leakage Test.
The Extended Leakage Test is not required for FETs.
To perform the Extended Leakage Test:

1. Perform the KEY LEAKAGE TEST described above. You will note th a t there are still four untested positions on the PERMUTATOR SWITCH for the SAME POLARITY TRANSISTOR.
2. Rotate the PERMUTATOR SWITCH through these rem aining four positions, and note the leakage at each position by depressing the LEAKAGE button. If you are testing a bipolar transistor, you should see two of the positions give  high (usually full scale) leakage. These are the forward leakage currents, Ibco, and Ibeo. The other two positions should read lower leakage. The two leakage paths tested in these last positions are Iceo and Ieco.
   A junction FET will give high leakage in all four rem aining switch positions since the channel of the FET is, for all practical purposes, a low-value resistor.
   See Applications Tip 2 for inform ation on w hen to use the Extended Leakage Test.

LOCATING LEAKAGE
If it is desired to find out which tran sisto r junction is showing high leakage, the following procedure may be used:

1. Locate the basing of the transistor using a reference book or by performing the “Lead ID Test.”

2. Connect the test leads of the TF46 in the following order:
   a. Green lead to Emitter.
   b. Yellow lead to Base.
   c. Red lead to Collector.

3. Repeat the Leakage Test to locate the high leakage position you wish to verify.

4. Refer to Fig. 12 to determ ine the leakage p ath read by each switch position.
   

OUT-OF-CIRCUIT PARAMETER TEST
The p aram eter test allows m easurem ent of the actual tran sistor gain for m atching or grading transistors and FETs. The Cricket Gain Test is used to determ ine if the tran sisto r has any gain and shows which PERMUTATOR SWITCH positions provide the proper basing connec­tions.
To measure tran sisto r param eters:

1. Perform the Cricket Gain Test as described previously. Note the two positions th a t indicate “good” gain on this test.

2. Select the type of tran sisto r under test – signal transistor, power transistor, norm al FET or enhancem ent FET. Generally, the correct tran sisto r type can be selected by m erely looking at the size of the device under test and  checking the type of circuit in which it is used. If the type of tran sisto r cannot be determ ined, refer to the “Identifying T ransistor Types” section below.
   

3. If the tran sisto r is being tested as a norm al Bipolar Junction Transistor (BJT):
   a. Depress the SIGNAL or OUTPUT TRANS button as indicated by the size of the transistor.
   b. Set the PERMUTATOR switch to one of the two positions th a t indicated good Cricket Gain.
   c. Depress the GAIN button and note the reading on the “B eta” Gain scale of the meter.
   d. Repeat the above test in the other position of the PERMUTA­ TOR switch th a t indicated good Cricket Gain.
   e. Use the higher of the two readings as the tran sisto r’s gain.

4. If the tran sisto r is being tested as an FET:
   a. Depress the NORMAL FET button.
   b. Set the PERMUTATOR switch to either of the two positions th at indicated good Cricket Gain.
   c. Depress the GAIN button and note the am ount of gain on the Gm Gain m eter scale.
   d. It is not necessary to repeat the test in the second position of the PERMUTATOR switch when testing FETs.

See Applications Tips 14 and 15 for inform ation on when to use the Param eter Tests.
DETERMINING TRANSISTOR TYPES
If the type of tran sisto r under test cannot be determ ined using other methods, the following procedure should be used:

1. Perform the “Out of Circuit Param eter Gain” test listed above testing the tran sisto r as a Normal FET.
2. If no gain reading (or a very low gain reading) is obtained at the conclusion of the test, the transistor should be tested as a “signal” transistor.
3. If a gain reading is obtained, repeat the test in the second position of the PERMUTATOR switch th a t gave a good Cricket Gain reading.
   a. If the two gain readings are the same, the device is an FET.
   b. If the two gain readings are different, the device is a germ anium transistor and should be re-tested as a “Signal” transistor.

DETERMINING TRANSISTOR BASING (LEAD IDENTIFICATION)
The TF46 will completely identify the type, polarity, and basing infor­mation of a transistor.
Example: Transistor shown as a PNP with red lead connected to Emitter, yellow to Base, and green to Collector.

To determ ine tran sisto r basing:

1. Perform the Cricket Gain test described previously. The polarity of the tran sisto r or FET is indicated by the position of the PERM U­ TATOR SWITCH at the conclusion of this test. If the pointer of the switch is to the right of center,  the tran sisto r is a PNP or P- Channel FET. If the pointer is to the left of center, the tran sisto r is an NPN or N-Channel FET.
2. Perform the Out-Of-Circuit Param eter Test. The basing of the tran­sistor is indicated by the position of the PERMUTATOR SWITCH at the conclusion of th is test.
   a. If the tran sisto r is a Bipolar Junction Transistor (normal transistor), the three elem ents of the tran sisto r are listed on the switch pointer, and the corresponding colors of the test leads are shown on the outside ring. (See Fig. 14.)
   b. If the tran sisto r is an FET, the Gate lead is indicated by the cen­ter color code on the outside ring. Since the Source and D rain of an FET are interchangeable in operation, the basing test will not determ ine these elements.
   See Applications Tip 13 for inform ation on w hen to use Lead ID.

DIODE TESTING
The TF46 checks the front-to-back current leakage of a diode. Since the resulting readings are given in microamps, the results may be compared to a spec sheet to determine if the diode is w ithin specs. The test will also determine the  polarity of an unknown diode. The test should be done out-of- circuit since current paths through external components in a circuit may cause false leakage readings.
To test a diode:

1. Connect the RED and GREEN test leads to the diode leads in either order.

2. Move the PERMUTATOR SWITCH to either of the two positions marked “diode.”

3. Press the LEAKAGE (DIODE) switch and note the am ount of leakage current on the “leakage” scale of the meter.

4. Repeat Step 3 in the other position of the PERMUTATOR SWITCH m arked “diode.”
   a. A good diode will show one high and one low leakage reading.
   b. A shorted diode will show high leakage in both positions.
   c. An open diode will show no leakage in either position.

5. To determ ine the polarity of the diode under test:
   a. If the highest reading was obtained in the “YGR” position, the cathode is connected to the RED lead.
   b. If the highest reading was obtained in the “YRG” position, the cathode is connected to the GREEN lead.

SPECIAL FETTESTS
Enhancem ent FETs m ay be tested for gain using th e special ENHANCEMENT FET test. These special FETs are indicated on a schem atic by the following symbol:

If the FET has a fourth “Substrate” lead, this lead should be connect­ ed to the G ate lead before performing the Out-of-Circuit P aram eter Gaintest. W hen testing out-of-circuit, use care in handling these insu­lated gate devices. See  Applications Tip 7 for details on handling MOSFETs. Dual-Gate FETs m ay be tested by testing each gate sepa­ rately. Since a dual-gate FET has four leads, it is necessary to know the basing inform ation before hook- up. Refer to the  schematic of the circuit associated w ith the tran sisto r or refer to cross-reference information for basing information.
To test a dual-gate FET:

1. Connect Gate 1 to the Source lead. Connect the TF46 to the G2, D, and G l/S leads and test as a norm al FET.
2. Connect Gate 2 to the D rain lead. Disconnect the previous connec­tion to Gate 1. Connect the TF46 to the G l, S, and G2/D leads and test as a normal FET. Idss tests are used for industrial grading or matching of FETs for critical  circuits.

To check for Idss:

1. Perform the Key Leakage test described previously.
2. Depress the Idss button and read the leakage on the Idss scale of the meter.

PROTECTION FOR TRANSISTORS UNDER TEST
The test currents of the TF46 tests have been chosen to provide the best balance betw een high testing accuracy, and protection for the device under test. The p aram eter tests have additional protection cir­cuits which prevent the  application of bias signals if the PERMUTA-TOR SWITCH is not in one of the positions that produced the Cricket Gain test. This m eans th a t the TF46 is completely safe for testing any tran sisto r or FET w ithout the possibility of dam  aging the transistor under test.

TESTING SCRS
Although the TF46 is not specifically designed to test silicon con­trolled rectifiers (SCRs) it will test m any types. The SCR specification that determ ines w hether or not it can be tested is the gate trigger volt­ age or current.

Two tests are available for SCR testing – “gain” (switching action) and leakage.
To test for SCR switching action:

1. Connect the three leads of the TF46 to the SCR as if it were a transistor.
2. Perform the Cricket Gain test.
3. An SCR th a t is being triggered by the TF46 and switching proper­ly will show one position as a good PN P transistor, and one as a good NPN transistor.
4. If no good readings are obtained, use a comparison test by testing a replacem ent SCR of the same type. If the replacem ent SCR tests properly, use the results of Step 3 as the test results.
5. If the replacem ent SCR does not test, the trigger characteristics are beyond the test signal levels supplied by the TF46. Proceed with the SCR leakage test.

Testing for SCR leakage:

1. Connect the RED and GREEN leads of the TF46 to the anode and cathode leads of the SCR under test.
2. Perform the Diode Test.
3. If the SCR is shorted, both positions of the test will show high leakage.
4. If the SCR is not leaky, NEITHER position will show high leakage.
5. An open SCR will not be detected unless it is one th a t can be test­ed using the Cricket Gain test.

CIRCUIT DESCRIPTION

THEORY OF OPERATION
The reliability of the Cricket Gain test depends on two im portant facts about transistors:

1. Over 90% of all tran sisto r failures result in a no-gain condition.
2. Any active gain component (including transistors) will invert a sig­nal applied at its input. Since virtually no combination of passive components will duplicate this action, an inversion check will not give false gain indications.

BLOCK DIAGRAM AND FUNCTIONAL DESCRIPTION

CRICKET GAIN TEST
There are three m ain sections to the Cricket Gain test. These sections are:

1. the test oscillator, 2) the perm utator test switch, and 3) the logic phase detector. The PERMUTATOR TEST SWITCH applies both input signal and tran sisto r bias to the tran sisto r under test in all pos­sible combinations of lead connections and polarities. The tran sisto rpolarity test is provided by reversing the bias. The PERMUTATOR TEST SWITCH also feeds the output of the transistor to  he phase detector. If the phase of the returning signal is 180° from th a t of the test oscillator, the phase detector feeds a signal to the speaker and relay drivers.
   Details of the phase detector are shown in Fig. 19. The signal from the test oscillator is fed to the base of the TR9. The second transistor, TR8, however, is normally biased on by the negative voltage at its base, shorting the output of TR9.  An in-phase signal (coming from a short­ ed transistor) will cause the bias to increase, which causes TR8 to rem ain on. If the signal at the base of the TR8 is out-of-phase, howev­er, TR8 will tu rn off during the positive portion of the retu  rningsignal, allowing the signal at the output of TR9 to pass to the speaker dri­ver and the relay driver.

The relay is used to prevent bias voltages from being applied to the tran sisto r during a param eter test if the PERMUTATOR SWITCH is not in a correct test position. The output of the relay also feeds the meter during the Cricket Gain  test to provide a visual indication of a good transistor.
If the GAIN switch is pressed with either the SIGNAL or OUTPUT TRANSISTOR TYPE buttons selected, bias is applied to the em itter through the relay contacts. A signal is applied to the tran sisto r base through one of the two precision resistors, R43 or R44. The output sig­nal is fed through one of the collector load resistors, R45 or R46. The voltage across the selected collector load resistor is m easured w ith the m eter circuit m ade of IC2C and IC2D and fed through the  meter bridge to the meter, which is calibrated to read the gain as Beta.

FET Gm Test
FET gain is determ ined by feeding the test signal through the voltage divider m ade up of R15 and R18. The signal is fed through C6 and R19 to lim it the current to prevent possible gate damage. The output is fed across R45, the source  load resistor. The voltage across this resistor is then m easured by the m eter driver circuit. The m eter is calibrated to read the micromhos of gain represented by the current passing through the load resistor. 

The PERMUTATOR SWITCH applies + or – 4.5 VDC through the m eter bridge. The current scale of the m eter is compressed by diodes CR13-15. The current is then fed to the base lead of the transistor under test. After passing through the transistor, the current passes through one of the collector load resistors to ground. The m eter is cal­ ibrated to read the current passing through this junction.
Idss LEAKAGE
The test voltages are fed through the relay contacts to prevent possi­ble damage if the PERMUTATOR SWITCH is not in the proper posi­tion. The Idssmeter bridge m easures the am ount of current passing through R49.

The power is controlled by TR1, 2, and 3. W hen the PU SH ON-OFF switch is “o ff, C2 is charged to battery potential, and C l is discharged through R2. W hen the switch is turned on, power is applied to the em itter of TR3. TR3 will not  conduct, however, unless TR2 is conduct­ing. The charge stored in C2 is applied to the base of TR2, which momentarily tu rn s it on, which in tu rn causes TR3 to conduct. Voltage from the output of TR3 is now fed through R5, causing TR2 to remain on. At th e same time, C2 begins to charge through the 22 Meg resis­ tor. When C2 is fully charged, TR1 conducts, shorting the base signal of TR2, causing it to tu rn off. This removes the base current path for TR3, which causes it to tu rn off, which removes power to the TF46.
SUPPLY SPLITTER
The TF46 requires a positive and negative 4.5 volts from the 9 volt supply. IC2A, TR5 and TR6 form a supply splitter, which produces a reference ground point, which is always h alf the b attery voltage.

MAINTENANCE AND SERVICE

INTRODUCTION
This M aintenance and Service section will help you m aintain the TF46 w ithin published specifications and assure years of useful appli­cation. The schematic, parts list, and PC board layouts are included on a separate sheet. Sencore’s  “Circuit Trace” schematic has the key sig­nal paths m arked for fast identification and tracing of power supply voltages, function paths, and signal paths as the various signals pass through the circuit and from board- to-board. Warranty  inform ation is printed on the back of the Quality Assurance tag attached to the instrum ent. Factory Service inform ation is located inside the back cover of this m anual.

CHECKING FOR BROKEN TEST LEADS
If the TF46 should fail to operate properly, check for a broken test lead.
To locate a broken test lead:

1. Connect all th ree test leads together.
2. Depress the LEAKAGE button and switch the PERMUTATOR SWITCH through all six positions of either polarity. All six posi­tions should read full-scale leakage.
3. If only two positions show full-scale leakage, one lead is open. To determ ine the open lead:
   a. Set th e PERMUTATOR SWITCH to either position reading full- scale leakage.
   b. The open lead is indicated as the EMITTER lead on the color indicators around the PERMUTATOR SWITCH.

If a lead should break, the break usually occurs w ithin V 2 ” of the E-Z Hook® connector. To repair a broken lead:

1. Give a sharp tug to the connector of the lead th a t has been deter­ mined to be open. If the break is near the connector, the wire will slide out of the insulation.
2. Re-strip the wire leaving about 1/2” of exposed conductor.
3. Open the E-Z Hook® by pulling the ball straight off the back of the connector. (See Fig.26.)
4. Resolder the test lead onto the hook, and slide the ball back into place.
5. Re-test the lead to m ake sure the problem has been corrected.

DISASSEMBLY
To obtain access to the inside of the TF46 for service or calibration:

1. Remove the 4 screws on the sides of the plastic case. Lift the plastic case away from the rest of the unit. At this point, all calibration controls may be reached.
2. Remove the 6 screws on the back of the TF46. The inside assembly may now be pulled out of the m etal portion of the case.
3. Remove the plastic strain relief holding the test leads.
4. Remove the 2 screws located in the lead storage compartm ent.
5. Disconnect the connector plug a t the top of the board.
6. The PC assem bly may now be pulled away from the front panel.
7. The connector plug m ay be reconnected to allow testing w ith all circuits connected.
   To reassem ble the TF46, reverse the above procedure.

CALIBRATION INSTRUCTIONS
NOTE: I f calibration should be necessary, adjust the leakage calibra­ tions before the gain calibrations. In order to assure accurate calibra­ tion, the metal meter cover door m ust be in its normal “open “position in front o f the meter  movement. The door m ay be removed from its track and placed in its normal “open” position without the case to allow access to the calibration controls.

Leakage

1. Connect a m eter capable of m easuring 3 mA in series w ith a 30 Kohm pot. Connect the Red and Yellow te st leads to th is series combination.
2. Set the PERMUTATOR SWITCH to the first position on the “N ” side (RYG).
3. Push the LEAKAGE function button and set the pot for a reading of 2.5 mA on the external meter.
4. Adjust the Full Scale calibration control (R38) for a full scale read­ ing on the TF46 meter.
5. Reset the external pot for a reading of .25 mA on the external meter.
6. Adjust the .25 mA calibration control (R39) for a reading of 250 uA.
7. Repeat steps 3-6 until both calibration points read correctly.

BetaCal
An approxim ate calibration may be obtained using the following pro­ cedure:

1. Connect an oscilloscope between the Red and Yellow test leads.
2. Set the PERMUTATOR SWITCH to the first position on the “N” side (RYG).
3. Select the SIGNAL TRANSISTOR tran sisto r type button.
4. Depress the GAIN button.
5. Adjust the Beta Cal control (R15) for an output signal of 1 VPP.

A more exact calibration may be obtained using the following proce­ dure:

1. Connect a tran sisto r w ith a known Beta to the three leads of the TF46.
2. Perform the Out-Of-Circuit P aram eter Gain test.
3. Adjust the Beta Cal control (R15) for the proper gain reading for the tran sisto r being tested.

FET Gm Cal
An approxim ate calibration may be obtained using the following pro­ cedure:

1. Connect an oscilloscope betw een the Red and Yellow test leads.
2. Set th e PERMUTATOR SWITCH to th e first position on the “N” side (RYG).
3. Select the NORMAL FET transistor type button.
4. Depress the Gain button.
5. Adjust th e Gm Cal control (R16) for a .4 VPP signal.

A more exact calibration may be obtained using the following proce­ dure:

1. Connect an FET with a known Gm to the three leads of the TF46.
2. Perform the Out-Of-Circuit P aram eter Gain test.
3. Adjust the Gm Cal control (R16) for the proper gain reading for the FET being tested.

APPLICATION TIPS

Due to the wide variety of tran sisto r types and circuits, there are a few transistors you may have questions about testing. Listed below are most of these special cases.

1. Circuits Requirin g “K ey L eak age” Tests
   Most transistors th a t fail, do so completely. These transistors will all be found by using th e Cricket Gain test. Two exceptions are tra n sis­ tors th a t are ju st startin g to go bad, and those in “critical-bias” cir­cuits.
   Critical-bias circuits fall into two m ain categories: 1 ) High frequency circuits, and 2) Direct-coupled circuits. In either type of circuit, any am ount of tran sisto r leakage can change the bias of the transistor enough to disrupt circuit  operation. These transistors should be test­ ed in-circuit using the Gain test. The circuit should then be tested for proper biases a t the tran sisto r leads as indicated in the schematic for the u n it on which you are working. If these  voltages are not as speci­fied, the possibility of leakage is very high. If your troubleshooting points to the tran sisto r – even though it has gain – it should be checked for leakage as explained in the Operation section. (SEE ALSO:
   Applications Tip No. 2).

2. Circuits Requirin g “E xten d ed L eak age” Tests
   Em itter/collector leakage can cause im proper circuit operation. While this type of leakage is rarely found in small signal transistors, it may appear in audio power output transistors, RF tran sm itter output transistors, and hi- urrent voltage regulator transistors. In these circuits, the voltages should be checked after the in-circuit gain test is per­formed. If the bias voltages are not correct, both the KEY and EXTENDED LEAKAGE TESTS should be performed. (SEE ALSO: Applications Tip No. 1).

3. Testin g H igh Frequency T ransistors
   See Applications Tip No. 1.

4. Testin g Direct-C oupled T ransistors See Applications Tip No. 1.

5. Testin g In tern ally Protected Transistors See Applications Tip No. 12.

6. Testin g Dual-G ate FETs
   Since a dual-gate FET has four test leads, it is necessary to know bas­ing inform ation to hook up the test leads for the test. The two gates of th e FET are tested independently. In the case of a dual-gate MOSFET, it is also necessary to  connect the untested gate lead to either the source or drain lead before the gain test is performed. This is because the second gate, if left open, will cause the FET to read “BAD”. Gate one should be connected to the source while  testing gate 2, and gate 2 connected to the drain while testing gate one.

7. Testin g M OSFETs (IG – FETs)
   W hen testing a metal-oxide device in circuit, no special precautions are necessary. If you test the device out-of-circuit, however, be sure to prevent possible damage to the gate due to static electricity. Keep all leads of th e device  shorted together by tw isting them or connecting a jum per betw een them before removing the MOSFET from the circuit.
   Connect the test leads and remove the short, and test the transistor.
   Then, reshort the leads again before removing the test leads.

8. Testin g “N oisy” T ransistors
   At tim es, an audio tran sisto r may check “GOOD” but cause distortion to the signal being amplified. The TF46 will help locate m ost of these transistors. The “Noise” m ay be caused by an in term itten t junction (see Applications Tip No. 9). At other tim es, high leakage is the cause.
   If the circuit param eters (voltage and signal) indicate the distortion is being caused in a particular transistor, it should be tested for leakage (see Applications Tips No. 1 and 2).

9. In termittent Transistors
   The combined gain and leakage test results in a 99.9% accuracy for all transistors tested. M any of the rem aining .1% (1 in 1000) transistors are interm ittent. The in term itten t may be therm al (unit operates for a short time then  cuts out) or mechanical (unit fails w hen bumped).
   Many of these interm ittents m ay be found by taking advantage of the test tone. Perform the GAIN test, and leave the TF46 in one of the test positions th a t reads “GOOD”. Now the tran sisto r may be heated, cooled w ith freeze spray, or tapped. If there is any change in the tone, the tran sisto r is in termittent and probably the cause of the circuit malfunction.

10. Testing Low Imped ance Circuits
    The few transistors th a t have very low-value AC sh u n t paths (fewer than 1 out of every 100 transistors) will not test in-circuit. In m any of these circuits, it is easy to disconnect the shunting path. In the case of a direct- coupled  audio-output transistor, the speaker is the load. By disconnecting the speaker, the tran sisto r m ay be tested in-circuit. In the case of an RF tran sm itter output, a low-resistance RF coil is usually the loading component and can be  isolated from the transistor.
    Or, in the case of a high-power voltage regulator, the power supply fil­ters are usually the cause of the loading, and can be elim inated by dis­ connecting one of the leads going to the capacitor.

11. One Position Tests (In-C ircuit)
    Since most transistors give a “GOOD” indication on two perm utator switch positions, m any technicians assum e all good transistors do.
    This is not the case, however. If the transistor being tested has a diode connected between two elements, or is direct-coupled to another tran sistor or I.C., it may only check in one position. The reason is that the diode action of the  second solid-state device may “short” out one of the two transistor outputs.
    When you encounter a one-position in-circuit test, refer to the schematic to see if the tran sisto r under test is direct-coupled to any other solid-state device. If so, the one position test indicates th a t the tran sisto r has gain. If there is any doubt, check the bias voltages. A final confirm ation m ay be m ade out-of-circuit. (SEE ALSO: Application Tips No. 1 and 12).

12. One Position Tests (Out-Of-Circuit)
    Since m ost transistors give a “GOOD” indication on two perm utator switch positions, m any technicians assum e all good transistors do.
    This is not the case, however. Some transistors have an extra diode junction built in for protection (as in a horizontal output transistor of a TV set) or to change its input characteristics (as in some IF transis­tors). This may cause only one switch position to give a “GOOD” test.
    If you have any doubt as to w hether a tran sisto r should read in one position or two, simply check a replacem ent transistor. If it reads on two positions and the suspected tran sisto r on one, reject the transis­tor. The EXTENDED  LEAKAGE test will also show an extra leakage path. Again if there is any question, compare the readings on a replacem ent transistor.

13. When To Use “Lead ID ”
    Most technicians find tim es w hen they would like to know the basing diagram of a transistor, but reference m aterial is not readily available.
    The most common tim es are: 1) A transistor has been replaced, and there is a possibility th a t it was not installed property, 2) A cross-ref­ erence tran sisto r is available, b ut the basing diagram is different from the original transistor, 3) An imported unit has transistors that are not shown in any cross-reference material. If any of these conditions arise, refer to the LEAD IDENTIFICATION section of the manual to determ ine tran sisto r basing. If a cross-reference transistor is needed, the basing inform ation plus the signal level and power supply ratings will usually give sufficient information to determ ine a cross-reference from the specifications included in most cross- reference books.

14. When To Use Parameter Tests
    The actual gain of a tran sisto r rarely changes unless the gain drops to zero. The Cricket Test will locate most bad transistors w ith the go/no-go test. The Param eter Test can be used, however, for testing tran sis­ torsin critical gain  circuits. The most common use of the Parameter Test is for m atching transistors (such as for audio outputs or balanced bridge circuits) or for grading transistors for use in critical circuits such as DC control circuits or IF stages.

15. In-C ircuit P aram eter Tests
    The Param eter Tests may be performed in-circuit, but th eir results will be questionable in most cases. The parallel paths formed by the circuits external to the tran sisto r may cause the gain characteristics of the tran sisto r to differ  from an out-of-circuit test. The in-circuit Param eter Test can, however, be used for in-circuit basing tests in m any circuits.

WARRANTY

Your Sencore instrum ent has been built to the highest quality standards in the industry. Each unit has been tested, aged under power for at least 24 hours, then, every function and range was retested to insure it m et all published  specifications after aging. Your instrum ent is fully protected with a one year w arranty (or three year w arranty on hand-held signal level meters) and Sencore’s exclusive 100% Made Right Lifetime G uarantee in the unlikely event a defect was missed. Details are covered in a separate document included w ith your instrum ent.

SERVICE
The Sencore Service D epartm ent provides all in and out-of-warranty service and complete recalibration services for Sencore instrum ents. No local service centers are authorized to repair Sencore Instrum ents. Factory service assures you of the highest quality work, the latest circuit improvements, and the fastest turnaround time possible. Most service repairs are completed within 72 hours of receipt.

Save the original shipping carton and packing m aterial for reuse should you ever need to ship your unit, or retu rn it to the Sencore factory for repair. If the original m aterials are unavailable or unfit for reuse, repack the unit according to  the following directions.
NOTE: Should you w ant to repair your own instrum ent, parts may be ordered directly from the Service D epartm ent. Any parts not shown in the parts list may be ordered by description.
We reserve the right to examine defective components before an in-w arranty replacement is issued.
SENCORE SERVICE DEPARTMENT
3200 S en co re D rive
S io u x F alls, SD 57107
Toll Free: I-8OO-SENCORE
FAX 605-339-7032
Fill in for your records:
Purchase D a te :__ Serial N um ber:__
Run Number:_____
Note: Please refer to the run number if it is necessary to call the Sencore Factory Service Department. The run number may be updated when the unit is serviced.

3200 Sencore D rive
Sioux Falls, SD 57107
Innovatively designed
Call l-SOO-SENCORE (736-2673) with your time in mind.
www.sencore.com
[email protected]

References

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