BK Precision 501A Instruction Manual Download

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single curve appears on the screen opposite to where
normal curves appear. This represents base-emitter
breakdown voltage of the device under test.

Matching two complementary devices may be ac

complished by inserting the PNP unit in the

right

socket and the NPN unit in the

left

socket. Simul

taneously switch both the 501 POLARITY and SOCK
ET  switches  in  unison  while  watching  the  CRT  dis
play.  When  the  devices  are  matched,  both  sets  of
curves  will  appear  to be  the  same,  only  opposite  in
polarity.  The  technique  is  similar  if  the  external
transistor  jacks  are  used  (Figure  15).

In-Circuit Transistor Testing

It is possible to use the semiconductor curve tracer

as an analysis tool for determining the condition of

a  transistor  in-circuit.  Although  quantitative  meas
urements  are  not  possible,  a  GO/NO-GO  test  of
device  can be performed.  The curves obtained using
this  method  most  often  appear  badly  distorted  due
to in-circuit impedances (capacitors, resistors,  induct
ors,  etc.),  but  if  properly  interpreted,  they  will  at
least  indicate  transistor  action.  However,  some  de
vices  will  not  produce  in-circuit  curves,  or  may  pro

duce curves that  are totally inaccurate.  For example,
a transistor  used  as a  series  pass  regulator  will test
"shorted"  because  the  large  value  filter  capacitors
around it act as a low impedance  to the  curve tracer
sweep  signal.  Another  example  is  where  all  the
curve tracer  step  drive  signal is shunted  away  by  a
very  low  in-circuit  base-emitter  resistance.  In  these
cases, it is best to  remove the device for out-of-circuit
testing.

The in-circuit probe supplied with this instrument

is ideal for making contact to transistors mounted
on P.C. Boards. Refer to the "In-circuit Probe" section
for more information.

When performing in-circuit tests use the fast set-up

markers on the 501 front panel as a starting point.

Since circuit impedances may shunt away base

drive  to  the  device  under  test,  it  may  be  necessary
to  re-adjust  the  STEP  SELECTOR  to  produce  curves.
The SWEEP VOLTAGE, however, should not  require

re-adjustment  throughout  testing.  Also,  it  is  not  rec
ommended  that  breakdown  voltage  tests  be  per
formed  with  the  device  in-circuit.

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Figure 15. Matching NPN and PNP Transistors

CAUTION:

COMPLETELY REMOVE POWER FROM THE

EQUIPMENT UNDER TEST.

Figure 16 illustrates some typical in-circuit curves

of  normally functioning transistors. The loops on the
NPN  curves  are  caused  by  capacitance  in  the  de
vice's collector leg.  Note,  though,  that  5 loops and  a
baseline  can be seen.  This indicates that every  base

step  produces  a  definite  collector  current,  meaning
the  device  is  probably  good.  The  PNP  curves,  taken
from  a  transistor  in  an  oscillator  circuit.  indicate
severe  leakage  even  though  device  has  none.  The
best  way  to  interpret  in-circuit  curve  displays  is
through  experience.  Practice  with  several  types  of
circuits and devices to obtain a general  idea  of  good
vs  bad in-circuit  semiconductors.

Greater precision for in-circuit testing with the

curve tracer can be obtained if the normal waveform
display  of  each  stage  is  available  for  comparison
to  the  results  obtained.  A  list  of  test  conditions
should  accompany  the  reference  display;  that  is,
sweep  voltage,  current  per  step,  oscillator  calibra
tion,  etc. Such  comparisons  go  one step  further  than
go  no-go  tests  and  allow  abnormal  conditions  such

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