Chevrolet 70 Series 1970, Service Manual

Page: 583 / 841

ENGINE ELECTRICAL 6Y-42
Figure 31 — Checking Transistors for Shorts
VOLTAGE-DIVIDER RESISTOR (R3),
(P art B , F ig . 30)
If one reading i s in fin ite or nearly in fin ite, or
if both readings a r e infinite or nearly in fin ite, the
r e s is to r i s open.
VOLTAGE DIVIDER RESISTOR (R5),
(P a rt C, F ig. 30)
If one reading is infinite or nearly infinite, or
if both readings a r e infinite or nearly in fin ite, the
r e s is to r is open.
POTENTIOMETER. (P arts D and E, F ig . 30)
If one reading is infinite o r nearly infinite in
P a rt D, the p otentiom eter i s open. If both readings
a r e infinite in P a rt E, the potentiom eter is open.
NOTE: When in stallin g a new potentiom eter,
lo c a te the adjusting le v e r in a v e r tic a l position
(fig. 28), turn the potentiom eter r e sista n c e adjust­
m ent to the m iddle position , then u se a sold erin g
iro n to m elt the le v e r into potentiom eter.
EMITTER-BASE RESISTOR
(OHMMETER CHECK NOT ILLUSTRATED)
Since th is r e s is t o r has been unsoldered from
the panel board at one end, m erely connect an ohm -
Figure 32—Checking Transistors for Opens
m eter a c r o s s the r e s is to r - an infinite reading in­
d ica tes an open. R ep lace if d efectiv e.
DRIVER AND POWER TRANSISTORS
(R efer to F ig. 31)
If both readings in Step 1 a r e z e r o , o r if both
read in gs a re v ery low and id en tical, the tr a n sisto r
i s shorted. S im ila rly , if both readings in Step 2,
o r in Step 3, a r e ze r o or v er y low and id en tical,
the tr a n sisto r is shorted.
DRIVER AND POWER TRANSISTORS
(TR -1 AND T R -2), (R efer to F ig. 32)
If both readings in Step 1 a re in fin ite, or if
both readings a r e v ery high and id en tical, the tran­
s is t o r i s open. S im ilarly, if both readings in Step
2
a re infinite o r very high and id en tical, the tran­
s is t o r is open.
REASSEMBLY AND FINAL CHECK
During a sse m b ly , coat with silic o n e g r ea se
both s id e s of the flat in su lators u sed betw een the
tr a n sisto r s and heat sink, and a lso the heat sin k on
the sid e on w hich the tr a n sisto r s a r e mounted. The
s ilic o n e g r e a se in c r e a s e s heat conduction.
CHEVROLET SERIES 70-90 HEAVY DUTY TRUCK SHOP MANUAL