Pulsar TCF-10B, System Manual, Page: 131 / 249

signal is applied to the bases of QN1A and QN2A.
Negative feedback is applied to the bases of
QN1B and QN2B. At the positive side (QN2), the
differential output from QN2A and QN2B is
amplified by QN2D and Q2. At the negative side
(QN1), the differential output from QN1A and
QN1B is amplified by QN1D and Q1. The
positive side power output transistor (Q6) is
driven by Q5; the negative side power output tran-
sistor (Q7) is driven by Q4.
The no-load feedback is from transformer (T1)
back thru the RC network of R21, C7, C2, C5 and
R18 to the junction of R16 and R17, for the
purpose of stability. The loaded feedback is
derived from a sampling resistor (R33, R35, R36,
R37, R38, and R39, all in parallel) and fed back
thru C28, C29 and R23. The overall no-load
voltage gain is approximately 282. The overall
loaded voltage gain is approximately 141. The
partial loaded gain, between C28/A28 and the
primary of T1, is approximately 38.
The alarm circuit (loss of RF signal condition)
consists of QN3, Q8, K1 and associated compo-
nents. The RF signal is monitored by C22, at T1
pin 1. The signal sample is amplified in QN3A and
fed to QN3B and QN3C (QN3B and QN3C are
configured as diodes). A voltage doubler is formed
from C30, QN3C and QN3B. The output of QN3B
drives QN3D, via R44 and R45. QN3D is
saturated for an input of 1 W to C22 (with
reference to T1 secondary). As QN3D saturates,
Q8 conducts, driving the front panel LED (D6,
power monitor), causing K1 to energize (or de-
energize), indicating loss of signal condition.
Jumper JU1 allows the selection of an open circuit
or a closed circuit for the loss of signal condition.
The +20 Vdc line (leading to the alarm circuit,
etc.) is filtered by C10, C11, L2, L4, C19, C20 and
C21. The -20 Vdc (leading to C2/C4) is filtered by
C12, C13, L3, C16, C17, C18 and L5.
12.3 10W PA
TROUBLESHOOTING
To check individual transistors, e.g., Q1 thru Q8,
QN1, QN2 and QN3, remove them first from the
PC Board. Ohmmeter measurements of the tran-
sistors while in the PC Board are misleading
because of other paths on the board.
You may remove the heat sink by unscrewing the
four (4) corner screws and the hold-down screws
for Q1 thru Q8. The 10W PA Module can operate
at no-load conditions without the heat sink for
short periods of time while you are trou-
bleshooting.
Page 12–2 January 1996
TCF–10B System Manual
Technologies, Inc.
!
CAUTION
THE 10W PA IS, BASICALLY, AN OP-AMP
PROVIDING VERY HIGH GAIN WITH NEGATIVE
FEEDBACK. TRANSISTORS Q1 THROUGH Q5,
Q6, AND Q7 ARE THERMALLY CONNECTED,
I.E., THEY ARE MOUNTED ON THE SAME PART
OF THE HEAT SINK. ANY FAILING TRANSIS-TOR
MAY AFFECT OTHER TRANSISTORS. CHECK
EACH TRANSISTOR SEPARATELY.
IF NO
FAULTS ARE FOUND, CHECK OTHER COMPO-
NENTS.
BE CAREFUL NOT TO MISPLACE SCREWS,
SPRING WASHER OR INSULATING WASHER
USED TO MOUNT Q1 THROUGH Q8. DAMAGED
SCREWS OR INSULATORS SHOULD NOT BE
USED.