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GENERAL
It is recommended that servicing of this receiver be done
by the factory service center. Special equipment and skills are
maintained at the factory to give fast and efficient service on
all of our products.
When retuming radio receivers to the factory for service,
include crystals, ac and dc power cables and telescoping
antenna. Disconnect cables and antenna, pack carefully and
include a trrief. detailed description of the difficulty you are
having.
The receiver circuitry is designed to utilize the best
feafures of four types of semiconductors: rectifier diodes,
conventional bi-polar transistors, insulated-gate field-effect
transistors and integrated circuits.
Servicing should not be
attempted by anyone who is not familiar with the manu-
facturer's recommendations and cautions relating to each of
these devices. The use of ohmmeters is particularly hazardous
since they can deliver voltages and currents large enough to
damage semicon duc tors.
Unusual circuitry in this receiver includes the automatic
tuning system. I.C. No.1 and the associated circuits generate
tuning voltages to track the antenna, r-f and oscillator circuits
as channels are scanned. Also, when the receiver is operating
in the
"L"
band, loading coil L-l is switched into the telescop-
ing antenna circuit.
Audio output power is measured with bursls of modula-
tion or by measuring the maxirnum excursion on voice
modulation as shorn on an oscilloscope. S'hen a continuous
tone is received. the output rrill start at full porver and then
decrease to approximatel,v half power to protect the output
inteEated circuit from overload. It rrill then retum to full
power for voice communications.
The LED indicators have a forward voltage drop of about
l.6v at 20ma. The current should not exceed 50ma. They are
polarized and may be damaged by a high reverse voltage.
When a channel lamp does not light, the failure may be either
the lamp or the switching I.C. If the channel n'orks, check the
lampi if not, check the I.C.
[hen
groups of lamps are out.
refer to the losic chart.
TRACKED TUNING SYSTEM
All tuned circuits in the RF sections are tuned by voltage-
variable capacitators which optimize the radio for each crystal
individually regardless of where it falls in any band. This tun-
ing is done automatically and allows the Bearcat IV to tune
all parts of any band without compromise.
Tracked tuning is accomplished by means of a DC voltage
a p p l i e d t o Y V C I . 2 . 3 , 4 , 5 . 6 .
T h i s v o l t a g e v a r i e s w i t h c r y s t a l
frequencv and is higher for higher frequency crystals. It is ad-
j u s t e d b r T l . T 2 . R l 7 a n d R 5 2 .
Facton- alignment of the RF and tracked tuning system in-
volves highlv specialized equipment and training not available
to nornral senice activities.
Because this unique feature is out-side the experience of
eren the most highlr- trained technicians. these adjustments
should onlv be made bv Electra.
BANDSWITCHING
Switches SWll-lB
are used to select the proper band for
each channel. These sn'itches, the channel scanning logic (IC6,
7 . 8 . 9 )
a n d h a n s i s t o r s w i t c h e s Q B , Q l l ,
a n d Q 1 3 g e n e r a t e
three l2\'
bandswitch signals-
U, U, and H. These signals
select the proper RF section (VHF or UHF) and switch in-
ductors in the VHF section to select L or H band. For any
selected channel. the logic diagram is as follows:
Band Selected
0 = OVDC
I = 12VDC
L
H
t-/T
BA\ DS \\' ITC H I\G FUN C TI ON S
(Q tt)
o n fo r L/ T b an d on l y
l.
Switches DB on (Grounds Tripler Tank)
2.
Switches QB off
3.
Switches Q{ on (Shifts tuning voltage upward)
{.
Switches Dl
on (Shorts antenna loading coil)
5.
-supplies bias roltages to Ql, Q2, Q3
(UHF Section)
(QB) on for [, or H band
Supplies drain voltage to Q6 (VHF amplifier)
(Ql3)
on for H band only
l.
Switches DB on (Grounds Tripler Tank)
2.
Supplies Gate 2 bias to Q7 (UHF \lixer)
3.
Switches D5 and D6 on (Shorts out low band coils
L9 a nd L l l)
4.
Switches Q4 on (Shifts tuning voltage upward)
SERVICING
I
I
0
0
I
0
0
0
I
U
- 1 0 -
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