Ehrhorn Technological Operations ALPHA 76A, Руководство по эксплуатации и техническому обслуживанию, Страница: 10 / 19

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SECTION 4
1. RF Amplifier S e c tio n : Tubes VI and V2 (and V3 in models 76PA and 76CA) are
parallel-connected in a grounded g rid configuration. +28 v o lts of cathode
bias i s applied via R115 and L3 to cut o f f plate current during non-transmit
periods; bias i s reduced to an operating level of 5.1 v o lts when the antenna
changeover relay i s actuated.
RF excitation i s applied to the tube cathodes via an input network, including
a broadband toroidal f e r r it e input matching transformer, which i s c a re fu lly
designed to present to the exciter a nominal 50 ohm load.
Changing t h i s input
network may re su lt in excessive input VSWR.
The plate output c i r c u i t i s a fu ll p i-L network co n sistin g o f C18, L4-L5-L7,
C19, and L6.
The pi-L provides harmonic suppression s u b s t a n t ia lly better than
that of a simple pi network, as well as somewhat better e ffic ie n c y in t h is
application.
An r f directional wattmeter, c o n sistin g o f L201 and associated components,
senses forward and reflected power in the r f output lin e for d isp la y on the
panel multimeter when selected by pushbuttons FWD and REFL, respectively.
Excitation r f voltage i s detected and f ilt e r e d by ALC r e c t i f i e r C209-C210-D203
and associated components. The drive level at which negative-going ALC voltage
i s generated ( i . e . , the threshold ALC le v e l) i s set by rear-panel potentiometer
R209, which appropriately back-biases D203.
Relay K4 switches the antenna s t r a ig h t through to the exciter when the ampli­
f i e r i s OFF or in STBY or receive conditions, permitting normal transceive
operation. K4 switches the ALPHA 76A into the transmit configuration when an
external s h o r t - c ir c u it i s placed across RELAY control jack, J3.
2. Power Supply; Transformer T1 i s a 1.5 KVA continuous-service unit (2.4 KVA
H i p e r s i l K-core for 76CA only) which supplies a l l required a-c operating voltages
for the amplifier.
High voltage d-c for the tube anodes i s r e c t if ie d by a full-wave bridge, D101-
D104, and f ilt e r e d by a 30 mfd., 2.7 KV capacitor c o n sistin g of C l01 - C l06 in
se rie s.
These computer-grade e le c t r o ly t ic capacitors are burned in and matched
at the factory for both d-c leakage and capacitance, to insure uniform voltage
d is t r ib u t io n and long service l i f e .
High voltage tap-change relay K3 selects high or low transformer voltage taps,
as controlled by front panel CW/SSB pushbutton switches.
Cover interlock switch S5 prevents actuation of the main power relay, K1, unless
the cover i s secured in place. A safety crowbar switch places a low-resistance
short across the HV supply (and discharges the f i l t e r capacitor bank) whenever
the cover i s opened. THIS IS A PERSONNEL-SAFETY FEATURE WHICH SHOULD NEVER BE
DEFEATED EXCEPT TEMPORARILY BY HIGHLY QUALIFIED SERVICE PERSONNEL!
THEORY OF OPERATION