Local Oscillator Circuits .(TR3-TR7
)
Germanium epitaxial mesa transistors (GM290) are used for all stages except TR3
which employs afurther AF239. TR3 &TR4are located in the RF Tuner Unit.
TR3 is the normal Local Oscillator for continuous tuning and is tuned by the fourth
section of the tunihg gang (C40 ), Output voltage is developed across the earth return
inductance from the tuned circuits which is marked "Z" on the circuit diagram. TR3 is
functional with the SYSTEM SWITCH S3 at "NORMAL" and is disabled in the other positions
by removal of the emitter supply voltage. Base voltage is retained at all times.
The remaining transistor located in the RF Tuner Unit is TR4. Its function is to
facilitate connection to the normal oscillator tuned circuits (via the isolating
resistor R17) from the alternative oscillator input at SK6.Operating conditions are
such that TR4will function as afrequency multiplier when required, multiplication by
factors of two or three being possible with normal drive levels. Oscillator circuits
L13-L16 offer some measure of protection against spurious drive signals especially when
using the receiver with external synthesised drive.
TR4is switched by interrupting the emitter supply at S3 when this is set to the
"NORMAL" position. Base voltage is retained as on TR3
.
The miniature socket SK6located on the RF Tuner Unit is fed via PL3 which termin-
ates ashort coaxial lead from TR6and TR7 in the separate Crystal Oscillator Unit.
Both these transistors are used as emitter followers and serve to isolate the separate
feeds from the internal crystal oscillator and the external oscillator drive source.
The Crystal Oscillator (TR5) employs aseries-mode circuit with output taken from
the collector via C56 .Up to eight crystals can be fitted at any one time, separate
tuned circuits being provided to permit accurate alignment of the oscillator circuit to
suit the crystal in use. The tuned circuits are adjusted when installing the crystals
and allow slight pulling of the crystal to bring the appropriate signal channel into
the centre of the i.f. passband. All circuits are identical and have values to suit
crystals in the basic range 37-88 MHz.
Actual crystal frequencies are calculated from (fs +10. 7), l/2(fs +10. 7)or
l/3(fs +10.7) MHzdepending on the part of the overall range in which the selected
signal frequency falls. Detailed information on calculation of crystal frequencies for
specific signal channels will be found on page 22 in the Section on "Operation".
Oscillator drive from the Crystal Oscillator is applied to the base of TR6 which
provides alow-impedance output to feed the alternative oscillator socket SK6 via PL3.
External oscillator drive is introduced to the Crystal Oscillator Unit via SK7 which
is linked to aBNC socket at the rear of the set by ashort length of coaxial cable
terminating in PL4. Input level should be of the order 500mV, isolation being provided
by the other emitter follower TR7 which feeds SK6 via PL3 and C60.
Switching of TR5, TH6 and TR7 is by S3C &S3D. Base and emitter supplies are re-
moved from TR5 except when S3 is set to one of the eight "CRYSTAL" positions. Base
voltage remains on the two emitter followers (TR6 &TR7) except at "NORMAL" when all
supplies are removed from the Crystal Oscillator Unit. The emitter voltages for TR6
and TR7 are applied only in the appropriate positions, i.e. to TR6 at "CRYSTAL" and to
TR7 at "EXT (OSC)".
Crystal Calibrator .(TR8 &TR9)
The crystal-controlled calibration oscillator (TR8 sGM378 )and its associated Tone
Oscillator/Modulator (TR9 sGET880) provide crystal check-points at 10 MHz intervals
throughout the tuning range of the receiver. Injection is to the primary of the inter-
stage coupling between RF Amplifier and Mixer by asmall probe attached to SK4. This
is fed from the Calibrator via ashort length of coaxial cable terminating in PL5.
-6-
