Racal Instruments RA-117 User Manual page 22

produce an output frequency of preisely 100 kc/s when the beat frequency oscillator
frequency control is set to zero. Bias is applied to this valve by C199 and R125.
39. The B.F.O. output is coupled to the diode detector anode via C215. The B.F.O. is
supplied with H.T. via S7 except when SYSTEM switch is in the CAL. or STANDBY
positions.
100 KC/S I.F. OUTPUT
40. The control grid of V17 is connected to the secondary of the first 100 kc/s I.F. trans-
former which feeds the stage with the 100 kc/s signal. The screen resistor R108 and
the cathode bias resistor R115 are of the same values as used in the scond 100 kc/s
I.F. amplifier, hence the A.V.C. characteristic of this stage is identical to that of the
main receiver. The anode load resistor R109 feeds the auto transformer L76 via block-
ing capacitor C189. This transformer provides a 70-ohms output at PL8 and PL9 for
external applications.
NOTE:PL8 and PL9 are connected in parallel, therefore only one 100 kc/s output is
available at 75-ohms, and to avoid a mismatch the other connection should be
made at high impedance.
CRYSTAL CALIBRATION
41. The crystal calibrator, controlled by the 1 Mc/s crystal or by the 1 Mc/s standard
input to V1, feeds signals at 100 kc/s intervals to the signal grid of the third mixer
stage to provide calibration check points. The calibration can only be carried out when
the V.F.O. switch S300 is set to the INT. position.
42. The 1 Mc/s signal, fed through SK2, is connected through PL2 and the grid stopper
R83 to the first grid of the mixer valve V13. The anode load consists of a 100 kc/s tuned
circuit (L70, C167) and is coupled to the control grid of V15 through the capacitor
C168. The anode load of V15 (L75, C117) is tuned to 900 kc/s and is coupledvia C178
to the third grid of V13. V15 is heavily biased so that it functions as a frequency
multiplier.
43. An output of 900 kc/s, appearing across the tuned circuit (L75, C177) is coupled
to grid 3 of V13 thereby producing a difference frequency of 100 kc/s relative to the
1 Mc/s input. The 100 kc/s output appears across the anode tuned circuit (L70, C167)
and is fed to the control grid of V15. The ninth harmonic is selected in turn by the
anode tuned circuit (L75, C177) of V15 and fed back to the third grid of V13 to
provide the beat frequency of 100 kc/s with the 1 Mc/s input. This crystal controlled
regenerative circuit is thus self-maintaining. The 100 kc/s output is obtained from the
coil L69 which is mutually coupled to L70 and fed via the octal plug (PL7) to the
cathode-follower V12A.
POWER SUPPLIES
44. The primary of the mains transformer is tapped to provide for inputs of 100 – 125
and 200 – 250V. To remove mains-borne interference the capacitors C224 and C225
are incluced. The secondary winding of T1 feeds a bridge-connected full-wave rectifier
MR4, MR5, MR6 and MR7 whose output is filtered by C206, L80 and C198 and fed
via the receiver muting relay RL1/1 to the SYSTEM switch S5. A 120-ohm resistor
Detailed Circuit Description 22