Racal Instruments RA-117 User Manual page 23

R124 is connected between the negative line and earths thus providing a negative 25V
d.c. supply for gain control purposes.
SYSTEM SWITCH
45. The following conditions exist for each setting of the SYSTEM switch. The link on
H.T. adaptor terminal is assumed to be in position.
(1) STANDBY
(2) MANUAL
(3)
(4)
(5) CHECK B.F.O.
"S" METER
46. The "S" meter is connected between the cathode of V14 and a point of preset (RV4)
positive potential. It is calibrated to provide an indication of signal strengh; a 1 V
signal provides a typical reading of between "S1" and "S3" and ascending "S" points
in approximately 4 dB steps. The variation in treshold is dependentupon the gain of
the R.F. stages. It should be remembered that only with the R.F./I.F. GAIN control
Detailed Circuit Description
S5A disconnects the H.T. from all stages and connects R119A
across the H.T. as a compensating load.
(a) The H.T. passes through S5A, S5B and S5C to all stages ex-
cept the calibration unit.
(b) S5F connects H.T. to the B.F.O. when S7 is switched on.
(c) The A.V.C. line is disconnected from the A.V.C. diode by S5D
and connected to the R.F./I.F. GAIN control (RV1) by S5E.
(a) (2)(a) and (2)(b) are applicable.
(b) S5D connects the A.V.C. line to the A.V.C. diode.
(a) H.T. is applied via S5A, S5B and S5F to all stages except:-
The R.F. amplifier (V3)
The first V.F.O. (V5)
The first mixer (V7)
The second mixer (V9)
The final 37.5 Mc/s amplifier (V10)
The B.F.O.
(a) (4)(a) applicable except that H.T. is also applied to the B.F.O.
via S7.
(b) (3)(b) applicable.
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