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RACAL RA.117 MANUAL RACAL RA.117 SERVICE MANUAL 9.8.1999 TECHNICAL SPECIFICATION INTRODUCTION INSTALLATION OPERATION BRIEF TECHNICAL DESCRIPTION DETAILED CIRCUIT DESCRIPTION MAINTENANCE SPURIOUS RESPONSES VALVE DATA FAULT DIAGNOSIS REPRESENTATIVE TEST DATA GENERAL SERVICING AND ALIGNMENT PROCEDURES DISMANTLING COMPONENT LIST 1 (RESISTORS, CAPACITORS, SWITCHES, PLUGS AND SOCKETS) COMPONENT LIST 2 (VALVES, CRYSTALS, INDUCTANCES, TRANSFORMATORS ETC.) LIST OF ILLUSTRATIONS: FIG.1 - REAR VIEW OF RECEIVER CHASSIS FIG.2 - FRONT PANEL, RA.117 FIG.3 - RA.117 BLOCK DIAGRAM FIG.4 - SIMPLIFIED BALANCING CIRCUIT FIG.5 - NOISE LIMITER CIRCUIT RACAL RA.117 MANUAL FIG.6 - TYPICAL SELECTIVITY CURVES FIG.7 - TERMINAL TAG STRIP FIG.8 - TOP VIEW OF RECEIVER FIG.9 - FIRST V.F.O. - TOP VIEW FIG.10 - FIRST V.F.O. - UNDERSIDE FIG.11 - SECOND V.F.O. FIG.12 - 1.7Mc/s OSCILLATOR / AMPLIFIER AND MIXER UNIT FIG.13 - B.F.O. UNIT FIG.14 - CRYSTAL CALIBRATOR UNIT FIG.15 - KEY TO UNDER - CHASSIS LAYOUT FIG.16 - R.F. ATTENUATOR FIG.17 - SUPPLY FILTERS FIG.18 - CRYSTAL OSCILLATOR / AMPLIFIER AND HARMONIC FILTER SYSTEM FIG.19 - SECOND MIXER FIG.20 - BAND - PASS FILTERS FIG.21 - 100 kc/s I.F. AMPLIFIER - RIGHT SIDE FIG.22 - 100 kc/s I.F. AMPLIFIER - LEFT SIDE FIG.23 - AUDIO STAGES AND POWER SUPPLIES RA.117 RECEIVER - SCHEMATIC DIAGRAM BACK TO RECEIVERS AND SCHEMATICS ! BACK TO TECHNICAL INDEX (FINNISH) ! BACK TO RA.117-RECEIVER PAGE (FINNISH) ! RACAL RA.117 SPECIFICATIONS TECHNICAL SPECIFICATION RACAL Type RA117 ----------------------------TECHNICALSPECIFICATION ----------------------------------------------------------------Frequency range: 1 - 30 Mc/s Stability: Afterwarm-up, overall drift is less than 50 c/s per hour under conditions of constant supply voltage and ambient temperature. Input impedance: (1) Wideband 2000-ohms approx. (2) Wideband 75-ohms. (3) 5 double-tuned circuits, 75 ohms. (a) 1 - 2 Mc/s (b) 2 - 4 Mc/s (c) 4 - 8 Mc/s (d) 8 - 16 Mc/s (e) 16 - 30 Mc/s Tuning: Effective scale lenght of approximately 145 feet, i. e. 6 inches of scale lenght corresponds to 100 kc/s Frequency increments remain constant over the entire range. Calibration: A 100 kc/s signal derived from a 1 Mc/s crystal oscillator having an accuraty of 5 parts in 10/6 provides check points at 100 Kc/s intervals. Sensitivity: A1 reception, bandwidth 3 Kc/s; 1µV for 18dB signal-to-noise ratio. A2 reception, 30% modulated, bandwidth 3 Kc/s; 3µV for 18dB signal-to-noise ratio. Intermodulation: More than 100dB down for interfering signals at least 10% removed from the wanted signal. Cross modulation: For wanted signal levels between 3µV and 1mV, an interfering signal 10 Kc/s removed and modulated 30% must have a level greater than 50dB above that of the wanted signal to produce a cross modulation of 3%. The ratio of wanted to unwanted signal is RACAL RA.117 SPECIFICATIONS improved up to 10% off tune, at the rate of 3dB per cent. Blocking: With similar conditions to those for cross modulation and unwanted signal f2 must be 60dB greater before the audio output of the wanted signal f1 is reduced by 3dB due to blocking. Selectivity: Six alternative i.f. bandwitchs are obtained by means of a selector switch. Filters detail are: -6dB -66dB (1) 13 kc/s 35 kc/s (2) 6.5 kc/s 22 kc/s (3) 3.0 kc/s 15 kc/s (4) 1.2 kc/s 8 kc/s (5) 0.3 kc/s Less than 2 kc/s (6) 0.1 kc/s Less than 1.5 kc/s Bandwidths 5 and 6 are obtained with crystal-lattice filters; differences in centre frequencies of these bandwitdth settings do not exceed 50c/s. I.F. ouput: 100 kc/s at 75-ohms impedance. Level 0.2 V approx, with a.v.c. in operation. Two outlets in parallel are provided. Image and spurious With wideband or tuned input, external image signals responses: are at least 60dB down. Internally generated spurious responses are less than 2dB above noise level in all cases. Noise factor: Better than 7dB troughout entire range. B.F.O. range: ±8 kc/s B.F.O. stability: With constant ambient temperature and supply voltage, drift after warm-up does not exceed 50 c/s. For input level variations from 10µV to 1mV, b.f.o. drift is negligible. Automatic volyme An increase in signal level of 20dB above 1µV control: improves the signal-to-noise ratio by 18dB. An increase in signal level of 100dB above 1µV increases the a.f. output by less than 7dB. A.V.C. time constans: Short: Charge - 25 milliseconds. Discharge - 200 milliseconds Long: Charge - 200 milliseconds Discharge - 1 second RACAL RA.117 SPECIFICATIONS A.F. response: With 13 kc/s bandwidth, response remains within ±4dB from 250 c/s to 600 c/s. A.F. output: 1. 2½-in. loudspeaker on front panel (switched). 2. Two headphone sockets in parallel on front panel. (see Note) 3. Three independent outputs of 3mW at 600-ohms at rear of chassis. 4. One output of 10mW at 600-ohms. Preset level is independent of A.F.GAIN control setting. 5. One output of 1W at 3-ohms. Note: The two headphone sockets are connected across one of the 600-ohms, 3mW outlets. Distortion: Not greater than 5% at 1W output. Hum level: With A.F.GAIN control at maximum, the hum level is never worse than 40dB below rated output (1W) Noise limiter: A series noise limiter circuit van be switched into operation to provide limiting at modulation levels exceeding 30%. Meter indication: Alternative switching for indication of signal carrier level, a.f. output level or "S" meter indication. Power Supply: 100-125V and 200-250V, 45-65 c/s. Power consumption 100W approx. Dimensions: Height Width Depth For rack mounting 10½ 19 20.1/8 in. (fitted dust cover) 26.7 48.25 51 cm. Fitted cabinet 12 20.½ 21.7/8 in. 30.5 52 55.6 cm. Weight: Rack mounted 62 lb. (28 kg). In cabinet 92 lb. (42 kg). RACAL RA.117 INTRODUCTION INTRODUCTION GENERAL DESCRIPTION. 1. The Communications Receiver Type RA.117 has been designed for use as a general purpose receiver which will provide a high order of selectivity and stability. The receiver covers a frequency range from 1.0 to 30.0 Mc/s. 2. A built-in crystal-controlled calibrator provides reference signals at each 100 kc/s division to permit exact alignment of the scale pointer. Two independent i.f. outputs, in parallel, at 100 kc/s are provided for external use if required. A number of audio outputs are available providing flexibility during operation; a small loudspeaker is fitted for monitoring purposes. 3. The receiver is designed to operate from 100-125 volts and 200-250 volts, 45-65 c/s main supply. The power consumtion is approximately 100 watts. CONSTRUCTIONAL DETAILS. 4. The receiver is designed for both bench (table) and rack mounting. The front panel is painted Light Battleship Grey (British Standard Specification 381C, colour 697) and has been carefully designed to minimize operator fatique. 5. The dimensions of the 1/8 in. thick front panel conform with the requirements for mounting in a standard 19 in. rack. 6. For bench mounting, the receiver is fitted in a robust steel cabinet which has a rear opening to enable the operator to gain easy acces to the fuses and the termination strips. 7. A dust cover is provided with both models. This may be removed from cabinet-mounted receivers in conditions of high ambient temperature. 8. The chassis and major modules are of cast construction thus ensuring maximum rigidity and effective electrical screening. Each receiver is supplied with three keys to facilitate removal of the control knobs, insulated trimming tool and coaxial terminations for aerial and i.f. connections. Extra sleeves can be provided with the terminations for alternative coaxial cable sizes. RACAL RA.117 INTRODUCTION RACAL RA.117 OPERATION OPERATION 1. References to the controls are in capitals and are in accordance with the panel titles adjacent to them (fig.2). 2. It should be noted that the method of operation of the receiver extremely simple, depens largerly upon the purpose for which the receiver is being embloyed. FUNCTION OF CONTROLS. 3. The front panel controls are described in the order in which they could be used for setting-up prior to use. POWER Makes and breaks the power supply to the mains transformer. R.F. RANGE MC/S This control enables the selection of one any of five antenna ranges plus two WIDEBAND positions, one of 75-ohms input impedance and other a high impedance input of 2000-ohms. R.F. ATTENUATOR This control enables the operator to reduce the level of all incoming signals when strong unwanted signals are present which cannot be rejected sufficiently by tuning the antenna. MEGACYCLES This cntrol selects the desired Mc/s frequency. The dial should be checked periodically to ensure that its setting is reasonably central with respect to the band in use. This is indicated by a reduction of signal or noise on either side of the correct setting. SYSTEM This switch provides facilities for STANDBY, MANUAL, A.V.C., CALIBRATION and CHECK B.F.O. BANDWIDTH The two crystal filters determining the bandwidth are adjusted to ensure that their centre frequencies are all within 50 c/s, thus any bandwidth can be selected without retuning the receiver. Six bandwidths are provided as follows:- RACAL RA.117 OPERATION 13 kc/s, 6.5 kc/s, 3 kc/s and 1.2 kc/s (L-C) 300 c/s and 100 c/s (crystal) A.F. GAIN The A.F. GAIN control adjust the audio output. KILOCYCLES This control selects the desired kc/s frequency. The calibration of this scale may be checked at 100 kc/s intervals by setting the system switch to the CAL. position and V.F.O. switch set to INT. B.F.O. The B.F.O. ON/OFF switch makes or breaks h.t. to the beat frequency oscillator. B.F.O. NOTE KC/S The b.f.o. is exatly tuned to a central point on the i.f. amplifier response when B.F.O. NOTE KC/S control is st to zero-beat with the calibrator. Having standardized the b.f.o. frequency, the frequency of an incoming signal may be accurately measured by setting the KILOCYCLES control to a zero-beat position; the b.f.o. should de detuned in order to produce an acceptaple note for c.w. reseption. R.F. TUNE If maximum sensitivity is not required, the antenna need not be tuned unless strong unwanted signals are present. It should be noted that the presence of very strong singnals anywhere within the spectrum may cause crossmodulation unless the aerial is tuned. Under these conditions, CARE MUST BE TAKEN TO AVOID TUNING THE INPUT TO THE INTERFERING SIGNALS instead of the signal required. Familiarity with the tuning controls will facilitate this. R.F./I.F. GAIN the R.F./I.F. GAIN control is operative both in the MAN. and the A.V.C. position of the SYSTEM switch. In the MAN.position of the SYSTEM switch the setting of the control should be always at a minimum consistent with satisfactory a.f. level. The following should be noted when the SYSTEM switch is in the A.V.C. position. Reducing the i.f. gain results in a reduction of a a.v.c. loop gain together with the a degraded a.v.c. characteristic. Therefore when in the A.V.C. position, it is desirable that the R.F./I.F. GAIN control be set to maximum. A possible execption of this occurs when receiving interrupted signals in which the carrier is periodically switched off; in this case , receiver noise could be trouble- RACAL RA.117 OPERATION some during the quiet intervals. A.V.C. The choice of time-constant depens conditions. The LONG time-constant (1 second) should be employed with the choice signals, the SHORT time-constant may be used with high speed telegraphy or voice. For hand (low) speed telegraphy, the MAN. position of the SYSTEM switch should be used (refer to R.F./I.F. GAIN) A.F. LEVEL The preset control sets the a.f. level in a separate a.f. stage for feeding a 600-ohms, 10mW line. It is unaffected by the position of the main A.F. GAIN control. IT IS MOST IMPORTANT that the A.F. LEVEL is not turned towards its maximum position unless the 10mW 600-ohms winding is suitable terminated. LIMITER When swithced into use, the LIMITER reduces the effects of noise peaks exceeding the level of a 30% modulated signal. It does not introduce noticeable distortion below a 30% modulation level. "S" METER With the METER switch in the R.F. LEVEL position the meter indicates the signal diode current. In the A.F. LEVEL position, the 10mW, 600-ohms output only is monitored. A calibration mark is provided at 10mW. SPEAKER The loudspeaker may be switched ON or OFF as required. The two telephone jack sockets remain in circuit in either position of the SPEAKER switch. The insertion of a telephone jack disconnects the loudspeaker. V.F.O. This switch should be set to the EXT. position when a external 3.6-4.6 Mc/s source is applied. PRELIMINARY SETTING-UP. 4. The instructions given below are concerned with tuning the receiver to a signal of known frequency. These instructions (1) to (8) apply with the V.F.O. switch in either position. (1) Set the power switch to ON. Allow a few minutes for the receiver to warm-up. (2) Set the R.F. RANGE MC/S switch to WIDEBAND. (3) Set R.F. ATTENUATOR to MIN. RACAL RA.117 OPERATION (4) Set A.F. GAIN control to its mid-position. (5) Set SYSTEM switch to MAN. (6) Set LIMITER and B.F.O. switch to OFF. (7) Select bandwidth of 3 or 6.5 kc/s. (8) Rotate the R.F./I.F. GAIN control to three-quarters of fully clockwise. FILM SCALE CALIBRATION 5. (1) Set the SYSTEM switch to CAL. (2) Select BANWIDTH of 3 kc/s. (3) Set the KILOCYCLES scale to that 100 kc/s point which is nearest to the frequency required and adjust the control accurately until a zero-beat note is obtained. Move the milled cursor slide on the dial escutheon so that the pointer coincides exactly with the selected 100 kc/s division. (4) Restore all other controls to the preliminary setting shown in para.4. above. B.F.O. CALIBRATION 6. (1) Set the B.F.O. to on. (2) Set the SYSTEM switch to CHECK B.F.O. (3) Adjust the B.F.O. NOTE KC/S control to zero-beat. (4) Restore all other controls to the preliminary setting shown in para.4. above. TUNING 7. (1) Set R.F. RANGE MC/S to the desired frequency band. (2) Set R.F. ATTENUATOR to MIN. (3) Set MEGACYCLES dial to the required integer (1 to 29). The position of maximum receiver noise will indicate the correct setting. (4) Set SYSTEM switch to CAL. (5) Set Bandwidth to 3 kc/s. RACAL RA.117 OPERATION (6) Set A.F. GAIN to mid-position. (7) Adjust KILOCYCLES scale to zero beat at the 100 kc/s point nearest to the desired frequency. (8) Adjust the milled cursor slide to coincide with this point. (9) Switch B.F.O. on. (10) Set SYSTEM switch to CHECK B.F.O. (11) Adjust B.F.O. NOTE KC/S control to zero beat. (12) Rotate the system switch to MAN. (13) Set KILOCYCLES scale to the required frequency and critically tune for zero beat in order to centralize the signal within the i.f. pass-band. (14) Adjust R.F. TUNE for maximum signal (or noise). For optimum c.w. reception, "off-tune" the b.f.o. to produce an acceptaple beat note. (15) Set the A.F. GAIN to its maximum clockwise position and adjust the output level with the R.F./I.F. GAIN control. (16) For m.c.w. or voice reception, switch B.F.O. off. (17) Set the SYSTEM switch to A.V.C. if required. (18) Set BANDWIDTH for optimum reception. "S" METER 8. The "S" meter should be correctly set to zero. 9. With no antenna connected, set the R.F. ATTENUATOR to MAX. Set the SYSTEM switch to A.V.C. Turn the R.F./I.F. GAIN control to the maximum clockwise position. NOTE: Unless the R.F./I.F. GAIN controlis in the maximum position, the "S" meter calibration is upset. 10. Remove the plated cap below the meter. Adjust the setting of the balance control (accessible trough the hole in the panel) by means of a screwdriwer until the meter reads zero. RACAL RA.117 TECH. DESCRIPTION BRIEF TECHNICAL DESCRIPTION 1. This section describes briefly, with the aid of the block diagram fig 3, the basic theory of operation. For a more detailed explanation of the receiver, DETAILED CIRCUIT DESCRIPTION, should be consulted. SIGNAL INPUT 2. The receiver is designed for an input impedance of 75-ohms for all positions of the R.F. RANGE switch except WIDEBAND; in the WIDEBAND position the input impedance is 2000-ohms. FIRST MIXER 3. Input signals between 0.98 and 30 Mc/s are via an r.f. amplifier and a 30 Mc/s low-pass filter to the first mixer (M1) where they are mixed with the output from a variable frequency oscillator VFO-1 (MEGACYCLES tuning). This oscillator has a frequency range of 41.5 to 69.5 Mc/s. The first i.f. stage is in effect a band-pass filter tuned to 40 Mc/s ±650 kc/s. Thus, according to the setting of VFO-1, any spectrum of signals 1 Mc/s wide and existing in the range 0.98 to 30 Mc/s cabn be mixed in M1 to produce an output accettable to the first i.f. band-pass filter. 4. It should be noted at this stage that the exact setting of VFO-1 is determined by conditions in the second mixer and harmonic mixer circuit ; These restrict the possible settings to position 1 Mc/s apart (e.g. 41.5, 42.5, 43.5 Mc/s, etc.). HARMONIC GENERATOR AND MIXER 5. The output from a 1 Mc/s crystal oscillator is connected to a harmonic generator. The harmonics derived from this stage are passed trough a 32 Mc/s low-pass filter and mixed with the output from VFO-1 in the harmonic mixer. This mixer provides an output at 37.5 Mc/s which is amplified before passing trough a band-pass filter tuned to 37.5 Mc/s with a bandwidth of ±150 kc/s. 6. The presence of this filter restricts the setting of VFO-1 to an exact number of Mc/s plus 37.5 Mc/s in order to give an output acceptaple to the filter and amplifier. As a result, the first v.f.o. must be tuned in 1 Mc/s steps. SECOND MIXER RACAL RA.117 TECH. DESCRIPTION 7. The 40 Mc/s first i.f. signal is mixed in the second mixer (M2) with the 37.5 Mc/s output from the harmonic mixer in order to produce an output consisting of a 1 Mc/s spectrum in the frequency range 2-3 Mc/s (second i.f.). 8. To clarity this method of operation, some examples of dial settings and intermediate frequencies corresponding to various incoming signals are tabulated below: Dial Settings Signal Freq. VFO-1 Xtal harmonic 1st I.F. 2nd I.F. Mc/s kc/s (fs) Mc/s (fo)Mc/s (nfc)Mc/s 4 1.000 5.0 44.5 7th 39.5 2.0 5 0 5.0 45.5 8th 40.5 3.0 18 600 18.6 58.5 21st 39.9 2.4 9. Frequency drift of VFO-1 within the limits of the 37.5 Mc/s filter bandwidth, does not affect the frequency stability of the receiver. A change in this oscillator frequency will alter the first i.f. to the same extent and in the same sense as the nominal 37.5 Mc/s signal from the harmonic mixer. Therefore the difference frequency from M2 will remain constant. THIRD MIXER 10. The 2-3 Mc/s receiver, which follows M2, is preceded by a pre-tuned bandpass filter. The 2-3 Mc/s output from the filter is mixed in the third mixer with either the output from the second variable frequency oscillator VFO-2 or an external signal within the frequency range of 3.6 to 4.6 Mc/s to provide the third intermediate frequency of 1.6 Mc/s. FOURTH MIXER 11. The 1.6 Mc/s intermediate frequency is mixed in the fourth mixer (M4) with the 1.7 Mc/s output from the 1,7 Mc/s oscillator/amplifier to provide the fourth and final intermediate frequency of 100 kc/s. FOURTH I.F. STAGE 12. The final i.f. stages are preceded by crystal lattice and L-C filters which provide six alternative bandwidths. Separate signal and a.v.c. diodes are employed and alternative switched time-constants give the optimum conditions for telegraphy and telephony reception. An additional i.f. amplifier is incorporated to give an independent output at 100 kc/s. A.F. STAGES 13. Two independent audio frequency stages are incorporated for either line output or headphone sockets and internal loudspeaker; each stage RACAL RA.117 TECH. DESCRIPTION is provided with a level control (see TECHNICAL SPECIFICATION). CRYSTAL CALIBRATOR 14. A crystal calibrator unit is incorporated to enable the scale of VFO-2 to be checked at 100 kc/s intervals when the V.F.O. switch is set to INT. position. These check points are obtained from a regenerative divider controlled by the 1 Mc/s crystal oscillator. http://koti.mbnet.fi/~ijl/117man4.html (3 of 3) [2/19/2007 11:57:04 AM] RACAL RA.117 DETAILED DESCRIPTION DETAILED CIRCUIT DESCRIPTION 1. Reference should be made to the circuit diagram at the end of this handbook. AERIAL CIRCUIT 2. A 75-ohms unbalanced aerial source is connected to the tuned r.f. amplifier trough a three-section 30 Mc/s low-pass filter and a fiveposition attenuator covering a range of 0 to 40 dB. Switch S2 selects wideband 75-ohms or wideband (high impedance) or any one the five double-tuned aerial coils L4-L8 for tuned operation. These aerial coils are aligned by means of dust iron cores. The aerial is tuned by a capacitor C18A/B which is switched out of circuit in both wideband positions. R.F. AMPLIFIER 3. The incoming signal is fed via C28 and grid stopper R25 to the grid of V3B; the r.f. stage (V3) employs a variable-mu, low-noise doubletriode; the two halves of the valve are connected in cascode so as to utilize the low-noise high-gain properties of the valve. A delayed a.v.c. voltage, derived from a shunt diode network, is applied to the grid of V3B when the signal level is approximately 10µV. The capacitors C40 and C41 ensure that the cathode is adequately decoupled over the wide frequency range. Ferrite beads have been fitted to the heater lead, connected to pin 4, the anode of V3A and the cathode of V3B adjacent to C41, to prevent parasitic oscillations occurring. 30 MC/S LOW-PASS FILTER 4. The amplified signal is passed to a 30 Mc/s low-pass filter which has a substantially flat responseover the frequency range. L27, C47 and R28 constitute the first 'L half Section' of the filter. The signal is then fed at low impedance (680-ohms) trough the coupling capacitor C74 and the grid stopper R45 to the control grid of V7, the first mixer stage. The input capacitance of V7 forms the capacitance to chassis betweeenL15 and L17 required to the filter network. NOTE: This capacitance is not critical, therefore no adjustment will be necessary should V7 be changed. FIRST VARIABLE FREQUENCY OSCILLATOR (VFO-1) RACAL RA.117 DETAILED DESCRIPTION 5. This circuit comprises a cathode-coupled Hartley oscillator stage (V5) which may be continously tuned over the frequency range of 40.5 to 69.5 Mc/s. The frequency determining components are an inductance L36 and a variable capacitanceC76. Alignment is accomplished by adjusting aluminium core of L36 and the trimming capacitor C77. The variable capacitor C76 is coupled to the Mc/s dial which is calibrated from 0 to 29 Mc/s. The anode load consists of L20, a compensating inductance which is wound on a 470-ohm resistor R18. The oscillator is coupled via C85 to the signal grid of the first mixer stage V7 and also via C42 to the control grid of the harmonic mixer V4. NOTE: The Mc/s dial calibration may be affected if V5 if changed. The necessary correction may be made by adjusting C77 with the Mc/s dial set to 29 Mc/s. FIRST MIXER (M1) 6. the outputs from the 30 Mc/s low-pass filter and the variable frequency oscillator VFO-1 are fed to the signal grid of the mixer stage (V7) which produces a signal at 40 Mc/s. The signal is then passed to a 40 Mc/s band-pass filter which forms the anode load of this stage. 40 MC/S BAND-PASS FILTER 7. The 40 Mc/s band-pass filter consists of eight over-coupled tuned circuits connected in cascade and is tuned by the trimming capacitors C21, C33, C43, C53, C61, C70, C79 and C88. This filter, which has a passband of 40 Mc/s ±650 kc/s, ensures that only the required 1 Mc/s spectrum of signals is passed to the second stage. This filter is deliberately set to a slightly wider passband than is theoretically required, to allow for possible drift in VFO-1. 1 MC/S CRYSTAL OSCILLATOR/AMPLIFIER 8. The frequency of the crystal oscillator V1 may be set precisely to 1 Mc/s by adjusting the trimming capacitor C2A. The crystal XL1 which is connected between the control grid and the screen grid is electron coupled to the anode. The anode coil L2 is adjusted to resonate at 1 Mc/s by means of a dust iron core. The fixed capacitors C9, C10 and C11 complete the tuned circuit. When an external signal is applied to socket SK3, the valve operates as an amplifier. 9. The output from V1 is capacitance-coupled to the harmonic generator V2 and via SK2 to a "T" adptor for feeding a 1 Mc/s input into the l.f. converter and also the control grid of the mixer valve V13. HARMONIC GENERATOR 10. The 1 Mc/s signal is fed via coupling capacitor C8 to the control grid of the harmonic generator V2. The h.t. is fed to the screen grid via RACAL RA.117 DETAILED DESCRIPTION R12 and is decoupled by C8A. Harmonics produced at this stage are passed to a 32 Mc/s low-pass filter. 32 MC/S LOW-PASS FILTER 11. The megacycle harmonics are fed trough a 32 Mc/s low-pass filter circuit to prevent harmonics other than those required from passing to the harmonic mixer (V4). Limited control over the cut-off frequency is provided by C7 which is adjusted to equalize the output from yhe filter at the frequencies corresponding to 28 and 29 Mc/s on the MEGACYCLE dial. HARMONIC MIXER 12. The outputs from the 32 Mc/s low-pass filter and VFO-1 are mixed in the harmonic mixer by applying the filtered megacycle harmonics to the suppressor grid and the output from the VFO-1 to the control grid. The 37.5 Mc/s output is selected by the tuned anode load, consisting of a fixed capacitor C50 and an inductance L28 which may be adjusted by means of a dust iron core, and coupled by C51 to V6. R36 is grid stopper. 2-STAGE 37.5 MC/S AMPLIFIER (1) 13. The anode load of V6 is a tuned circuit consisting of a fixed capacitor C67 and an inductor L33 Which is tuned to 37.5 Mc/s. Frequency adjustment is by the dust iron core L33. This stage feeds the amplified signal via C68 to the following stage V8. The 37.5 Mc/s signal is then passed to the 37.5 Mc/s band-pass filter. The anode load of this stage is provided by this filter. 37.5 MC/S BAND-PASS FILTER 14. The 37.5 Mc/s band-pass filter consists of eight under-coupled tuned circuits arranged in cascade. These filter sections may be tuned by C24, C35, C45, C55, C63, C72, C81 and C91 respectively. This filter, which has a passband of 300 kc/s, allows for possible drift in VFO-1. The narrow passband and high rejection to frequencies outside the passband prevent spurious signals from reaching the second mixer stage (V9). 37.5 MC/S AMPLIFIER (2) 15. The filtered 37.5 Mc/s signal is further amplified by V10 before being passed to the second mixer stage (V9). To prevent interaction between the 40 Mc/s band-pass filter and the 37.5 Mc/s tuned circuit (L50 and C113) and to enable either circuit to be adjusted without affecting the other, a balancing circuit is included which is shown in simplified form in fig.4. The 40 Mc/s signal is introduced into the 37.5 Mc/s tuned circuit at a point of zero r.f. potential since L50 is centre tapped and C108 is adjusted to be equal to the total of the capacitance of V10 anode to chassis. C107 and the RACAL RA.117 DETAILED DESCRIPTION input capacitor of V9. NOTE: The anode load of V10 is adjusted to 37.5 Mc/s by adjusting the dust iron core in L50. The balancing circuit will be affected if V9 or V10 is changed. SECOND MIXER (M2) 16. This mixer (V9) produces the second intermediate frequency of 2-3 Mc/s by mixing the 40 Mc/s i.f. and the 37.5 Mc/s signal. The tuned circuit formed by L300, C300 remove the 37.5 Mc/s frequency whilst the other tuned circuit formed by L301, C301 remove the 6 Mc/s frequency so that only the second i.f. is passed to the 2-3 Mc/s band-pass filter preceding the third mixer. 2-3 MC/S PRE-TUNED BAND-PASS FILTER 17. This filter consists of two pre-tuned band-pass filter sections. The characteristic impedance of the filteris 1000-ohms. THIRD MIXER 18. The output from the 2-3 Mc/s band-pass filter is resistance-capacitance coupled to the signal grid of V25 together with the output (3.6-4.6 Mc/s) from the second v.f.o. amplifier V11 when the V.F.O. switch (S300) is set to the INT. position. With the V.F.O. switch set to the EXT. position, V11 operates as a buffer amplifier. This mixer (V25) produces the third intermediate frequency of 1.6 Mc/s. The signal is then fed to a 1.6 Mc/s band-pass filter which forms the anode load of this stage. 19. The 1.6 Mc/s band-pass filter consists of two double-tuned i.f. transformers, the first section of the filter is formed by C320, L306, L309 and C325 and the second section by C332, L313, L314, C334. This filter has a bandwidth of 13 kc/s. SECOND VARIABLE FREQUENCY OSCILLATOR (VFO-2) 20. The second variable frequency oscillator, covering a frequency range 3.6 to 4.6 Mc/s, is an electron coupled Hartley circuit embloying one half of double-triode V12. The oscillator frequency is determined by an inductance L55, two fixed capacitors C303, C305, a trimming capacitor C306 and a variable capcitor C301. The KILOCYCLES scale which is calibrated between 0 and 1000 kc/s is coupled to this variable capacitor. 21. The output from VFO-2 is resistance-capacitance coupled to the grid of V12A, a cathode-follower stage. With the V.F.O. switch set to the INT. position the output from V12A is fed via PL305 and PL300A to the control grid of the second v.f.o. amplifier V11. In the EXT. position the external 3.6 to 4.6 Mc/s signal is fed ti V11. RACAL RA.117 DETAILED DESCRIPTION FOURTH MIXER 22. the output from the 1.6 Mc/s band-pass filter is directly coupled to the signal grid of a pentagrid valve V26; it is mixed with a 1.7 Mc/s signal from V27 fed via the coupling capacitor C339 to the oscillator grid of V26. The resistor R68 completes the d.c. path from this grid to earth. The 100 kc/s output from this mixer stage is then fed via SK6, PL6 to the crystal filter unit. 1.7 MC/S CRYSTAL OSCILLATOR/AMPLIFIER 23. The frequency from the crystal oscillator C27 may be set precisely to 1.7 Mc/s by adjusting the trimming capacitor C337. The crystal XL300 which is connected between the control grid and the screen grid is electron coupled to the anode. When an external signal is applied to socket SK303A the valve operates as an amplifier. The output from this circuit is fed via C339 to the oscillator grid of the fourth mixer V26. CRYSTAL FILTER 24. Six alternative switched i.f. bandwidths are available as follows:100 c/s ) Crystal 1.2 kc/s ) 300 c/s ) 3.0 kc/s ) 6.5 kc/s ) L - C 13.0 kc/s ) 25. In the crystal positions the fourth mixer anode is connected to L48 in the crystal filter. L47 and L49 provide a balanced output which is tuned by capacitors C109 and C110. In the 100 c/s position, the balanced output is connected via crystals XL2 and XL5 to the first tuned section of the 100 c/s L-C filter. The differential trimmer C118 is the phasing control for this bandwidth. XL3, XL6 the capacitor C119 form a similar circuit for the 300 c/s position. Damping resistors R64 and R65 are connected across the tuned circuits to obtain the required bandwidth. 100 KC/S L-C FILTER 26. This filter consists of four tuned circuits arranged in cascade. In the L-C bandwidth positions, the signal is fed to the tuned circuit formed by L61 and the combination of the capacitors C145, C146, C146A and C147. The second section consists of L62 and L63 in series with C152, C152A and C153. The final section consisting of L68 and L71 in series with C161 and C162, is damped by the series resistors R86, R87A and R88 according to the bandwidth. In the L-C positions the output is taken from a capacitive divider formed by C161 and C161A with C170, to equalize the gains in the L-C and crystal bandwidth positions. RACAL RA.117 DETAILED DESCRIPTION 27. The L-C banwidths are obtained by varying the degree of coupling between each section of the filter in addition to the damping resistors in the final stage. The capacitor C175 is included to compensate for the effective reduction of the input capacitance of V14, appearing across the tuned circuit, when switching from crystal to L-C positions. 28. To maintain the input capacitance of the L-C filter, in the crystal positions, a trimming capacitor C148 is switched into circuits. This trimmer is adjusted to be equal to the output capacitance of V26 and the screened cable. In the crystal bandwidth positions, the L-C filter is operating in its narrow bandwidth positions, i.e. 1.2 kc/s. NOTE: The 470-kilohm damping resistors R77 and R80 are disconnected except during filter alignment. FIRST 100 KC/S I.F. AMPLIFIER 29. The output from the L-C filter is passed trough a coupling capacitor C164 to the control grid of the pentode amplifier valve V14. This grid is returned via R96 to the a.v.c. line which is filtered at this point by R102 and C173. The screen potential is derived from a potential divider formed by R93, R97 and RV4. This stage is coupled to the second i.f. amplifier and the i.f. output stage by a double tuned transformer having an over-coupled characteristic. SECOND 100 KC/S I.F. AMPLIFIER 30. The signal from the first i.f. tranformer is fed trough the grid stopper R114 to the control grid of the second i.f. amplifier. H.T. is supplied to the screen via the dropping resistor R113 and is decoupled by C181. The anode load is tuned circuit consisting of L77, C192 and C191. This circuit is heavily damped by R112. The secondary winding L78 and L79 is tuned by C195 and C195B with R120A as a damping resistor. The output is fed to the diode detector anode. DIODE DETECTOR 31. The low potential end of L79 is connected through the r.f. filter (C209, R128, C210, C219 and C211) to the diode load R130. With the meter switched to R.F. LEVEL, the meter indicates the detector diode current. The resistor R131 is incluced to complete the diode detector circuit when the meter is switched out of circuit. NOISE LIMITER 32. The noise limiter diode (pins 2 and 5 of V21) is connected in a series circuit to operate at approximately 30% modulation. its operation is explained with reference to Fig.5. RACAL RA.117 DETAILED DESCRIPTION 33. The d.c. path from point A is trough R134, R135, the diode and R137. The a.f. signal path from detector diode load is through C216, the diode and C218 when S8 is open. In the presence of a signal, a negative potential varying with the depth of modulation, will be developed at point A thus causing the diode to conduct. The negative potential at B, will be lower than at A and will be maintained at a constant level due to the long time constant of R134 and C217. R135 allows the cathode potential to vary in sympathy with the modulation provided the modulation depth does not exceed 30%. The potential appearing at the cathode of the noise limiter diode therefore consists of a steady negative potential with the modulation superimposed. When noise impulses corresponding to high modulation peaks appear at point A and via C216 at point C, the voltage across the diode changes sign thereby causing the diode to stop conducting and open-circuit the a.f. signal path. With S8 in the OFF position the limiter is inoperative. A.V.C. AND T.C. DIODE 34. The signal appearing at the anode of V16 is passed through the capacitor C139 to the anode of the a.v.c. diode. The diode load is formed by R116. A positive potential derived from R120, R121 and R122, supplies the required a.v.c. delay voltage to the cathode of this diode.When A.V.C. switch is in the SHORT position and the SYSTEM switch set to a position in which the a.v.c. is operative, i.e. A.V.C., CAL. or CHECK B.F.O., the anode of the a.v.c. diode is connected to the a.v.c. line via L81 and R127. The choke L81 is tuned by C203 to a frequency slightly below 100 kc/s so that is presents a small capacitance at 100 kc/s, thus R127 is prevented from shunting the diode load. When the signal level falls, the capacitors C182 and C173 discharge through R118, R127 and L81 into the diode load resistor R116. The a.v.c. potential is brought out via R123 to the tag strip at the rear of the receiver for external use if required. With the SYSTEM switch set to the MANUAL position, the a.v.c. line is connected to the R.F./I.F. GAIN control RV1, thus the gain of the 100 kc/s amplifiers may be varied by adjusting the negative potential applied to the a.v.c. line. AUDIO OUTPUT 35. Audio frequencies are applied to the control grid of V23B via RV2 the A.F. GAIN control. The output transformer (T2) provides four separate outputs as follows: 1W into 3-ohms, and three windings supplying 3mW into 600ohms. 36. The internal loudspeaker (which may be switched out of circuit by operating S11) is connected across the 3-ohm winding. The headphone jacks JK1 and JK2 are connected across one of the 600-ohms windings. A.F. LINE OUTPUT 37. The audio frequencies are also applied to the grid of V23A via RV3, the A.F. GAIN LEVEL control; this control presets the level from output