The ARC 51 BX MODULES

Transcription

1 The ARC BX The ARC-BX is a compact UHF military aircraft transceiver, made by Rockwell-Collins around. The transceiver has 00 Xtal controlled channels between MHz and 00MHz. Channel distance is 0kHz, audio bandwidth is khz, and the modulation is AM. MODULES The ARCBX has a modular construction with a vertical septum in the middle, with modules on each side. The septum distributes all signals, cooling air, and motion to the tuned elements and crystal selector switches. The modules are Frequency control Three rotary knobs on the remote control panel select the tens of MHz ( positions), units MHz (0 positions) and khz (0 positions, in 0kHz steps) Each rotary switch position is transferred by contacts to the ARC-BX. The receiver is a triple superhet, the first knob controls the first local oscillator (LO), etc. There are = Xtals, selecting x 0 x 0 = 00 channels. The dials can be set anywhere from , so 00 channels, however the first 00 channels ( up to MHz) are not used in the official spec. MIL-R-D. During transmit (key down), the oscillator frequencies are added backward from third to first LO using the same IF filters as in the receive mode. This requires a lot of relays. A A A A A A A A A Rcvr / RF preamp First and second IF amplifier ( with nd and third local oscillator) Third IF amplifier Modulator / Audio amplifier Spectrum Generator ( First local oscillator) RF power amplifier Guard receiver Power Supply Mechanical Tuning 0-00 MHz Modulator preamp Power amp st IF 0-0 MHz nd IF.-. MHz audio AM det d IF 00 khz ARC-BX st LO 00-0 MHz nd LO.-. MHz d LO. -. MHz pos - 0 pos 0-0 pos 00- The RF preamp is mechanically tuned from - 00 MHz BW MHz The e IF is mechanically tuned from 0 0 MHz BW 00 khz The e IF is mechanically tuned from. -. MHz BW 00 khz The e IF is fixed, centered around 00 khz. BW 0 khz (0-0 khz) With all dials at minimum, the receive frequency is = 0.00 MHz and the transmit frequency is = 0.00 MHz With all dials at maximum, the receive frequency is =. MHz and the transmit frequency is =. MHz

2 More on next pag P -00 MHz Receive +0V K Antenna Transmit 0W +0V J-J +0V Z V J J V 00 Z V Z V Z V Z V Z 0p 0 k AM +0 K V 0 00 A RF Preamps st LO k 00 : p J 0. 0k 0 p k V k 00 k P J 0 p V J V 0 J 0 00 p V First LO 00-0 MHz in steps of 0 MHz p +0V 0 k J P P AGC K A A Coax relay K +dbm ( Vrms) All relay contacts shown in receive position J 0 A A RF Power Amplifier J d LO To d IF (00kHz) Testpoint, M series resistor and 00pF feedtru not shown 0 p 0 dbm J J 0 V 0 J nd LO - MHz 00k A J J u 0. nf A A R bias First & second IF First IF 0-0 MHz ARC-BX RF part For heaters and oven see power supplies sheet dec 0 kb n J k - V +V K

3 ARC BX UHF part The UHF part of the ARC has modules: A The RF Preamp A The RF Power Amplifier A The Spectrum generator ( First LO) These modules are the only ones with tubes, most planar (ceramic) triodes. All have tuned circuits, driven by the mechanical tuner. All relays in the UHF parts diagram are controlled by the Push To Talk (PTT) switch, and all are drawn in the receive position. Frequency control Three rotary knobs on the cockpit panel select the tens of MHz ( positions), units MHz (0 positions) and khz (0 positions, in 0kHz steps) Each rotary switch position is transferred by contacts to the ARC-BX. RF Power Amplifier The Power amplifier is driven by the carrier signal from the RF preamp module. Each stage has about 0dB gain, providing W drive level to the final tetrode. The output level to the antenna is approx. 0W (not less than W at.vdc supply voltage) The final amp is amplitude modulated on the anode and g voltage, the latter is a divided copy of the anode voltage. The average anode voltage is 0V, the average g voltage is V. The CW anode current is 0mA. The anode is coupled to the antenna by a variable capacitor. This is a strip of flexible copper, moved by an individual made camwheel to maintain constant output power over the entire band. The receiver is a triple superhet, the first knob controls the first local oscillator (LO), etc. There are = Xtals, selecting x 0 x 0 = 00 channels. The dials can be set from , so 00 channels, but the first 00 channels ( up to MHz) were not in the military communications band During transmit (key down), the oscillator frequencies are added backward from third to first LO using the same RF and IF filters as in the receive mode. This requires a lot of relays. First Local Oscillator. Also called Spectrum Generator The first LO is tuned from 00MHz to 0MHz in 0MHz steps. The Xtals are placed in a temperature controlled oven.(yellow box on top of the picture, isolation removed). Each Xtal can be trimmed individually over khz.. The First LO has planar triodes type 0 as oscillator and tripler or quadrupler, followed by two pencil triodes to filter side products. The output level is +dbm ( V rms) Knob Xtal LO frequency. MHz 00 MHz * * * 0. 0 Picture of the first local oscillator RF Pre Amplifier. In receive mode, this module has the first two stages to amplify the antenna signal (between uv and 0.V), and to provide mirror rejection. The next two stages are in the RF power amplifier module. In transmit mode, V is the mixer from a) the first local oscillator and b) the sum of second and third LO. The RF preamp amplifiers filter the carrier signal for the final amplifier. The tuned UHF circuits have a constant impedance, as both the capacitor and the single turn inductor are variable. This way, each has to vary only : for the -00MHz tuning range.

4 A p K K k 0 p 00k Q J Q N N Q n N k k k AVC Tx A A A 0 +0kHz. MHz +V 0mA k pf 0 A pf 0 st IF 0-0MHz pf 00p Vpp RF V 0pF 00p k MHz 00k n Q N0 00 Q N0 0pF J K k k pf 00k 0pF 0 0p nd IF filter.. MHz p 0p J n p 00k 0p 00p Vpp RF p MHz.uH 0p 00p k K +V / 0mA Q N n Q N k k 00kHz output Signal to third IF p see text +V K F K K K A Coils V /0mA MHz 0 d LO & IF K st IF nd LO. MHz. MHz 0 MHz. MHz nd LO 0 d LO Clockwise= higher freq Red= unused positions..... MHz..... MHz..... MHz ARC-BX First and second IF amplifier feb 0 kb

5 ARC First IF Amplifier This module sets the MHz and 0.MHz with : - The first IF amplifier ( tuned, at 0-0 MHz) - The second Local Oscillator (.. MHz) - The second mixers for receive and transmit - The second IF filter, tuned at.-. MHz - The third Local oscillator, at.-. MHz - The third mixer for receive and transmit. Mechanical tuning The wheels on the rear side of the module set the tuning elements. All rotate freely in either direction. From the full turn, only 00 degrees are active. The remaining 0 deg, marked red in the picture below, are used to return the permeability tuning cores to other extreme position. From top to bottom the wheels are: - Third LO (0 º steps ) - First IF ( º steps ) - Second LO ( 0 º steps) Each LO wheel selects Xtals. The second LO wheel also drives a rotary switch for individual tuning of the corresponding Xtal. The third LO wheel also tunes the second IF bandfilter. The indications in the picture below refer to the index hole. This hole points to the printed "0" when the dials are set for XX.00 MHz The various tuned filters and Xtal oscillators realize the MHz,- and sub MHz dials at the cockpit control panel. Three relays reconnect the IF filters in transmit mode, in order to add the local oscillator frequencies to compose the carrier frequency. To make the transmit frequency equal to the receive frequency, the first, second and third LO frequencies shall be added to the last IF frequency of 00kHz. To circumvent the 00kHz addition, the third LO Xtal bank is split in two halves that are swapped during transmit. This trick shifts the third LO frequency by 0.MHz with respect to the dials, as required. IF circuits First IF This -stage amplifier is permeability-tuned with coils between 0 and 0MHz with % bandwidth (0.MHz db). The gain is 0dB when the AVC line is below V. The first IF stage is a N, NPN transistor with a dual base connection. The extra base connection is used to decrease the gain when the AVC voltage rises to +V Second LO The nd LO has 0 Xtals from. to. MHz in MHz steps. Each Xtal frequency can be shifted individually over khz. The Vpp second LO signal is applied either to the second receiver mixer Q or the second transmitter mixer Q. Second IF The second IF is only a bandfilter, with a high input impedance and low output impedance. The filter is used from left to right in receive mode, and from right to left in transmit mode, as the third LO frequency is then in the middle of its passband. Third LO In the original ARCX, the third LO had 0 Xtals for 0 channels. With the introduction of the ARC- A and B types having 00 channels, a second bank with 0 Xtals was added for the interposed 0kHz channels. Selection between these is done with relay K, controlled by the last digit (either 0 or ) of the selected frequency. Xtals Bank 00kHz IF nd IF 00kHz IF Xtals Bank 0 Horizontal in this plot is the position of the 0.MHz digit on the control panel. Vertical are the 0 Xtal frequencies of the third LO (yellow dots) and the permeability tuning of the second IF bandpass filter ( red line), all in MHz. As can be seen, the LO frequency is either 00kHz above or below the second IF in receive mode. During transmit, the upper and lower Xtal banks are swapped, bringing all third LO frequencies in the middle of the passband of the second LO bandfliter. Dial Xtal d LO K on nd IF XXX.00.0 MHz.0 MHz XXX.0. x. XXX XXX.. x.0 XXX XXX.. x. XXX XXX.. x. XXX XXX.. x. XXX XXX.. x. XXX XXX..0 x. XXX XXX.. x. XXX XXX.. x. XXX XXX.. x.. MHz In this picture of the open unit you see from left to right: - The third IF bandfilter and third LO - The first IF in the middle - The second LO is just outside the right side of the picture, beyond the feed-thru capacitors. Testpoints A rectified sample of the output of the LO's is available on J and J, and should be approx..v dc in all Xtal selections. A rectified sample of the sum of LO and LO is available on J in transmit mode. After adding the LO- frequency, this will be the Xmit carrier frequency. With keydown, V is present at J. Problems a) The four yellow, hermetic-sealed relays have pure-tin coatting on the inside, and might develop tin whiskers over 0 years In my unit, the base of Q was shorted to ground by several whiskers inside K, I measured 00 ohms to case both from the contacts and from each coil end! b) There was 00 ohms series resistance between the.0mhz Xtal and the rotary switch. The plated-thru was not soldered well. c) The third mixer Q oscilates on some channels at 0kHz. This gives sidebands in the transmission. pf from its base to ground helps.

6 Q N Q N 00kHz Q N k Q N k Q N k k k 0p +V 0k Q Q Q N0 N N0 AVC 0 uf R N0 (V) J AVC 0mA k k k k k 00.MHz 00 0p k Q N k k 0 Q N k uf k N uf k k k Q0 N0 Q N +0V Rx Q N0 k Q N k Q N 00 Q N k Q N 00 n uf k Q N 0k N Q N +0V Q N k Q0 N k Q N N00 (0V) k 0 0 Q N +V Rx 0mA P - S +V - V P-a squelch P-L RF preamp AVC SQUELCH R0 0 k k All diodes N unless otherwise noted LIMITER R k 0 uf CR 0 ARCBX d IF AMPLIFIER Januari 00 / kb Main out Aux out - Main output to Modulator/audio ampl - Aux output IF AGC RF preamp AGC Repeater control

7 ARC Third IF amplifier module The third IF amplifier provides the majority of the gain of the ARC- receiver. The module includes the third IF amplifier at 00kHz, and the squelch circuits. On each side of the module there is a circuit with components soldered directly on posts. One side has the IF amplifier, detectors and AVC circuit. This part is always powered by V. The circuits on the other side are the audio preamplifier and squelch circuits. These circuits are powered only during receive. The IF amplifier has no tuning elements. The amplifier starts with a Collins mechanical filter at 00kHz with 0kHz bandwidth (0-0 khz). This is sufficient to separate the next channel, 0kHz higher or lower, and gives khz audio bandwidth at the auxiliary output. The headset audio bandwidth is further limited to or khz by a second order filter in the modulator audio amplifier An input signal of 0uV is amplified to V on the collector of Q, and when 00% amplitude modulated, gives Vpp audio on the main and auxiliary audio outputs of this module AVC The x amplifier Q-Q-Q provides the AVC control signal. On testpoint J, this signal is.v without RF signal, +V at 00dBm RF signal, rising to +.V with 0dBm RF signal. The AVC control signal reduces the gain in the first and third IF asmplifiers. With more than 0dBm input, Q stops conducting, and a negative voltage on pin reduces the gain of the RF preamplifer as well. Audio amplifier The x amplifier Q0-Q-Q is not powered in transmit mode. Its output is the auxiliary output ( P-L), and the squelched output to the modulator/audio amplifier. Squelch circuit Diode CR is the squelch audio switch. With sufficient RF signal, Q-Q and Q are off, and Q0 is on. In this case, or when the squelch is disabled, the cathode of CR is low, and CR conducts, transferring the received signal to the audio amplifier. Also, Q and Q conduct, pulling the carrier detect pin P-S low. An automatic retransmit station can be made with two ARC sets when each carrier detect output P-S is connected to the key-down input P-P of the other set., and the audio in/out signals are crossconnected.

8 + V + Tx 00k.u P-V Tuning tone 00Hz Q N Sidetone gate on main N connector K MAIN GUARD *) Carbon mike bias J J µf MUTE k µf µf 0 + V R0 MIC xµf µf k + V k µf k Q. N µf 0uF 0mA k 0 0 Q. NA 0 x 0.µF +.V ma Q. N T 0 // R 0nF 0nF 0mH K x R Q / Q N00 (+) : T N + 0V K K To Transmit tube P- R P-E K MIC Test Jack J MIC T k µf +.V + 0mV 0 R *) 0mH See txt x 0. uf Test Jack J MIC TEL 0 *) connections drawn for ARC BX ( dynamic MIC, 00 Ω headset, khz) MIC impedance Ω (carbon) or 0 Ω (dynamic) Headset impedance 0 Ω or 00 Ω Audio bandwidth kc or kc ARC BX MODULATOR jan 00 kb

9 ARC Modulator This module is the audio amplifier in both receive and transmit modes. In Transmit mode, it provides the plate modulating signal to the RF power amplifier, and a side tone to the headset. Inputs - Main: is a Vpp audio signal from the third IF module. In Transmit mode and during tuning, this signal is blanked - Guard: a Vpp audio signal from the guard receiver, blanked during tuning and transmit - Mike: This balanced input is Xformer coupled. The sensitivity can be set from 0.Vpp to Vpp with R0. Normal setting is Vpp for a carbon microphone or 0.Vpp for a dynamic mike. - MUTE. This input is open in receive, but shorted to ground in transmit mode without carrier. This way, the pilot does not hear his own voice ( the sidetone) in the headset, indicating a fault in the transmitter. Tuning tone. During tuning, an 00Hz signal from the inverter enters the MUTE input. Audio amplifier. Transistors Q, Q and Q form the audio amplifier with two outputs: ) for the headset telephone, ) for the modulator The amplifier has heavy feedback, stabilizing the current in Q. The DC feedback sets I-Q at 0mA. The AC feedback reproduces every mvac at the base of Q into ma ac in the primary side of the audio transformer T. With a load of 00Ω on - + -, the gain is 00 x. /(.+.) = x. With the turns ratio of T, setting of R0 and k series R to Q, the voltage gain was x. The maximum current variation in Q is 0mApp, giving Vpp (Vrms) into a 00 ohm load is 0.W as specified. Without a load, the signal is heavy distorted. Transformer data T Microphone transformer Winding / / / Ratio (+) : Resistances (+) : 0 Ω Remark: allows 0mA dc primary current from a carbon mike T Audio output e transformer Winding / // / / Turns ratio. : (+ ) : (.+.) Resistance (.+.) (+) Ω Remark: allows 0mA dc primary current from class A driver Q 0mH 0mH 0mH *) 0mH khz 0 Ω x 0. uf x 0. uf Test Jack J Test Jack J T Modulator transformer Winding / / / Ratio (+ ) : Resistances ( ) 0Ω Remark: allows 0mA dc sec. current from RF power amplifier. Nominal load impedance k Ω TEL Audio bandwidth filter The headset output filter is one of two circuits below, depending on headset impedance. The filter is flat until khz ( option 0 0 P- R TEL 0 Ω P- R 00 Ω Modulator Operated in Class B. Output peak power is W, peak input dc current is.a.

10 Q N Q N +V / 0mA input uf N0 Q N +V Q N Q N Q N Q N Q N MHz 0.p p k Q N k k 0. MHz n k k k k. MHz k k k k k Tuneable from MHz p +V +V J. MHz Q N +V +V N0 doubler 00k k. 00p J -.V. p NA.u Q N.u N0 NA 0k Q0 N 0R Q N 0R k u R R 0 N +0V k k k 0k Q n 0.uF Q n 00 0k 00 N k k 0 x N noise amplifier and squelch gate k k 0 0 k u Audio output RT- / ARC- BX Guard Receiver (A) April 00 kb

11 ARC BX Guard Receiver This is one of the first fully solid state UHF receivers, a masterpiece from Collins. The guard (emergency) receiver is switched on and off with a relay in the supply line; its audio output is permanent connected to the modulator/audio amplifier module. The receiver is a single conversion superhet, with RF stages, a mixer and IF stages at 0. MHz. The oscillator has a.mhz Xtal, a doubler provides the.mhz local oscillator frequency. The gain of the RF stages and the mixer is controlled by the Vce of the transistors. IF amplifier The IF stages operate at 0. MHz and have the rare N dual base transistors. One of the base connections is used to control the gain of the - th IF stage. After detection, a dc-coupled audio amplifier provides the audio signal as well as the gain control signals for the RF and the IF stages. For low input signal, the RF gain is not reduced. Above a certain level, set with R, the RF gain is reduced to prevent blocking with strong input signals. Noise amplifier and squelch gate This circuit is identical to that in the third IF amplifier of the main receiver, but built into a much smaller submodule.

12 ARC Mechanical tuning system The ARC can be tuned from 0.00 to. MHz in 00 steps of 0kHz each. The first 00 steps (until MHz) are not part of the military UHF band. The control head transmits each digit of the frequency with contacts ( V/Adc). The first decimal is or ( contacts), the 0MHz, MHz and 0.MHz decimals each are sent as a -out-offive code, and the last digit is a single contact, representing either 0 or, making a total of contacts. The ARC is a triple superhet, with local oscillators and IF frequencies. The first two decimals set the first local oscillator in steps of 0MHz each, the middle digit sets C- / ARC 00MHz 00MHz 0MHz MHz 0.MHz 0 khz K M Z U V W X T a b c d Y f g h j S P P X b V U T c J L M K N a Y F Z H S the second LO in 0 steps of MHz each, and the last two digits set the third LO in 0 steps of 0kHz each. The mechanical tuner provides rotating shafts. Three control the local oscillators; their positions are compared with the manual settings of the control panel. The remaining shafts are a linear combination of the LO shafts, and tune the RF and first IF frequencies. The second IF is tuned together with the third LO, and the third IF is fixed at 00 khz. The mechanical tuner has one dc motor, and three electromechanical clutches. When a decimal is changed on the control head, the corresponding clutch is activated, and an auxiliary contact of the clutch starts the motor to rotate the output shaft. When the codes coincide, the clutch is deactivated, S S : S S K K : K K K Clutches K,K and K are Ω / V st + d IF AMPLIFIER +0kHz (d LO) Motor +V K 0 deg (0 deg used) + 0 deg (00 deg used) + 0 deg (00 deg used) but the motor rotates until the next click-stop of that output shaft. Two turn ambiguity The RF shaft makes one complete turn, of which half is used to set the variable capacitors in the RF preamp and RF power amplifier. The LO- shaft makes two complete turns, of which one turn is used to select the Xtals in the spectrum generator and (after : reduction) to set the variable capacitors. The other half turn of RF and the other full turn of LO- are skipped automatically, adding a few seconds to the tuning time. The LO-, IF- and LO- shafts make one full turn, of which 00 degrees are used. Both LO and LO shaft positions are divided in click stops, of which 0 are used. The LO- 0 deg (0 deg used) 0 deg (00 deg used) 0: 0: +V K st LO Preamp & power amp First IF d LO shaft selects the 0.MHz steps. The 0kHz steps inbetween are made with a separate relay. The IF- shaft is a copy of the LO- shaft, the 0º steps are filled in with º microsteps, reduced from the LO- shaft. The RF shaft is a : reduced copy of the LO- shaft. The 0º steps are filled-in with a 0: reduced copy of the IF- shaft Waterfall control Changing the 0MHz setting activates clutch. Changing the MHz setting activates clutch, and clutch as well. When you change the 0.MHz setting, all clutches are activated, giving the longest tuning delay. Warning The index pin is sufficient to locate all shafts, except for LO-..When replacing the mechanical tuning unit or the spectrum generator, make sure that the variable capacitor in the spectrum generator is at maximum capacitance when the index pin is in the marked zero position.

13 POWER SUPPLY MODULE N N - µf - V µf N - -V F J K A B T X Y Not used +0V A uf 0.uF External Fan V /0.A 00Hz 00 rpm 0 0 R oven A K Spectrum A C B A ºF ºC +. V x R +.V zener diodes.v, N0 0W V.A. Q. N00 Q Internal fan 0.uF +.V V V V V x 0 Q. N R +0V R 0V 0 R 0 CR N V V RF power 0 RF +V F 00 Hz 0V x µf 0uF Q. N 0.A 0 x N L R Q. N 0, +V max 0 uh K or A k CR N0 0.H /0Ω HEATERS 0 Tuning tone K Antenna Coax relay K K.µF µf Rx A / sec +V 0uF 0 K See RF part for coax connections 0k 0 k Guard Rx +0V / 0mA Mechanical tuner K K K K +0V K LO Coax Relay RF Power amp K Sidetone gate Q 00k N Tx.u 0.H / Ω 0uF K K Tx mode P uf +0V Modulator TEL MIC 0 d IF 0 First & nd IF RF Preamp K K tuning S Z K 00k K K Aux. out K Push to talk R D U V E L a b H c C P P connector pin RFI filters not shown ARC BX POWER SUPPLIES / kb

14 ARC Internal wiring and power supply. The ARC-BX is powered by. +/-0.V.dc With the oven off, the dc input current is: - Receive: A - Transmit ( CW) A - Transmit 0% AM A Tuning forces receive mode, and adds A for a few seconds. The guard receiver + K adds 0. A The Xtall oven adds A for 0 sec. every minute to maintain the constant temperature of the first LO Xtalls. The external fan runs when the internal chassis temperature rises above F ( C). This adds another 0.A to the dc current. Operation is possible down to Vdc input, but with reduced RF output power and modulation depth. Modules A Rcvr / RF preamp A First and second IF amplifier ( with nd and third local oscillator) A Third IF amplifier A Modulator / Audio amplifier A Spectrum Generator ( First local oscillator) A RF power amplifier A Guard receiver A Power Supply A Mechanical Tuning Heater circuit The tubes have the following heater data : Heaters: 0 0.A; : 0.A; 0 : 0.A; : 0.A; : 0.A, all at.v, and for the. 0.A at.v Current in the heater chain is.0 A, supplied from the 0V constant voltage source Q There are testpoints at the.v,.v and.v level. The heater current for the RF power amplifier tube is the base current of Q ( Q is normally off) Three.V/0W power zeners save heaters when a module is removed for tests or in case of a broken filament. Without heaters, the zener current is 0.A. With all modules placed, the zener current is zero, and the heaters current is.a Transient blanker The aircraft DC grid might have severe overvoltages, typical up to 0V for 0. sec. The transient blanker Q+Q prevents damage in this case. Transistor Q is normally fully conducting, passing the V input directly to the main dc rail in the chassis. Transistor Q is normally off. When the input voltage rises above V, Q starts to conduct, blocking Q. Reception during such a surge is still possible, while the A loadcurrent is supplied via the ohm bypass resistor, giving V drop from up to 0V input is V Transistors Q and Q ( N) are germanium PNP types for V/ 0A. Dc/dc inverter The inverter runs on 00Hz, defined by the saturation of the driver transformer. The output voltages are +0V for the RF PA, +0V for all other tubes, -V for the RF preamplifier AVC, and -V as bias for the RF PA. The dc high voltage outputs are boosted by V during transmit to compensate for the extra load current. This way, the dc voltages stays constant at 0V and 0V dc. Relay circuits Relays in chassis Coil Contacts K Power-on relay 0Ω x make, A / 0V contacts K Transmit relay 0Ω x make, A / 0V contacts K Antenna T/R 0Ω x change over, coax K First LO relay 00Ω x change over, coax K Tuning in progress 00Ω x change over ( used) K Guard Rx enable 00Ω x change over, x make used K AGC and modulator 00Ω x change over K External ADF ampl. 00Ω x HV change over. Relays in modules, all gold plated A K disconnect antenna 00Ω x change over, x make used A K input first IF 00Ω x change over,, K select a nd mixer 00Ω x change over, x make used,, K swap d LO Xtal 00Ω x change over,, K +0kHz relay 00Ω x change over T/R switching K is powered by the Press-To-Talk switch, as long as there is no tuning in progress. K switches the HT for the RF power amplifier, completes the microphone circuit, gives a V boost to the plate voltages, reconfigures the IF amplifiers to add all local oscillator frequencies, and actuates relay K. Relay K enables the modulator, disables received audio signals from the main and guard receivers, and disables the first mixer in receive mode in the RF power amplifier module. Relay K, operated in parallel with K, applies 0Vdc plate voltage for the AN/ARA- directional antenna, or is used for a testset. To prevent overload of the power supply during transmit, the 0vdc is only available in receive mode. Coax relay K connects the first LO output to the transmit mixer, or, when energized, to the receive mixer in the RF power amp module. K is the only energized relay in receive mode. Audio signals are switched with relay contacts, by application or removal of dc supply voltage to the relevant circuit parts. Modulator/audio mute A small detector is attached to the antenna T/R switch, including transistor Q. When key-down does not result in sufficient carrier signal to the antenna, then Q will turn on, shutting down the complete modulator. The pilot will not hear his own voice then as sidetone, warning for a malfunction in the ARC. The mute input is also used to present an 00Hz tone to the headset during tuning. ARCA and ARCB These non-x versions have a -phase, V-00Hz power supply for the fans, the high voltage and the tuning tone. The power supply module has a -phase transformer instead of the inverter. All heaters and the oven are still supplied from the Vdc bus, like the relays and other electronics. The 00Hz ac voltage (V square wave) is used for both the internal and external fans, for a tuning tone, and to make +V for the transient blanker.