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5.6 RECEIVER ALIGNMENT ANDADJUSTMENT ( MSR-5050A / 6700A)
AII modules and assemblies in the transceiver are of high reliability, solid state design. Adjustments and alignment are seldom, if ever, required. However, if a module or component replace mentor performance indicates the need for adjustment or alignment, the following procedures are Pro-vided. Table 5.2 provides a list of recommended test equipment.
Before performing adjustments or realignment, it is suggested that Section 4 of this manual be reviewed for a more complete understanding of the receiver.
5.6.1 PRELIMINARY ADJUSTMENTS
a) Connect an RF signal generator to the transceiver antenna connector J34 on the rear panel.
b) Connect an audio voltmeter to rear panel connector J36, Pins 21 and 22, to measure the 600 ohm audio output. Set transceiver mode to USB.
c) Remove the transceiver top cover and the card cage top cover.
Refer to Figure 5.7 for locations of modules and receiver adjustments. The receive path consists of the following boards (from antenna to speaker):Half Octave Filter board, High Pass Filter board, High Level Mixer board, IF/Filter board, Audio Squelch board, Audio 1/0 board and Speaker Driver board. Some auxiliary functions exist on the Interface and Keypad/ Display boards. No receiver adjustments are contained on the Audio1/0, Half Octave Filter, High Pass Filter Speaker Driver boards.
The High Level Mixer board has adjustments for internal component tolerances and should not be readjusted, if components are replaced on this board, it may be put back in service by adjustingC33, C35 and L14 for maximum receiver audio
Output with 0.5 pVrms rf input to the receiver. Access to adjustments requires that this board be mounted on an extender board (Mackay P/N601098-536-001).
Before making further adjustments, check the following jumpers:
1) Audio/ Squelch board - JP1 on E1 should be between pins 1 and 2. This allows muting and
Squelch of speaker audio, but not 600ohm audio.
2) Interface board-JP1 on J4 should be betweenpins1 and 2. JP1 causes channel scan to stop on signals and resume scan after a preset delay.
5.6.2 DC VOLTAGE ADJUSTMENTS
DC voltages generated in the radio are 5 V, 9 V, 13 V, 14 V, 24 V and 26.4 V. The 5 V and 26 V sources are generated in the power supply module The 9 V and 14 V sources are generated on the dual regulator module mounted on the rear of the card cage. The 9 V source is set to +9.2 V by R3on the Mother board, Precise setting of the 14 V source is not required. The 13 V regulated supply and adjustment is located on the 125 W power amplifier (see paragraph 5.7.8.2).
The 24 V supply is on the Reference board and is only used on the Major Loop board.
5.6.3 AGC THRESHOLD ADJUSTMENT
Set the MSR 8050A frequency to 11.6 MHz, USB FAST AGC. Adjust the rf signal generator frequency to peak the audio response and increase the level until the front panel meter barely moves Adjust R35 on the IF/Filter board, if necessary, to cause initial meter movement at 7 u-V rf input.
5.6.4 DELAYED AGC, METER AND 600OHM AUDIO ADJUSTMENT
With power off and ohm meter from ground to E3 on the Audio Squelch board, adjust R80 (squelch) for 2.7k ohms.
With ohm meter from ground to R53 pin 3 (top terminal), adjust R53 (DAGC GAIN) for 100 ohms.
Turn power on. Connect d.c. voltmeter from ground to TP3 on the Audio Squelch board.
Adjust the generator to 11.601 MHz at -47 dBm, radio to 11.6 MHz, USB. Adjust R52 (DAGCGAIN) for 0.2 VDC drop at TP3 with generator on versus generator off. Increase generator level to+13 dBm. Vary the generator frequency d: 5 kHz monitoring the speaker audio for oscillation. If oscillation is present, adjust R53 (DAGC GAIN) on the Audio Squelch board CCW until stable.
Adjust the generator frequency to produce 1 kHz audio tone. Vary the generator level from +13dBm to -90 dBm. The audio level should changeless than 3 dB. Adjust R33 clockwise if necessary.
Set the generator level to -73 dBm (50 u-V). Adjust the Audio Squelch board R64 (600 ohm audio) for 0 dBm out into 600 ohms (J36 pins 21 and 22). Set the generator level to -7 dBm (100 mV). Adjust the Audio Squelch board R44 (SMET) for full scale indication on the front panel meter (100dBttV).
5.6.5 SQUELCH
No adjustments are normally required as R80 on the Audio Squelch board is factory set for a Com-promise between threshold sensitivity and noise immunity. However, if the receiver will be Operating in a high radio noise environment, R80 may be adjusted slightly CCW to reduce squelch breaks on noise.
5.6.6 SCAN ADJUSTMENTS
5.6.6.1 Signal Capture Mode Select
With the front panel RF GAIN/ SQUELCH control pushed in (SQUELCH disabled), the SCAN will be stopped by either CW or voice signals above the set threshold. When the RFGAIN/SQUELCH control is pulled out, the SCAN will only be stopped by voice signals above the set threshold. The SCAN may stop momentarily on strong CW Signals, but will continue to scan after about two seconds. SCAN will stop and remain on a Channel with a voice signal.
In SCAN, the receiver normally operates in a SCAN/ DWELL/ DETECT/ DELAY/ CONTINUE mode. Changing JP1 on the Interface board from pins 1 and 2 to pins 2 and 3 will cause the SCAN to stop and stay on a channel as long as a threshold signal remains. The jumper is accessible from the top of the Interface board.
5.6.6.2 Scan Rate/ Dwell Time Adjustment
Scan rate may be adjusted from 0.21o 3 Hz (dwell time of 5 seconds to 0.3 sec) using R33 of the Keypad/ Display board (accessible through the front panel hole - SCAN RATE).
5.6.6.3 Scan Threshold Adjustment
The threshold level for stopping a SCAN is normally set to 2 uV by R45 on the Interface board. To adjust to a different threshold, tune the receiver to 11.6 MHz, adjust rf signal generator frequency to produce a 400 Hz tone, and adjust the generator rf output to the level desired. Press the SCAN ON button and adjust R45 to barely light DS1 on the Interface board.
5.6.6.4 Scan Dwell Adjustment
This is normally set at two (2) seconds by R34 on the Keypad/ Display board (this is the time On-channel if a threshold signal is present). R34 may be adjusted to any desired delay time between0.5 sec (CCW) to 5 sec (CW). This is accessed through a hole in the front panel. If, during SCAN, a channel is noted to consistently break scan be-cause of a large unwanted CW signal, pull the SQUELCH control to the OUT position(SQUELCH ON).
5.7 TRANSMITTER ALIGNMENT AND ADJUSTMENT
5.7.1 REQUIRED TEST EQUIPMENT
See Table 5.2 for a list of recommended test equipment.
5.7.2 TEST SETUP
1) Remove the top cover and inside card cage top cover of the MSR 8050A.
2) Connect the Audio Combiner/Key Box (Figure 5.6) to the front panel microphone connector. Be certain that the CARBON/ DYNAMIC switch on the MSR 8050A Mother board is in the DYNAMIC position.
3) Connect the audio oscillator to the audio combiner.
4) Connect the rf voltmeter probe directly to the antenna jack J34 using a "tee" connector. Do not connect the voltmeter to any other part of the transmission line between the transceiver and the load or erroneous readings will result. Do not use a coaxial cable between the "tee" and the rf voltmeter, Connect the dummy load to the "tee" using any convenient length of 50 ohm coaxial cable.
CAUTION
Connect the rf voltmeter probe ground lead to the chassis so that it does not inadvertently cause a short circuit on the Half Octave board.
5.7.3 BOARD JUMPER SETUP
Before attempting alignment, check the option jumpers on the Half Octave board and the Transmit Modulator board.
5.7.3.1 H alt Octave Board
JP1 should be in the normal upper (Pins 1 and 2) position at E1.
5.7.3.2 Transmit Modulator Board
Jumpers JP1 (Carbon/Dynamic mic) and JP2 (Center tap key) may be in either position. JumperJP3 should be in the upper position, Pins 2 and 3, for "Compressor In". Jumper JP4 (Half octave board old and new) should always be in the left hand position between pins 2 and 3.
5.7.4 OUTPUT POWER ADJUSTMENTS
Since the oscilloscope is used only for waveform monitoring and absolute value is not important, the scope probe may be placed next to the Half Octave Filter board at any convenient point and capacitive coupling should provide adequate Signal for monitoring.
NOTE
Some adjustments interact. The adjustments should only be made in the order given here.
5.7.4.1 ALC Threshold Adjustment
Set the power level to 2. With the transmitter UNKEYED, set R83 on the Transmit Moudlator board for 4.00 VDC measured at TP6.
5.7.4.2 ALC Detector Adjustment
On the Transmit Modulator board, adjust R51 (DSB output) and R17 (carrier) fully CW (max. output). Set radio frequency to 1.6 MHz in CW mode. Key the transmitter and adjust R11 (ALC CAL) on the Half Octave Filter board for 79 VRMS rf output as read on the rf voltmeter. Unkey the transmitter. Set the radio frequency to 29.9 MHz in CW mode. Key the transmitter and adjust C2 (ALC COMP) on the Half Octave Filter board for 79 VRMS rf out-put. Recheck results at 1.6 MHz (and 29.9 MHz, if necessary).
5.7.4.3 Low Power Adjust
Set the radio to 1.6 MHz in CW mode and set power level to 1. Key the transmitter and adjustR84 on the Transmit Modulator board for 35VRMS rf output.
5.7.4.4 Transmit Modulator Board Output Level Adjust
Set the frequency to 29.9 MHz in CW mode and set power level to 2. Key the transmitter and adjust R51 CCW until the rf voltage just starts to fall, then turn back CW 1/8 turn.
5.7.5 AMPLITUDE MODULATION EQUIVALENT (AME) ADJUSTMENTS
Turn R44 (AM MODULATION) on the Half Octave Filter board fully CCW. Set the frequency to1.6 MHz in AM mode. Key the transmitter with no modulation and adjust R13 (AM CARRIER) on the Half Octave Filter board for 42 VRMS rf output. Unkey the unit.
Connect a load and 30 dB power attenuator to the MSR 8050A rf output connector J34 and, using a convenient length of 50 ohm coaxial cable, connect the Spectrum Analyzer to the attenuator output.
CAUTION
Do not connect the Spectrum Analyzer direct1y to the transmitter rf output or severe damage will result. Adjust the Analyzer Controls so that the carrier and a tone 1 kHz away can be seen.
Set the frequency to 29.9 MHz in AM mode. Increase the level of a 1 kHz modulating tone (fed through the mic connector) until the Compressor LED DS1 on the Transmit Modulator board is illuminated. Key the transmitter and adjust R17 (CARRIER) on the Transmit Modulator board until the output monitored on the Spectrum Analyzer shows the sideband 1.5 dB below the Carrier. Note that the sideband tone is 1 kHz above the carrier frequency. Unkey the unit.
Set the frequency to 1.6 MHz in AM mode. Key the transmitter. Adjust R44 on the Half Octave Filter board CW until the rf output meter reads 79VRMS. Unkey the unit.
5.7.6 A3A (REDUCED CARRIER) ADJUSTMENTS
Connect the load with 30 dB attenuator to the transceiver rf output connector J34 and connect the Spectrum Analyzer to the attenuator output. Set the audio oscillator frequency to 1 kHz and increase its amplitude until the compressor LED DS1 on the Transmit Modulator board is illuminated. Set the radio frequency to 1.9 MHz in A3Amode. Key the transmitter and adjust R49 on the High Pass Filter board so that the carrier is 16 dB below sideband output as observed on the Spectrum Analyzer. Unkey the unit.
Set the radio frequency to 4.0 MHz in A3A mode and key the transmitter. Adjust R48 on the High Pass Filter board so that the carrier is 16 dB below the sideband output. Unkey the unit.
Set the radio frequency to 19 MHz in A3A mode and key the transmitter. Adjust R47 on the High Pass Filter board so that the carrier is 16 dB below the sideband output. Unkey the unit.
Set the radio frequency to 29.0 MHz inA3A mode and key the transmitter. Adjust R46 on the High Pass Filter board so that the carrier is 16 dB below the sideband output. Unkey the unit.
5.7.7 VSWR AND CURRENT LIMIT ADJUSTMENTS
Set the radio frequency to 29.9 MHz in CW mode. Connect the digital voltmeter (DVM) between TP2 on the Half Octave Filter board and ground. Key the transmitter and adjust C30 on the Half Octave Filter board for a null (minimum voltage).Unkey the unit.
Connect two 50 ohm dummy loads in parallel, using the shortest coaxial lines possible, and connect with a short coax to the transceiver antenna jack J34. Set the radio frequency to 2.0 MHz in CW mode and key the transmitter. Adjust R35 (current limit) on the Half Octave Filter board for an rf output of 45 VRMS as read on the rf voltmeter. Unkey the unit.
5.7.8 PA ADJUSTMENT
Normally, adjustments to the solid state power amplifier are not required. If a component replacement or operation indicates a need for adjustments, the following adjustments can be made.
5.7.8.1 Test Set-Up
Terminate the transmit output, 1A2J34 in a 50 ohm, 125 watt load. Install a thru-line watt meter (Bird or equivalent) in series with the output for these adjustments. Remove the four screws that attach the amplifier module to the Rear Panel assembly.
Carefully position the PA module in a flat position on the test bench, Insure that all wires and harnesses are attached to the exciter and that no electrical short circuit of the exposed PA circuit board or wiring can occur to the chassis or other metal objects. The exciter power amplifier assembly can be safely operated in this position for short periods.
5.7.8.2 13 VDC Regulator Adjust
Set the transmit frequency to 5.2 MHz, the mode to USB. Key the radio using the microphone PTT but DO NOT speak into the microphone. Using a DC voltmeter connected between E1 and GND, adjust R2 for +13.2 VDC at E1. Unkey the radio.
5.7.8.3 Output Stage Bias Adjust
Key the radio with no modulation. Adjust R44 (DRIVER BIAS) for .68 VDC measured between the base of Q6 or Q7 and ground. Adjust R47 (FINAL BIAS) for .68 VDC measured between the base of Q8 or Q9 and ground.
5.7.8.4 Drive Stage Bias Adjust
The exciter settings are the same as 5.7.8.3. Connect an oscilloscope across the 50 ohm load and connect the audio combiner key box Apply two equal audio tones, 700 and 2300 Hz, and key the exciter. The RF output pattern (shown below) on the scope should depict the standard two-tone pattern (similar to an AM modulation pattern with100% modulation). Adjust R44 until the area between peaks just touches the reference line.
5.7.8.5 Over current Adjustment
Change the transmit frequency to 29.999 MHz, the mode to CW; and key the exciter. 125 watts should be indicated on the wattmeter. Adjust theR16 over current adjust until the output power starts to decrease. Slowly adjust R46 until full power returns. Leave R46 adjusted to this Setting. Remove power from the radio. Reinstall the PA module on the Rear Panel assembly.
5.7.9 POWER SUPPLY MODULE
See Figures 5.9, 5.10 and 5.11.
5.7.9.1 Troubleshooting
5.7.9.1.1 Troubleshooting Chart
Table 5.5 describes the LED status indicators that are visible through the top cover of the power supply. Table 5.6 outlines symptoms and Probable causes of some power supply failure.
5.7.9.2 Use of Extender Cables
The power supply can operate in a partially disassembled mode through the use of optional extension cables. Follow disassembly procedures in Section 5.7.9.3 until the FET Drive board has been removed. Reconnect the FET Drive board to the Control board (P4 to J4 and P5 to J5) outside of the power supply. Use the extension cables to connect P1, P2 and P3 of the FET board to J1, J2 and J3 of the Transformer board. Lay the Control board/ FET board assembly in front of the power supply with the FETs face down. Use the last extension cable to connect J1 of the Control board to P1of the Input board, In all cases, be careful not to twist the cables or reverse any of the connections. Do not operate the supply at high current output for long intervals because the cooling fan is not connected in this state.
Finally, the power supply can be turned on and off by moving jumper P2 from J2, pins 2 and 3, to J2pins 1 and
2.5.7.9.3 Power Supply Removal
To remove the power supply from the radio, follow the steps below.
1. Disconnect AC power cord from rear of radio.
2. Remove top and bottom covers of radio. Top cover is held by two (2) quarter-turn fasteners and bottom cover is secured by ten screws. Remove four (4) Pan head screws that fasten power supply to radio Mother board.
3. Disconnect power supply wiring harness from radio by removing P1-4 from their mating Connectors.
4. Remove two (2) screws that secure steel top shield to top of power supply. Remove this shield.
5. Remove power supply unit from radio chassis. Steel bottom shield may now be removed also.
5.7.9.4 Disassembly of Power Supply
These instructions assume the power supply has already been removed from its radio. Refer to the Installation section of the radio manual for this procedure.
5.7.9.4.1 Top Cover Removal
Remove the 14 screws that fasten the power supply top cover to its chassis. Note that some units are equipped with a steel shield over the top cover. This shield is held by two of the 14 top cover screws and must be removed first. lift the top cover from the chassis.
Notice the large cylindrical capacitor C101. It is secured to the side panel by a c-clip that is fastened with a screw. This screw should not be removed.
5.7.9.4.2 Side Panel Removal
1. Remove the three (3) screws that fasten the side panel to the fan bracket.
2. Remove the two (2) screws securing the side Panel to the input bracket Next, lay the power supply on its side and remove the five (5) screws that hold the side panel to the bottom position of the main chassis (A3A5).
3. Return the supply upright and remove the five (5) screws that fasten the side panel to the internal FET heat sink. Do not remove the screw holding the c-clip, but gently pull the side panel outward until the wiring harness can be lifted up and over the top of the input bracket. While pressing the bottom of C101 toward the fan, slide the side panel away from the fan until the c-clip slides off of the end of C101.
5.7.9.4.3 Control Board Removal
The Control board can be removed with disconnecting it from the wiring harness or C101.
Remove two screws on either end of the Control board. You may have to loosen the black output ground lead at terminal E41o remove one of the screws. Unplug the fan at connector J3.
Gently work the control upward until the three PC mount connectors (J2, J4, J5) are free. The Control board, C101 and the wiring harness can now be draped over the side wall of the main chassis. If the Control board must be totally removed, disconnect C101 at terminals E1 and E2.
5.7.9.4.4 FET heat sink Assembly Removal
The FET heat sink Assembly can be removed by gently pulling it upward until the three connectors (P1, P2, P3) between it and the Transformer board are disenaged.5.7.9.4.5 Final Disassembly Further disassembly is usually not required, but careful visual inspection at this state will show the remaining disassembly steps to be self-evident.
5.7.9.5 Power Supply Reassembly
5.7.9.5.1 General
These instructions apply to a power supply whose FET Heat sink Assembly and Control board have been removed, but whose Transformer board and Input board are still installed in the main chassis along with the input and fan brackets.
5.7.9.5.2 FET Heatsink Assembly Installation
The FET Heat sink assembly plugs directly onto the Transformer board by aligning P1, P2 and P3of the FET board with J1, J2 and J3 of the Transformer board, and carefully pressing downward to engage the connectors.
5.7.9.5.3 Control Board Installation
Carefully work the Control board underneath the lugs of the main chassis side panel and align J4and J5 of the Control board with P4 and P5 of the FET drive board. Align J1 of the Control board with P1 of the Input board and gently press the Control board into place. Replace the two screws that secure the Control board in place.
5.7.9.5.4 Side Panel Replacement
Place the output wire harness in its place between C101 and the Input board and route it through its normal exit point a notch in the input bracket. Align the edge of the side panel nearest the fan with its corresponding place on the fan bracket and carefully press the side panel into place, guiding C101 into its c-clip. This will require a mode rate amount of pressure. Be sure the wiring harness and C101 are in place and reinstall all of the screws into the side panel.
5.7.9.5.5 Top Cover Replacement
Align the holes of the top cover with the Corresponding holes in the main chassis and install the screws holding it in place. Tighten all screws securely. If the power supply has a top shield, place it over the top cover of the supply before installing the final two screws that hold it in place.
5.7.9.6 Adjustment Procedures
5.7.9.6.1 General
The power supply is factory preset and should need no further adjustment, but a procedure is presented here for use in the event an adjustment is necessary, It is assumed the technician has a variable load with current indicator. AII adjustments are made on the Control board. Adjustment tolerances can be found in Performance Specifications, Paragraph 5.9.15.
5.7.9.6.2 Switching Frequency
The Switching Frequency is approximately 40 kHz. It can be measured with an oscilloscope atTP10.
The frequency is set at the factory to optimize current limit circuit performance and should not be readjusted.
5.7.9.6.3 +26V Output Adjustment
The +26V output should be set to 26.4 VDC while loaded to 1.5 amps. Potentiometer R37 is for this adjustment.
5.7.9.6.4 +5V Output Adjustment
While the +26V output is loaded to 1.5 amps, adjust the +5V output for +5.2 VDC. This adjustment is made with potentiometer R9, located on the FET Drive board. An access hole is provided in the Control board near J5.
5.7.9.6.5 Temperature Threshold Adjust
The over-threshold temperature of the FET Heat sink should be set to +82"C. This is done by adjusting potentiometer R2 while measuring the DC V at TP4. The adjustment voltage is 355 mV.
5.7.9.6.6 Primary Current Threshold
This adjustment is made with pot R6 by slowly increasing the loading of the +26V out-put until the primary limit indicator (D55) just begins to turn on, and adjusting R6 so that this occurs at a loading of 18 amps. This ensures proper voltage fold back at a load current of about 20 amps.
5.7.9.7 Periodic Maintenance
5.7.9.7.1 Intake Air Filter
The intake air filter should be inspected often and cleaned when necessary. A clogged air filter will cause inefficient performance of the power Supply, resulting in higher operating temperature sand possible failure. When the over temperature threshold is reached, the power supply FAULT indicator will come "ON", but the power supply will still be operational. A signal is sent to the host radio which will disable the transmit function if this condition persists.
5.7.9.8 Options and Accessories
5.7.9.8.1 Extension Cable Kit
The optional extension cable kit, P/N 600289-700-001, can be used as a troubleshooting aid, allowing operation of the power supply in a partially disassembled state. This allows access to PC boards and components that otherwise are in accessible during operation. The kit consists of a set of three cables (P/N 600891-540-001) and a set of one cable (P/N 600891-540-002).
5.9.6 TRANSMIT MODULATOR BOARD.
1A16 See Figure 5.41 for the schematic of the Transmit Modulator board.
The Transmit Modulator board processes audio inputs and generates the double sideband Sup-pressed carrier (DSSC) rf signal at the transmitter's IF. This DSSC signal is then filtered to remove the unwanted sideband, shifted to the desired transmit frequency and amplified.
The following circuits are located on this board: audio input, audio limiter, modulator, control and ALC.
5.9.6.1 Modulator Section
The Modulator M1 is a double-balanced ring mixer. The 5 MHz third LO signal is applied to the LO port of the mixer and audio is applied to the IF port. The resulting DSSC signal is obtained from the rf port, pin 1. Because of the inherent balance of the mixer, the carrier is suppressed 50 dB with no adjustments required.
5.9.6.2 Double Sideband Amplifier
The DSSC signal is amplified by Q4. The gain of the amplifier may be varied by R51 which, by controlling the current through PIN diode CR29, varies the amount of negative feedback in the amplifier.
5.9.6.3 Third LO Amplifier
The 5MHz third LO is amplified by transistor Q5 and injected into the modulator M1.
5.9.6.4 Carrier Injection
5.9.6.4.1 AME Mode
In either AME ( full carrier ) and A3A (Partial Carrier) operation, some carrier must be replaced in the SSB signal following unwanted sideband removal. Since the unwanted sideband is removed in the IF filters, and the IF filters are narrow enough to remove at least 15 dB or so of carrier frequency, the carrier must be re injected f0110w-ing this operation. The carrier is then routed to the IF Filter board where it is mixed with the SSB signal after the filters. Transistor Q12 on the Transmit Modulator board is a switch used to turnoff the carrier output when it is not wanted in CW and SSB modes, In the AM Transmit mode, the/AMT 1ine wi11be1ow, making U4pin 11ow. Turning off Q6 (removing the short to ground on the third LO line) and turning on Q12. This allows the third LO signal to be sent to the IF Filter board for carrier injection. Carrier level is determined by the current through PIN diode CR8, which is set by control R17.
5.9.6.4.2 A3A Mode
In the A3A mode, the /A3A line is low, making U4pin 14 high, which turns on Q12. This allows the third LO to be sent to the IF Filter board for carrier reinsertion. Q6 will be on so no third LO signal can flow through CR8, and the carrier level is entirely controlled by the current through PIN diode CR9.The control voltage for CR9 is located on the High Pass Filter board.
5.9.6.5 Audio Compressor
The audio compressor provides two benefits: it limits the maximum audio level applied to the modulator, preventing distortion; and it increases the average transmit power by compressing the voice amplitude range. The compressor consists of U1, Q1 and Q2. U1A is the audio amplifier stage with its gain controlled by the resistance of FET Q1. The amplifier receives input from two sources: the front panel microphone and the rear panel 600 ohm inputs. When Q1 is off, the overall gain of U1A is roughly unity. When Q1 is turned on completely, the gain is reduced about 26 dB. The gain is controlled by a feedback loop through U1C to keep the output level of U1A constant. The attack time for the compressor is about 4.7msec and the release time is about 1 second.U1D drives an LED to indicate when there is sufficient audio to activate the compressor. The output of the compressor is a buffer amplifier U1 Band Q2, which supplies audio to the modulator.
The gain of the buffer amplifier can be altered to reduce the DSSC level during AME and low power modes of operation. When the radio is in any mode except low power or AM, the output ofU4A and U4B is high, keeping CR6 and CR7 turned off. When the /AMT line goes low, U4B
Output will go low, turning on CR6, which reduces the output level by about 7.5 dB. When the low power signal is high, U4A output goes low, turning on CR7, which reduces the output level about 6dB. Both the low power and AM transmit Conditions are not permitted together and are Prohibited by the transceiver's software.
5.9.6.6 Audio Input Circuits
The purpose of the audio input circuits is to condition and amplify the microphone and 600ohm line inputs.
5.9.6.6.1 600 Ohm Line Input
The 600 ohm balanced line audio is applied to transformer T1, the output of which is terminated with 600 ohms (R3 and R4 in parallel), and protected from high voltage spikes by back-to-back
zener diodes. The audio is fed into U2A for amplification. The amplifier's gain is adjustable via R1 to accommodate input levels from -26 dBm to 0 dBm. The amplifier output is then fed to audio gate U6A, which is on, allowing audio to pass un less/TUNE or/CW is low. This latter condition prevents modulation from the 600 ohm line during either TUNE or CW operation.
5.9.6.6.2 High Level Dynamic Microphone Input
The dynamic microphone input is first filtered to remove any rf present on the line, then is amplified by U2B. This amplifier has three different gain ranges, depending upon which microphone input is used. Microphone gain is adjustable via R58. The amplifier output goes to gate U6B, which is on, allowing audio to pass unless/TUNE or /CW is low.
5.9.6.6.3 Low Level Dynamic Microphone Input
The other microphone input (for carbon or dynamic microphones) can be set via jumper JP1 to accommodate either a carbon or a low-level dynamic microphone.
5.9.6.6.4 Carbon Microphone Input
When jumper JP1 is in the 1-2 position, the input is set for a carbon microphone. DC for the carbon element is provided by Q9, a well-filtered emitter follower source.
5.9.6.6.5 Side tone Output
Amplifier U2C provides a side tone output de-signed to drive the earpiece of a handset. The amplifier picks up the microphone amplifier Out-put and applies it to the earpiece. The side tone level is adjustable via R67.
5.9.6.6.6 Center Tap Key
An arrangement allowing radio keying by application of a DC voltage on either side of the 600 ohm audio line is provided by Q10. This feature is furnished to comply with the TADIL-A high speed data specification. If a DC voltage is detected at the center tap of the 600 ohm input transformer, Q10 will turn on, pulling the key line to ground, and keying the transceiver. This feature may be disabled by placing jumper JP2 in the 2-3 Position.
5.9.6.6.7 1 kHz Input Circuit
The 1 kHz tone used to modulate the transmitter during CW operation is fed to audio gate U6D. Gates U6D and U6C are toggled together and are turned on only when /CW is low, /PTT is low, and/ TUNE is high. These conditions occur only when the transmitter is keyed in CW mode. The output of audio gate U6E is fed to the compressor input. Use of the compressor eliminates the need for atone level adjustment control. The CW side tone audio is taken from gate U6D and fed to the Audio/ Squelch board to provide CW side tone duringtransmit.
5.9.6.7 Control Circuits
This section describes the various control circuits used on the Transmit Modulator board.
5.9.6.7.1 Internal Power Supplies
Transistor Q3 provides a switched +9 VDC source available only during transmit. It supplies power to the third LO amplifier Q5 and DSSC amplifierQ4. U5 provides a regulated +5 VDC for use on the board. The regulated +5 VDC is applied to voltage follower U2D, which is the signal "ground"(at +4.5 VDC) reference for the operational amplifiers.
5.9.6.7.2 Compressor Control
The audio compressor may be disabled by either an external circuit (TTL low pin 34) or a jumper (JP3 pin 1 to 2) on the board. The disable arrangement, however, prevents disabling when/CW is low, so that the CW tone level remains controlled. U4D is used for the disable function.
5.9.6.7.3 /PTT, n-UNE and /CW Control
The /PTT command comes from the front panel, is filtered to remove rf, and is fed to the /PTT comparator, U3C. This works in conjunction with the /CW comparator, U3D, during CW operation to shape the keying of the 1 kHz tone to control the rate-of-rise of the CW envelope and limit its Over-shoot.
The /CW command is applied to the /CW Comparator, U3D, which controls transistor Q11. In CW operation, Q11 cuts off audio gates U6A andU6B, inhibiting other audio inputs to the Compressor. When Q11 is turned on, Diode CR22 is turned off allowing the /PTT command to key the transceiver 1 khz CW tone.
The purpose of the /TUNE command is to set the Tx Modulator board for coupler tuning – AM carrier with no modulation as a coupler rf source. When /TUNE goes low, it also pulls the /MUTE line low, muting the receive audio, disables the 1kHz tone, and inhibits the microphone and 600ohm audio.
5.9.6.8 Automatic Level Control (ALC) Circuitry
The purpose of the ALC circuit is to keep the transmitted output power (PEP) constant in SSB,CW, FSK, DATA and A3A modes without regard to changes in signal path gain, temperature or load impedance. In addition to the internal ALC circuitry, an external ALC port is provided for output level control when the transmitter is driving the MSR 1020 1 kW linear power amplifier. The ALC is a voltage type, which holds the rf output voltage constant, regardless of load impedance.
If the load is 50 ohms, 125 watts of power will be produced. If the output impedance is different from 50 ohms, a different amount of output power will be produced as the output voltage is held constant at 79 VRMS. A VSWR sensing circuit on the Half Octave Filter board decreases the output to protect the power amplifier if the load impedance is too high or too low.
5.9.6.8.1 Internal ALC
The internal ALC circuit is enabled whenever the/AMT line is high, indicating that the transmitter is not in AM mode, sending U4B output high, cutting off diode CR15. Conversely, when the /AMT line is low, diodes CR14 and CR15 are both turned on, disabling both the internal and external ALC Circuits.
The internal ALC gets its input from the ALC Detector located on the Half Octave Filter board. This input is a voltage proportional to the rf output voltage of the transmitter, and is adjusted so that it is +6.0 VDC when the rf output voltage is 79VRMS. A reference voltage for the ALC amplifier U3A establishes the ALC threshold.
During operation at power level 2, diode CR16 is off so that the threshold level is determined by ALC control R83. During low power (Power level1) operation, diode CR16 is turned on by U4A, putting low power adjust control R84 in parallel with R83, letting R84 now control the threshold level. Note that since the adjustments interact, R83 must be adjusted first.
The output of the ALC amplifier U3A is applied to complementary amplifiers Q7 and Q8 whose output is the Transmit Gain Control (TGC) line. This line is set up so that a TGC voltage of +6 VDC produces no gain decrease in the transmitter, while a voltage of +1 VDC produces the maximum gain decrease. The gain control circuit is located on the IF Filter board.
When the ALC voltage from the ALC detector exceeds the reference voltage, the output of U3Adecreases, causing the TGC voltage to drop, reducing the transmit gain and the rf output. Loop feedback causes the TGC voltage to settle Some-where between +1 and +6 VDC, wherever 79VRMS output is produced. Because the open loop gain of the ALC control loop is very high, the final rf output voltage depends only on the ALC detector characteristics, not on the transmit Signal path gain.
5.9.6.8.2 External ALC
The external ALC is applied to amplifier U3B. Its output is also applied to the TGC amplifier, Q7and Q8, for control of the transmitter output as described in the previous paragraph.
5.9.6.8.3 Automatic Carrier Control (ACC)
The ACC circuit is responsible for controlling the transmitter output average power in AME mode. The circuit itself is located on the Half Octave Filter board, but uses Q7 and Q8 to control the TGC line voltage. While the ALC circuitry controls the Peak Envelope Power of the transmitter, the ACC controls the average output power to maintain good AM linearity and keep the carrier level constant.
5.9.11 AUDIO/SQUELCH BOARD, 1A17
See Figure 5.52 for the schematic of the Audio Squelch board.
The Audio Squelch board is used to process receive functions only.
It accepts the 5 MHz output from the IF Filter board and performs the final detector function of converting the intermediate frequency signal into usable audio intelligence.
A product detector is used in SSB, CW, FSK and DATA modes, while an envelope detector is used in the AM mode.
Two separate audio outputs are provided: one ultimately ends up at the rear panel as a balanced 600 ohm output and the other is amplified by a speaker driver and appears at the transceiver front panel speaker and phone Out-put.
Located on the board are an input IF amplifier, envelope detector, AGC circuitry, a delayed AGC circuit, product detector, third LO amplifier, 600ohm audio amplifier, speaker/ side tone circuits and squelch circuits.
The AGC has fast attack and front panel selectable fast, medium and slow decay times.
The squelch circuit is syllabic, responding to the syllabic characteristic of human speech, with a front panel enabling control.
5.9.11.1 Input IF Amplifier and Envelope Detector
Q1 is a common emitter amplifier with 36 dB gain connected through an impedance-stabilizing pad to the product detector, M1, and to the gate of FET Q2.
This latter stage is a source follower buffer amplifier which drives envelope detector Q3.
The detected envelope is routed to U2, which selects either the product detector output in SSB, CW, FSK or DATA, or the envelope detector signal in AM.
The envelope signal is also applied to the AGC circuit at U1A and U1C.
5.9.11.2 Product Detector and Third LO Amplifier
The 5 MHz third LO signal for the product detector is supplied at -15 dBm.
Transistor Q4 amplifies this to about +7 dBm for the product detector.
The 5 MHz IF signal input to the product detector is about -10 dBm.
The detected audio is applied to the analog switch U2.
5.9.11.3 Speaker Audio and 600 Ohm Audio
The selected envelope cr product detector audio signal from switch U2 is amplified by U4A. The audio levels from the two detectors are matched so that they are the same for an SSB (100%modulated) signal as they are for a 100% modulated AM signal. The AC ground reference for most of the analog circuitry is established at 1/2the supply voltage by U5A. This eliminates transient outputs in the audio during switching. The600ohmaudiosigna1isroutedthroughtheMUTE/ SQUELCH switch U2 unless bypassed byjumperJP1 from E1, pins 1 to 2. R64 provides a level control for the 600 ohm line audio provided by the line amplifier, U4B and Q10. The line amplifier is capable of supplying +10 dBm when loaded with600 ohms.
The speaker audio goes through analog switchU2 and is buffered by amplifier U4C before being sent to the front panel Speaker Driver board. ACW side tone signal from the Transmit Modulator board is injected into the buffer amplifier.
5.9.11.4 Squelch Circuits
The syllabic squelch circuit is basically a pulse count discriminator and integrator, which Produces a gating output for the slowly changing, i.e. syllabic, frequency content of the audio signal. The gating output has a fast attack and slow decay characteristic.
The audio signal is first amplified by U6B, the squared in U6C to produce a square wave input to U7A for signals as weak as the receiver noise level. U7A and U7B form a mono stable multi vibrator with an output pulse width of 0.5 msec. A differentiator and integrator produce a DC voltage output proportional to the average frequency of the audio signal. U6A is an integrator with a time constant of 73 msec, which reduces output due to fast (greater than 5 Hz) frequency changes. U5D is an absolute value amplifier which produces anegative output when the peak voltage changes from R80 exceed the forward voltage drop of either CR17 or CR18. The one-shot multi vibrator, formed by U7C and U7D, is triggered throughCR16 and resets in about 2 seconds, unless retriggered. The positive output from U7CthroughCR15 forces comparator U5B high, which in turn 5-147
enables the 600 ohm audio gate U2B and the speaker audio gate U2C.
The squelch gates may also be enabled through comparator U5B by comparator U5C, which is driven by the front panel SQUELCH control and provides a degree of squelch threshold control.(In the MSR 8050A only an OFF/ON signal is obtained from the front panel control.)
5.9.11.5 AGC Circuits
U1 A is an amplifier/peak detector which amplifies (gain of 50) signals from Q3 and applies them toU1 B. The attack time is established by R95 andR22 with C23 to less than 10 msec. The decay time is determined by the discharge time constant of C231hrough R23 for3.0 seconds in slow AGC, through R39 for 200 msec in medium AGC, and through R38 for 30 msec in fast AGC.
U1C is identical to U1A, but with a gain of 200 and a fast charge and discharge time. The outputs of both amplifiers are compared in U1D. The output of U1C is normally higher than that of U1A, so U1D output is low. If the signal level suddenly falls, the output of U1C follows, but the output ofU1A remains high due to the slow decay of C23.
The output of U1D then goes high, switching onQ6 or Q7 (whichever has been selected by open collector comparators U3A and U3B). U3A orU3B is selected by voltage level presented to the board at pin 38. If the input voltage exceeds +7.7 VDC, both comparator outputs are low, both Q6 and Q7 are turned off and the AGC decay is slow. If the input voltage is between +3 VDC and +7.7VDC, U3A output is high and U3B output is low, turning on Q6, and the AGC decay is medium. If the input voltage is below +3 VDC, both Comparator outputs are high, turning on both Q6 and Q7, and the AGC decay is fast. An external ground onU1B through CR10 will dump the AGC voltage, putting the receiver in maximum gain. A DC voltage through CR3 allows the AGC to be Over-ridden by the RF GAIN control. U1 B is the AGC output amplifier. A logic high on pin 29 disconnects the signal driven AGC allowing the receive gain to be determined only by the RF GAIN d.c. input at pin 34.
The AGC controls the gain of the IF Filter board amplifiers, maintaining a constant audio output level over a wide rf input level range. A short on pin 26 (DATA) reduces the AGC threshold (lower input level for constant output) from about -90dBm to -100 dBm. The S-meter output is adjustable by R44. A delayed AGC signal to control High Level Mixer board gain is produced by current amplifier U4D and Q9. Delayed AGC offset is adjusted with R52. Delayed AGC current gain is set with R53.
5.9.11.6 Receive Voltage Gate
Transistor Q5 applies +9 VDC to the third LO amplifier Q3 and to IF amplifier and detector Q1 ,Q2 and Q3 in receive when the /Rx line goes 1ow.
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