Ericsson_Monogram_VHF_Maxon_SM-4150_PM150_serv.pdf

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1、? LBI-38864B Maintenance Manual Monogram Series VHF MOBILE RADIO MODEL MGM 148 the CALL indicator will no longer be illuminated. TRANSMITTING 1.Turn the radio on and select the desired channel. 2.Pick up the microphone and listen briefly to insure the channel is clear. Alternatively, leave the micro

2、phone on-hook and depress the MONITOR button (2). 3.Depress the PTT switch on the side of the microphone. Hold the microphone one to two inches from the mouth and speak in a normal tone of voice. The TX indicator should be illuminated. 4.Release the PTT switch when the message is completed. MONOGRAM

3、 SERIES LBI-38864B GENERAL OPERATION Page -9- Nov. 94 DIGITAL CHANNEL GUARD Channel Guard provides a means of restricting calls to specific radios through the use of a CTCSS (Continuous Tone Coded Squelch) or DCS (Digitally Coded Squelch). The tone frequencies range from 67 Hz to 250.3 Hz and there

4、are 83 programable codes. Channel Guard tone frequencies and codes are software programmable. Both tone frequencies and digital codes may be mixed on each channel. The frequencies and codes are shown in Tables 1 and 2. A Channel Number display that does not flash, indicates that Channel Guard is not

5、 programmed. A flashing Channel Number indicates that Channel Guard is programmed and disabled. Table 1 - Channel Guard Tone Frequencies Standard Tone Frequencies in Hertz 67.071.974.477.079.782.585.488.5 91.594.897.4100.0103.5107.2110.9114.8 118.8123.0127.3131.8136.5141.3146.2151.4 156.7162.2167.91

6、73.8179.9186.2192.8203.5 210.7218.1225.7233.6241.8250.3 Digital Squelch Codes 023114174266411506703 025115205271412516712 026116212274413523723 031122223306423526731 032125225311431532732 036131226315432546734 043132243325445565743 047134244331446606754 051143245332452612 053145246343454624 05415225

7、1346455627 065155252351462631 071156255356464632 072162261364465654 073165263365466662 074172265371503664 Table 2 - Primary DCS Codes MONOGRAM SERIES LBI-38864B GENERAL OPERATION Page -10- Nov. 94 CIRCUIT ANALYSIS RECEIVER RF Amplifier Incoming signals from the antenna jack are routed backwards thro

8、ugh the transmitter lowpass filter in PIN diode switch D12. In receive mode, D12 conducts allowing a low impedance path through the diode to the receiver front end circuitry. The receiver RF amplifier section is comprised of two bandpass filter sections separated by an amplifier based around Q1. The

9、se two filters allow signals at or near the operating frequency to pass but provide strong rejection of the mixers spurious response frequencies. The first filter section is a two pole design formed around RF transformers T1, T2 and their associated circuitry. This filter is followed by the RF ampli

10、fier transistor Q1. This device is a dual-gate MOSFET which, with its low noise figure, yields good receiver sensitivity while showing strong resistance to overload from strong signals. The output of Q1 drives a three-pole filter section formed around T3, T4 and T5. The output of the RF amplifier st

11、age is routed to the first mixer. First Mixer and First IF Amplifier The action of the first mixer transistor Q2 is to convert incoming signals at the operating frequency to the frequency of the first IF which is 21.4 MHz. The output of the mixer is at a frequency which is equal to the difference be

12、tween the frequency of the incoming signal and the local oscillator. In this radio, the local oscillator signal is chosen to be 21.4 MHz below the operating frequency. The device chosen to perform the mixing operation is Q2, a high performance JFET. The incoming signal is applied to the gate of Q2,

13、and the local oscillator signal is filtered by transformers T6, T7 and associated circuitry before being supplied to the drain of Q2. The difference frequency signal at 21.4 MHz exits the mixer at the source of Q2 drives the first IF filter XF1 and XF2. XF1 and XF2 form a 4-pole monolithic crystal f

14、ilter pair which in part determines the selectivity of the radio. The output of the crystal filter is routed to the first IF amplifier formed around Q3. RF transformer T8 and T9 provide proper matching of the crystal filters to insure good pass band response and selectivity. Second Mixer, Second IF,

15、 and FM Detector The output of Q3 is applied to the input (pin 16) of IC2. IC2 is a single conversion FM receiver on one integrated circuit chip. The signal at the input is routed straight to a mixer which converts the incoming signal to the second IF frequency of 455 kHz. The second local oscillato

16、r is formed with crystal X1 and circuitry within IC2. The output of the second mixer is at pin 23 which is connected to a ceramic bandpass filter CF1 and centered at 455 kHz. This filter, along with XF1 and XF2, determine the adjacent channel selectivity of the radio. The output of CF1 drives a high

17、 gain IF amplifier chain internal to IC2 which in turn drives the quadrature detector. The output of the detector is amplified and exits IC2 at pin 9. Audio Detected audio from IC2 passes through a lowpass filter formed around L7, C53 and C52 which removes IF frequency components at 455 kHz. The aud

18、io signal then passes through buffer amplifier transistor Q6 before being filtered by a two section, 4-pole high pass filter (IC3) This filter removes DCS and CTCSS low frequency tones from the recovered audio. Transistors Q7 and Q8 act as switches around volume control VR2 to mute the audio during

19、squelched receive operation. The audio signal is finally routed to audio power amplifier IC5 and then to the speaker. MONOGRAM SERIES LBI-38864B CIRCUIT ANALYSIS Page -11- Nov. 94 Squelch The presence of an RF carrier is determined by noting the level of ultrasonic noise at the detector output (pin

20、9) of IC2. When a carrier is present, the noise level drops. The audio at pin 9 of IC2 is filtered by a two-pole bandpass filter formed around L5, L6 C48 and C50. This filter passes at and near an audio frequency of 50 kHz. This frequency is high enough that voice audio and its harmonics will not ca

21、use improper squelch operation. The output of the filter is routed to an amplifier internal to IC2. The output of the amplifier drives the squelch detector D7 and D8. The DC voltage at the detector output is amplified and filtered by Q4. The output of Q4 sends its squelch signal to the microprocesso

22、r. When the microprocessor determines that a valid carrier exists, it sends an unmute signal to the audio switch transistors Q7 and Q8. TRANSMITTER Audio The microphone audio is amplified, pre-emphasized and peak limited by circuits within IC113. The output of the limiter is routed through RV4, the

23、microphone; deviation control. Input CTCSS and routed DCS signals are routed through RV5, the CTCSS/DCS deviation control. Both signals are summed and lowpass filtered to remove high frequency components from the limiter which could cause channel splatter. When the microprocessor enables the TX 8 Vo

24、lt supply, analog gate IC115 delivers the modulation signal to the VCO transistor Q112. RF Driver and Power Amplifier Diode D9 acts as a switch allowing the RF signal from the phase locked loop frequency synthesizer to pass through to the RF driver and power amplifier during receive. Buffer amplifie

25、r Q9 amplifies the carrier to the level needed by the driver amplifier stages. The driver amplifiers, of which the last 2 stages are gain controlled by the automatic power control, drive the final amplifier stage formed around Q12. The final amplifier boosts the carrier level to the power level set

26、by the automatic power control. The carrier signal passes through the automatic power control directional coupler, the RF output lowpass filter, and then is routed to the antenna connector. Automatic Power Control The automatic power control directional coupler samples a portion of the forward RF po

27、wer output to determine the RF level. Diode D10 rectifies this RF sample and produces a DC voltage which is proportional to the RF output level. This DC signal is summed with the voltage set from the power output control RV2. This voltage is compared with a voltage derived from the TX 8 Volt supply

28、and the difference is amplified by IC4. The output is further amplified by Q14 and Q13. The output voltage on Q13 is the supply voltage for the RF driver stage. The output of the RF driver is proportional to its supply voltage. This completes a negative feedback loop which results in constant output

29、 power over supply voltage and temperature variations. FREQUENCY SYNTHESIZER The phase locked loop (PLL) frequency synthesizer section is responsible for generating the RF signal at the carrier frequency during transmit and at the local oscillator frequency for the receiver during receive. A PLL fun

30、ctions by comparing the output frequency of a voltage controlled oscillator (VCO) with a fixed frequency reference. An error signal is generated which drives the control input of the voltage controlled oscillator to force its frequency to match the reference. The PLL based frequency synthesizer has

31、a digital frequency divider inserted between the output of the VCO and the frequency comparison circuitry. As this divider number is varied, the output frequency of the VCO varies as well with a frequency step size equal to the reference frequency (5 kHz in this radio). This allows a large range of

32、frequencies to be generated with one well controlled oscillator signal, the reference. PLL Integrated circuit IC108 contains most of the digital circuitry to form a PLL frequency synthesizer. This includes a reference oscillator, programmable variable frequency divider, a modulus control counter, a

33、phase/frequency comparator and a frequency lock dector. The operation of this integrated circuit is controlled by the radios microprocessor through a serial data line. MONOGRAM SERIES LBI-38864B CIRCUIT ANALYSIS Page -12- Nov. 94 Reference Oscillator Crystal X3, varactor diode D119, a thermistor/res

34、istor network and oscillator stage of IC108 from a temperature compensated 10.24 MHz oscillator. This frequency is divided by 2084 to generate the 5 kHz reference frequency for the PLL frequency synthesizer. This reference determines the frequency stability of the overall radio. Voltage Controlled O

35、scillator FET transistor Q112 and its associated circuitry form a grounded gate oscillator which is voltage tuned by varactor diodes D120 and D121 and which is bandswitched by diodes D122 and D123 (D126 and D127 for Low band). The VCO output is buffered and isolated by Q113, Q114 and Q119. Audio mod

36、ulation is applied to the source of Q112 to produce frequency modulation during transmit. Dual Modulus Prescaler The internal dividers within IC108 are not able to operate at the VCO output frequency. To alleviate this problem, part of the overall frequency division necessary between the VCO and the

37、 phase/frequency comparator is placed external to, and controlled by, IC108. IC109 divides the VCO frequency by 64 or 65, determined by the state of IC108 pin 6. This produces a lower frequency which can be further divided by IC108. By strategic timing when to divide by 64 or 65, the overall divisio

38、n will be that necessary to put the VCO on the correct frequency. Loop Filter Resistors R316, R323, R324, R325, R326 and capacitors C263, C264 and C265 form the loop filter. The purpose of the loop filter is to filter out the 5 kHz reference frequency products from the output of phase/frequency comp

39、arator IC108 and to determine the dynamic operation of the overall loop. R315, C262, Q107 and Q108 act to speed up operation of the synthesizer loop during channel changes and during frequency transition (receive to transmit and transmit to receive). Out-Of-Lock Detector IC108 contains a circuit whi

40、ch compares the timing difference of the 5 kHz reference frequency and the divided down VCO frequency. The output is a 5 kHz pulse whose duration is equal to the timing difference. R312 and C295 filter this pulse and average it producing a DC voltage which is proportional to the pulse width. When th

41、e loop is in lock, this voltage is zero, but when the loop is in lock, this voltage is zero, but when the loop is out of lock, it rises to a level which will forward bias Q106. The output of Q106 drives the microprocessor. The microprocessor will not allow the radio to transmit unless the synthesize

42、r is in lock. This is to prevent out of band signals from being transmitted. MONOGRAM SERIES LBI-38864B CIRCUIT ANALYSIS Page -13- Nov. 94 PREVENTIVE MAINTENANCE To ensure high operating efficiency and to prevent mechanical and electrical failures from interrupting system operations, routing checks

43、should be made of all mechanical and electrical parts at regular intervals. Preventive maintenance should include the following checks: CONNECTIONS Ground connections to the voltage source should be periodically checked for tightness. Loose or poor connections to the power source will cause excessiv

44、e voltage drops and faulty operation. When ground connections are not made directly to the battery, the connection from the battery to vehicle chassis must be checked for low impedance. A high impedance may cause excessive voltage drops and alternator noise problems. ELECTRICAL SYSTEM Check the volt

45、age regulator and alternator or generator periodically to keep the electrical system within safe and economical operation limits. Over voltage is indicated when the battery loses water rapidly. Usage of 1 or 2 ounces of water per cell per week is acceptable for batteries in continuous operation. A w

46、eak battery will often cause excessive noise or faulty operation. MECHANICAL INSPECTION Since mobile units are subject to constant shock and vibration, check for loose plugs, nuts, screws and other parts to make sure that nothing is working loose. ANTENNA The antenna, antenna base and all contacts s

47、hould be kept clean and free from corrosion. If the antenna or its base should become coated or poorly grounded, loss of radiation and a weak signal will result. ALIGNMENT The transmitter and receiver meter readings should be checked periodically, and the alignment touched up when necessary. Refer t

48、o the Alignment Procedure in this Service Manual. FREQUENCY CHECK Check the transmitter frequency and deviation. Normally, these checks are made when the unit is first put into operation, after the first six months, and once a year thereafter. MONOGRAM SERIES LBI-38864B PREVENTIVE MAINTENANCE Page -

49、14- Nov. 94 DISASSEMBLY TOP AND BOTTOM COVERS There are no screws used to secure the top and bottom covers. Both top and bottom covers are removed with a flat blade screwdriver or similar tool. On each side of the radio there are two small slots (one at the top and one at the bottom). Insert the screwdriver into the slot and gently pry the lip of the cover out from the radio. Without removing the screwdriver from the slot and in the same motion, pry the cover up. Both covers can be removed from either side of the radio. FRONT PANEL 1.Remove the (6) M3 x 6 machine screws (3 on t