TECHNICAL MANUAL FOR COMPACT VHF DSC RM2042

3 SERVICE RM20423.6.3 ADJUSTMENT OF RECEIVER (MODULE 3)ADJUSTMENT OF FIRST LOCAL OSCILLATORThe 1st. local oscillator is adjusted by the coil L6-3.1. Connect a frequency counter by means of a passive probe to the tap of the two capacitors C89-3 and C90-3.2. Adjust the coil L6-3 by a plastic or ceramic stick until the frequency is 141.225 MHz ±100 Hz.ADJUSTMENT OF SECOND LOCAL OSCILLATORThe 2nd. local oscillator is partly build into the integreted IF circuit and is adjusted by the external trimmercapacitor C34-3. The oscillator is adjusted by measuring the frequency of the down converted signal atthe 2nd. IF = 450 kHz.1. Apply an unmodulated carrier with the frequncy 156.525 MHz to the antenna terminal and adjustthe output level of the RF-signal generator to 0 dBm (EMF: 113 dB/µV).2. Connect a frequency counter by means of a passive probe to the limiter output (pin 7) at U1-3.3. Adjust C34-3 until the frequency is 450 kHz ±25 Hz.NOTE! Be careful not to press the trimming capacitor, while the adjustment is performed.The trimming capacitor is constructed with a ceramic plate and is therefore easilydamage by pressure.ADJUSTMENT OF FRONT-END FILTERS AND MIXER TRANSFORMERSThe front-end filters contains four adjustable coils and the mixer includes two adjustable transformers.All components are adjusted to maximum meter deflection at the field stregth meter output and the mixeroutput transformer TR2-3 is afterwards adjusted to minimum distortion.1. Apply an unmodulated carrier with the frequncy 156.525 MHz to the antenna terminal and adjustthe output level to -80 dBm (EMF: 33 dB/µV).2. Connect a multimeter to the field strength meter output, pin 13 at U1, for measuring the DC-voltage.3. Adjust the coils L1-3, L2-3, L3-3, L4-3 and the two transformers TR1-3 and TR2-3 to maximummeter deflection.4. Connect a distortion meter to the output of the deemphasis filter, pin 8 at U3.3-3.5. Modulate the RF-carrier with a 1 kHz tone to a peak diviation of 3 kHz. Increase the RF-carrier levelto -30 dBm (EMF: 83 dB/µV).6. Adjust the output transformer TR2-3 to minimum distortion and control that the distortion is lessthan 3%.ADJUSTMENT OF AF OUTPUT LEVELThe AF output level from the detector is adjusted to match the RX AF input level from the connected VHFtranceiver.1. Connect a RF-signal generator to the antenna input and adjust the carrier level to -50 dBm (EMF:63 dB/µV). Modulate the carrier with 1 kHz to a peak frequency diviation of 3 kHz.2. Connect a multimeter to the analog switch output, pin 14 at U6-3 for measuring the AC-voltage.3. Adjust the trimpot R66-3 until the output level is 260 mV RMS±5mV.PAGE 3-89403