Midland / Alan 87

Funzionamento del circuito:

PLL Circuitry. PLL is an abbreviation of the phase-locked loop which is fundamentary composed of a closed loop feedback citcuit. The feedback component is the balance of frequency drift and the PLL circuit acts to cancel it out. To detect out the frcquency drift of the PLL output, a fixed reference ocillator (10 kHz, 1/1024 divided down from 10.240 MHz) is compared constantly with the input frequency (10 kHz). The input frequency is obtained by dividing the VCO frequency. A functionalI block diagram is provided below under "PLL Circuit" for the easier understanding.

Off-Set frequency oscillator IC10. The off-set-frequency oscillator IC10 oscillates at 14.460 MHz for all band. Switching between these oscillating frequencies is made by biasing the diodes D34 to D36.
The off-set frequency signal is obtained at IC10 Oscillator and flows into IC10 Mixer where it is beat with the VCO signal. The VCO signal is obtained from the following:
f _vco = f_off-set + N_fr
Where, f _vco = VCO frequency, N = programming code for divider output, and fr = referency frequency step (10kHz).

i.e.: At channel #1 in band A, and band corresponding N code is 91:
f _vco = 14.010 + 91 x 0.01 = 14.920MHz.

Since the mixer output determined by two factors the off-set frequency output (dependent on band selector switch) and the VCO output, the mixer output contains the subtracted frequency of 0.91 to 2.25 MHz. These frequencies appears in pin #2 of IC5, and divided by the programmable divider in IC5 down to 10 kHz which is compared with another 10kHz signal obtained from the reference oscillator TR24 (10.240MHz).
The VCO output is mixed with the off-set frequency signal and applied to the TX mixer IC9 through band-pass filters.
i.e. At channel 1 in band A, and AM band, the TX mixer IC9 accepts 14.920 + 10.695MHz = 25.615MHz is TX frequency.
When receiving channe 1 in band A (26.065MHz), the 1st RX mixer TR18 accepts 15.370 local ignal at its base, and converts down it down to 10.695MHz IF (for AM/FM mode, this is the 1st intermediate frequency). TR18 off-sets the 10.695MHz signal so that TR30 can oscillate at 10.6975MHz for LSB.

Taratura

Strumenti necessari per la taratura:

Predisposizione iniziale dei comandi:

Collegamenti del microfono:

 Lista delle frequenze dei canali

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Caratteristiche tecniche dei componenti:

MC145106P PLL Frequency Synthesizer
S042P TX Mixer
KIA6410S Oscillator, Mixer and Amplifier
uPC1028H FM IF Amplifier and Detector
AN612 Balanced SSB Modulator
TA7222 Audio Power Amplifier
NJM4558 Dual Operational Amplifier
TA6324 Quad Operational Amplifier
2SC1969 RF Finale Transistor (SuperStar 3900)
2SC2312 RF Finale Transistor (Alan 87)
2SC2166 RF Driver Transistor
2SC2086 RF Pre-Driver Transistor
2SB754 LF Power Transistor
2SA473 LF Power Transistor

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Modifiche Alan 87:

Activeren bandenschakelaar:

Schroef de bak open. Leg de bak met de knoppen naar u toe, met de soldeerzijde baar boven.
Verwijder de oranje draad tegen het voorfront.
Leg toestel op de soldeerzijde, met de knoppen naar u toe. U ziet dat op de bandenschakelaar een paarse draad is doorgeknipt, de losse paarse draad hangt in de draadboom. Herstel deze draad, ofwel soldeer de draad opnieuw op de bandschakelaar.

Modificatie voor 10 kc: Draai de bak om met de soldeerzijde naar boven, met de knopen naar u toe. Herstel breuk in printbaan. U kunt dit doen door of de print op deze plek te solderen, of de oranje draad die over was te gebruiken op beide  eindpunten van dit stukje printbaan. kies wat voor u het makkelijkste is. Meest rechtse schakelaar zonder tekst wordt nu 10kc schakelaar. Schakelaar omhoog is standaard.

Voor meer vermogen:

VR 13 voor AM. advies: maximaal 15 Watt
VR 12 voor SSB. advies: maximaal 25 Watt

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Come migliorare il guadagno in ricezione:

Quieting of AM reception and improving gain of incoming signals is a common request from radio operators. In the first stages of the HF input 2SC1674 transistor can be found. This transistor is responsible for the amplification of a small detected signals. A problems exist if the transistor itself is noisy as is such the case of the 2SC1674 when compared to other low noise packages. Along with the amplification of the incoming signals is transistor noise. Replacement of this transistor with a higher gain, lower noise transistor greatly improves the signal to noise ratio of your receiver.
We will use an 2SC2999 transistor that has higher gain lower noise characteristic. Replace the 2SC1674 (TR17) with an 2SC2999 (or similar low noise and high gain transistor) to achieve this improved signal to noise ratio.
Re-Adjust L6 and L7
The gain will improved with more than 6dB with the same signal to noise ratio.

Ultimo aggiornamento: giovedě, 01 maggio 2008 12.25 +0200

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