Authored by VE3EFJ
10.1 SPR-4 MODS AND TECH
The 50 kHz bandpass network coils accessible from the rear chassis panel tune sharply and are critical for proper sensitivity. Align with the bottom plate in place. It isn't easy, but this is the only way to get a good alignment. Do not disturb any of the wiring around this area of the receiver.
The board behind the S meter contains the delayed AGC for the RF amplifier and the S Meter driver logic. If you have problems setting up the AGC in exact accordance with the instructions, replace Q14, the AGC FET amp. The AGC bus goes right to the gate of Q14 - any leakage at all will ruin the AGC. An MPF102 will work fine.
The AGC in the SPR4 reduces the IF amplifier gain by setting gate 2 of the IF amplifiers from slightly positive, through zero and then negative. On extremely strong signals, gate 1 of the RF amplifier is driven positive from the S Meter circuitry. This positive voltage in gate 1 causes the RF amplifier to draw more current and increase the voltage drop across the source resistor, cutting the device off. Its a technique called 'forward AGC' and aids in reducing overloading on strong signals. The noise blanker in the TR7 uses a similar technique.
The AGC S Meter transistor is an FET. An MPF102 will work nicely, providing you device select. Essentually you need to select an FET that provides a minimum of 2.25 volts across a 4.7 K source resistor. The following IC that drives the S Meter (and eventually the RF amp AGC) is a CA3053. You can readily substitute a CA3028, A or B into this circuit.
Nominal AGC voltage to the gate of Q14 is around .6 volt. If it is higher than this, the receiver will appear to be 'hotter', but it will overload on strong signals. Most of the gain control AGC in the receiver comes from the RF amplifier past a certain threshold.
Across the AGC time constant pins install a 1 Meg 1/4 watt resistor. This seems about right, but you may want to play a bit by sweeping the receiver in the CW position across the crystal calibrator. Check the decay time by watching the S meter.
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