Drake Mods - SPR Mods

R.L. Drake

Drake Mods - SPR Mods
Authored by VE3EFJ

10.1 SPR-4 MODS AND TECH

Sensitivity
Measure the input resistance with an ohm meter at the antenna termi- nals on any band but 'A' or 'B'. It should be about 2 ohms. If it is not, likely the ground pin on the input matching coil has become unsoldered. This will be hard to get to and you'll be required to unscrew the slide switch on the back panel immediately above it. For some reason this solder connection seems to fatigue over the years; perhaps the area is stressed during assembly. On 40 meters and above an SPR-4 will still 'hear' a lot of signals with the input link coil ungrounded. All thats coupling the antenna in this case is just stray capacitance and is surprisingly adequate. If you have this problem, you'll notice 80 and 160 not to be too perky, yet the receiver aligns OK.
IF alignment
The 50 kHz IF transformers tune broadly.
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.
AGC
The AGC in the SPR-4 is somewhat unique. The AGC controls on the back of the S Meter affect its strong signal performance. The AGC line is very high impedance. You cannot measure it successfully without dragging it down.
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.
AGC CW Time Constant
Later model SPR4 receivers allowed for an owner to use a faster time constant on CW than the default SSB one. On about the middle of the function switch gang you will see two pin connectors at the top with the radio on its back. If you short the two pins, the AGC decay time is extremely fast. There are options other than shorted or open, however.
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.
Alignment Tricks
Drake wants you to measure the AGC bus during alignment. A volt meter will drag the bus down. Don't bother - there already is a volt meter there - the S meter. Use it instead during alignment, but keep the input signal level to the S3 to 5 level.

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