Mfj Deluxe Rf Preselector Ii

An unusual combination of accessories in one cabinet has been released by MFJ, a company noted for their large catalog of accouterments for radio communications.

The new 1040 Deluxe Rf Preselector IF houses both a flexibly receiver preselector and an adjustable-delay relay to protect the delicate circuitry when used with a transceiver. Very thoughtful and very effective.

Designed with applications agility in mind, the 1040 features at least 20 dB of preamplifier tion. More important, the 1040 doesn't introduce considerable circuit noise as do some other rf preamplifiers.


Q is sharp, awarding the listener with good out-of-band rejection of unwanted signals. Gain may be continuously varied by a potentiometer or attenuated by 20 dB in one increment by pressing a button.

Circuit protection is automatic; when transmitted rf is detected by a high-gain sampling circuit, the antenna relay is automatically switched, bypassing the preamp A frontpanel pot may be adjusted to lunethe receive delay—a sort of VOX—to prevent constant relay dropout during SSB excursions. Delay may be varied from 0 to about 2 seconds.

The dc amplifier used to control the relay consists of two di--ectcoupled bipolar transistors; sampling from the antenna circuit is through a 12-pF capacitor and rectified by two 1N34 diodes.

The preamplifier circuit is built around a 40673 dual-gate MOSFET Preselection is accomplished in lour bands (1-54 MHz), switching among four standard rt chokes, each of which becomes part of a tuned tank when connected across the 320-pF main tuning capacitor

Rear-apron connectors in a row (SO-239s and phono jacks in parallel) allow selection between two antennas and two receivers by front-panel pushbuttons.

When the 1040 is switched on, a red LEO signals the status. When switched off, the antenna circuit automatically bypasses the electronics so that the rig feeds the antenna straight through.

The preselector may be powered by an external source of 9 to 1fi volts dc or by its companion ac adapter.

Our Test

We were impressed at the small size and large flexibility of the MFJ-1040. Two tests were performed, one with a general coverage receiver, the other in a full transceive mode.

First, the 1040 was connected to the antenna input of a Kenwood FM0GG receiver; a 135-foot dipole was connected to the input of the preselector

With the preselector stilt switched off, signals came into the receiver business as usual But with the preamplified preselector switched on and adjusted to frequency, dead bands came alive! Keep in mind that 20-dB gain is equivalent to more than 3 S-units, and that amount of signal brought up out of the noise can be heard!

Tuning the receiver through-oui the range of the 1040 (up to 30 MHz —we did not try it through 54 MHz although there was no question that it would work there), we determined that its gain was high and its selectivity was sharp. Out-of band images and intermod were re^ duced considerably and in many cases eradicated. The continuous adjustability ot the gain stage made custom preamplifi-cat+on a snap.

The contacts on the antenna-selecting push-buttons showed good isolation; high level signals were barely detectable wnen the receiver switch was pushed to an alternate output.

A rear-apron jack provides the ability to remote-control the unit from a pair of shorting (or grounding) contacts in the transceiver.

Next, the 1040 was connected to the station rig, a Triton IL Since the preselector is rated to withstand 350 Watts PEP, we weren't concerned that we might cause injury pumping 150 Waits or so into the unit.

Trying the combination first on ten meters, we were astounded at the increase in signal levels without a corresponding increase in background noise, Double-checking this performance, we cranked the Triton s gain control down to just below signal threshold; switching the 1040 on, signals came in loud and clear!

We repeated the exercise on all five bands with equal success»

Next, we decided to try to burn the unit out with rf! During transmit, the relay responded instantly Releasing the mike button. we heard the reassuring sound ot the relay drop back to receive, accompanied by signals once again.

Transmitting again, we varied the settings of the VOX delay control as we spoke into the microphone and then released the button. Delay times were adjustable from 0 to roughly 2 seconds. which would accommodate any amateur mode: CW, AM. FM. SSB, RTTY. and even SSTV!

For older rigs of questionable sensitivity, or even when used with modern rrgs where threshold signals must be improved, the MFJ 1040 is hard to beat Its preselection shoutd improve the performance of virtually any receiver or transceiver operating within its design range. For further information on this S99.95 product, contact MFJ Enterprises, PO Box 494r Mississippi Slate MS 39762. Reader Service number 476.

Robert B. Grove WA4PYQ Brasstown NC

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