MFJ MFJ-751B User Manual

MFJ-751B Owner's Manual

Tunable CW Audio Filter

INTRODUCTION

Thank you for purchasing the MFJ-751B Tunable CW Audio Filter. The MFJ-751B consists of a four stage, switch selectable band pass CW filter, using selected components that will make “cleaning up” CW signals effortless and easy. Featuring razor sharp selectivity with extremely steep sided skirts and a Tunable Notch, makes even the weakest signal stand out. Also, the MFJ-751B has a built-in 1 watt audio amplifier that will easily drive headphones or an external speaker. With the MFJ-751B you “bring up” any hard to hear signal out of a “band pile up” for easy listening, or get rid of unwanted, annoying signals. The MFJ-751B also features a headphone output that will allow the use of standard mono headphones. The MFJ-751B is powered from any 9-18 volt DC power supply.

OPERATING INSTRUCTIONS

You may use the MFJ-751B with any communications receiver or scanner. You can also use the MFJ-751B with a ham-radio transceiver. The MFJ751B requires a 9-18 volt DC source.

There are a few items that you will need to operate the MFJ-751B. We have provided a list of these items below for your convenience.

•Communications Receiver, scanner, or Ham Radio transceiver with proper cables.

•9-18 volt DC power supply

•External speaker with a 3.5mm mono plug

•Mono Headphones with 1/4" phone plug. (optional)

Receiver or Scanner Operation:

As mentioned in the above, you can use the MFJ-751B with a communications receiver or scanner. A BFO, or Beat Frequency Oscillator, or a Fine Tuning control will assist you in fine tuning the received signals.

1.The filter requires audio from the external speaker or headphones output of the receiver or scanner. Apply the audio signal to the INPUT jack on the back of the unit. The INPUT jack is a RCA phono jack, which requires an RCA phono plug. The center pin of the RCA phono is positive, while the outer shell is ground.

2.Connect the POSITIVE lead of the external speaker to the TIP of a 3.5mm mono plug. Connect the NEGATIVE lead of the external speaker

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MFJ-751B Owner's Manual

Tunable CW Audio Filter

to the SLEEVE of the 3.5mm mono plug. Plug the 3.5mm mono plug into the EXT SPEAKER jack on the back of the unit.

3.Connect the POSITIVE power supply lead to the CENTER conductor of a 2.1mm coaxial DC plug. Connect the NEGATIVE power supply lead to the outer SLEEVE of the 2.1mm coaxial DC plug. Before plugging in the power to the MFJ-751B, set POWER switch to the OFF position (out). If you choose to use a pair of headphones, then DO NOT plug the headphones into the filter until the POWER switch is set to the ON position (in).

4.Turn the receiver volume all the way down, then set the POWER switch to the ON position (in). Set NOTCH/PEAK switch to the NOTCH position (in). Now turn the receiver up slightly so you can hear the received signals on the external speaker or headphones. The signal you are listening to is the "filtered" signal. Using the tuning knob on the radio, tune in a signal you want to listen to. Then use the FREQUENCY and SELECTIVITY controls to clean up any unwanted or annoying interference.

The maximum filter settings are when the FREQUENCY and SELECTIVITY controls are set fully clockwise. This is the highest "Q" setting for the filter. Note that at maximum selectivity you may experience a "ringing" effect; this is normal. The most usable setting for the SELECTIVITY control is when it is about 10-15% less than the maximum setting.

The PEAK/NOTCH switch is a dual function switch. The IN position is NOTCH, while the OUT position is PEAK. How you use the MFJ-751B dictates what position this switch will be in. Placing the switch in the PEAK position allows you "peak" desired signals; bringing them up where they are more readable and out of the SSB noise. Using the filter with the switch in the NOTCH position allows you to actually notch or null out nearby signals and interference.

Use the FREQUENCY control to "peak up" desired signals while the NOTCH/PEAK switch is in the PEAK position. While the NOTCH/PEAK switch is in the NOTCH position, the FREQUENCY control can be used to "notch" or "null" unwanted interference.

You can also use a pair of mono headphones with a 1/4" mono plug attached. Connect your headphones to the HEADPHONES jack on the rear panel.

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