MFJ MFJ-752C User Manual

MFJ-752C SIGNAL ENHANCER II

INTRODUCTION

The improved MFJ-752C SIGNAL ENHANCER II is comprised of two tunable audio
filtering systems designed to clarity and remove inter fering signals from both voice ( SSB or AM) a nd
code (CW or RTTY) signals. This i s accomplished by a primary filter which is a two-section, fou r­pole variable filter which has PEAK, NOTCH, LOW PASS, and HIGH PASS functions, and by an
auxiliary filter which is a single section two pole PEAK or NOTCH filter. These two independent filters
allow you to remove interference while enhancing the desired signal.

INSTALLATION

1.

Connect any 9-18V DC supply to the power jack on the back of the MFJ-752C. The adapter

must have a 2.1 mm coaxial plug with the center pin position and the sleeve ground. An
optional AC adapter, MFJ-1312B, 9-18VDC adapter is available from MFJ. A 12 VDC, 300mA
supply is required to obtain the full two watts audio output.

2. Using the proper connector, connect a shielded cable from the external speaker or headphone

jack of the transceiver or receiver to the MFJ-752C. Use an RCA phono plug to connect to the
filter input. There are two inputs on the MFJ-752C allowing connection to two receivers for
greater convenience. The input is selected by the push button switch on the back of the MFJ­752C.

3. Connect a speaker to the filter using shielded wire with a RCA phono plug at the filter speaker

output. Fo r be st results, us e a 4 to 16 ohm speaker.

4. Headphone operation is accomplished by plugging headphones into the phone jack on the back of t he

MFJ-752C. A mono plug or a stereo plug will work for headphone operation. With stereo
headphones, only one ear of the phones will work.

FRONT PANEL CONTROLS

POWER SWITCH:
Selects power OFF, power ON,
the OFF position. NOTE: The noise limiter in not ON when the power switch is in the ON position.

PRIMARY FILTER:
Function Selector: Sel ects PEA K, NOTC H, LOW PA SS (LP), or HIGH PASS ( HP) in the

primary filter.

Selectivity Control: Adjusts the bandwidth of the primary filter from 40 Hz to almost flat. Frequency

Control: Adjusts the center frequency of the peak, notch and the cutoff frequency of the
LOW PASS and HIGH PASS circuits approximately 300 Hz to 3000 Hz.

SSB

Noise Limiter and CW Noise Limiter. The filter is bypassed on

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AUXILIARY FILTER:

Function Switch: To select the PEAK function push button to the IN position. To select the

NOTCH function push button to the OUT position.

Frequency Control: Adjusts the center frequency of the auxiliary peak and notch circuits between

approximately 300 Hz to 3000 Hz.

Selectivity Control: Adjusts the bandwidth of the auxiliary circuits from 40 Hz to almost flat. To select

the PEAK function push IN this control. To select the NOTCH function push out
this control.

PRIMARY FILTER OPERATION

PEAK:
Turn Function Selector to PEAK. Set the SELECTIVITY control to about the 10 o'clock position.
Adjust FREQUENCY control for maximum response to the desired signal. Selectivity can be increased
by turni n g the S ELECTIVITY contro l c lock wise . FREQUENCY and SELE CTIVI TY control should be
adjusted alternately to obtain maximum selectivity and best signal intelligibility. The PEAK mode can be
used on CW to narrow in on a signal and greatly reduce unwanted signals. Peak mode is also useful on
wider bandwidths such as RTTY and SSB. Adjust SELECTIVITY until the desired signal just fills the
passband and interfering signals are reduced.

NOTCH:
The depth of the NOTCH filter is approximately 70 dB. Notch frequency is adjustable from
approximately 300 Hz to 3000 Hz. The primary notch filter can be set to the NOTCH mode by setting
the Function Selector to NOTCH. Set SELECTIVITY control to the 9 o'clock position. Adjust
FREQUENCY control to remove the CW signal or heterodyne. Increase SELECTIVITY (clockwise)
to narrow the bandwidth of the notch. Readjust FREQUENCY control if necessary. The unwanted
heterodyne should become weaker and the desired signal become clearer as notch selectivity is
increased. At high selectivity on NOTCH, the FREQUENCY control tuning is very critical and
the notch frequency is hard to locate. If the SELECTIVITY control is rotated past the 3 o'clock
position clockwise, the NOTCH depth will decrease. NOTCH mode is very helpful in removing
heterodynes, stations tuning up on your QSO, or CW signals on or near your frequency. The NOTCH
can be used very effectively on AM when two stations are heterodyning one other. The heterodyning can
be reduced by notching out the interfering tone.

LOW PASS:
LOW PASS cutoff is adjustable from approximately 300 Hz to 3000 Hz with the FREQUENCY
control. The SELECTIVITY control adjusts the steepness of the cutoff. LOW PA SS mode is useful in
reducing annoying high frequency static, hiss and splatter. When selectivity is increased beyond the 2
o'clock position, a peak will form at the frequency set on the FREQUENCY control. The more the
selectivity is increased, the more pronounced the peak will become. This peak may be preferred for CW
operation when extra audio is needed and low frequency signals are not a problem. Because circuit gain
rises in the LOW PASS mode, the input audio level may need to be reduced to prevent overdriving the
filter. Gain on frequencies below the set frequency will drop as selectivity is increased. Therefore, when
using the LOW PASS mode for SSB, the SELECTIVITY control should not be set beyond the 2
o'clock position.

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