MFJ MFJ-752D User manual

INSTRUCTION MANUAL
CAUTION: Read All Instruction Before Operating Equipment
MFJ ENTERPRISES, INC.
P.O. BOX 494, MISSISSIPPI STATE, MS 39762, USA
925- 0037D- 752D- REV 1 COPYRIG HT 2010 MFJ ENTERPRISES, INC.
MFJ SIGNAL ENHANCER II
Model MFJ-752D

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MFJ-752D SIGNAL ENHANCER II
INTRODUCTION
The improved MFJ-752D SIGNAL ENHANCER II is comprised o two tunable
audio iltering systems designed to clarity and remove inter ering signals rom both
voice (SSB or AM) and code (CW or RTTY) signals. This is accomplished by a primary
ilter which is a two-section, our-pole variable ilter which has PEAK, NOTCH, LOW
PASS, and HIGH PASS unctions, and by an auxiliary ilter which is a single section two
pole PEAK or NOTCH ilter. These two independent ilters allow you to remove
inter erence while enhancing the desired signal.
INSTALLATION
1. Connect any 9-18V DC supply to the power jack on the back o the MFJ-752D. 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 rom
MFJ. A 12 VDC, 300mA supply is required to obtain the ull two watts audio output.
2. Using the proper connector, connect a shielded cable rom the external speaker or
headphone jack o the transceiver or receiver to the MFJ-752D. Use an RCA phono
plug to connect to the ilter input. There are two inputs on the MFJ-752D allowing
connection to two receivers or greater convenience. The input is selected by the
push button switch on the back o the MFJ-752D.
3. Connect a speaker to the ilter using shielded wire with a RCA phono plug at the ilter
speaker output. For best results, use a 4 to 16 ohm speaker.
4. Headphone operation is accomplished by plugging headphones into the phone jack
on the back o the MFJ-752D. A mono plug or a stereo plug will work or headphone
operation. Plugging in headphones will not mute the speaker jack.
FRONT PANEL CONTROLS
POWER SWITCH:
Selects power OFF, power ON, SSB Noise Limiter and CW Noise
Limiter. The ilter is bypassed on the OFF position. NOTE: The
noise limiter in not ON when the power switch is in the ON position.
PRIMARY FILTER:
Function Selector: Selects PEAK, NOTCH, LOW PASS (LP), or HIGH PASS (HP) in
the primary ilter.
Selectivity Control: Adjusts the bandwidth o the primary ilter rom 40 Hz to almost lat.
Frequency Control: Adjusts the center requency o the peak, notch and the cuto
requency o the LOW PASS and HIGH PASS circuits
approximately 300 Hz to 3000 Hz.

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AUXILIARY FILTER:
Function Switch: To select the PEAK unction push button to the IN position. To
select the NOTCH unction push button to the OUT position.
Frequency Control: Adjusts the center requency o the auxiliary peak and notch
circuits between approximately 300 Hz to 3000 Hz.
Selectivity Control: Adjusts the bandwidth o the auxiliary circuits rom 40 Hz to almost
lat. To select the PEAK unction push IN this control. To select
the NOTCH unction 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 or maximum response to the
desired signal. Selectivity can be increased by turning the SELECTIVITY control
clockwise. FREQUENCY and SELECTIVITY 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 use ul on wider bandwidths such as RTTY and SSB.
Adjust SELECTIVITY until the desired signal just ills the passband and inter ering
signals are reduced.
NOTCH:
The depth o the NOTCH ilter is approximately 70 dB. Notch requency is
adjustable rom approximately 300 Hz to 3000 Hz. The primary notch ilter 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 o the notch. Readjust FREQUENCY control i 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 requency is hard to
locate. I the SELECTIVITY control is rotated past the 3 o'clock position
clockwise, the NOTCH depth will decrease. NOTCH mode is very help ul in
removing heterodynes, stations tuning up on your QSO, or CW signals on or near
your requency. The NOTCH can be used very e ectively on AM when two
stations are heterodyning one other. The heterodyning can be reduced by
notching out the inter ering tone.
LOW PASS:
LOW PASS cuto is adjustable rom approximately 300 Hz to 3000 Hz with the
FREQUENCY control. The SELECTIVITY control adjusts the steepness o the
cuto . LOW PASS mode is use ul in reducing annoying high requency static,
hiss and splatter. When selectivity is increased beyond the 2 o'clock position, a

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peak will orm at the requency set on the FREQUENCY control. The more the
selectivity is increased, the more pronounced the peak will become. This peak
may be pre erred or CW operation when extra audio is needed and low requency
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 over driving the ilter. Gain
on requencies below the set requency will drop as selectivity is increased.
There ore, when using the LOW PASS mode or SSB, the SELECTIVITY control
should not be set beyond the 2 o'clock position.
HIGH PASS:
HIGH PASS mode is set by turning the Function Selector to HIGH PASS (HP). To
remove unwanted low audio requencies, set the SELECTIVITY and
FREQUENCY controls or the desired response.
AUXILIARY FILTER OPERATION
PEAK:
To set the auxiliary ilter in the peak mode, simply push the FUNCTION button to
the IN position. Set the SELECTIVITY control to about center range and adjust
the FREQUENCY control to the desired signal. Adjust SELECTIVITY and
FREQUENCY controls alternately or the best signal and the desired bandwidth.
In the peak mode, the auxiliary ilter can be used to urther enhance the primary
ilter operation. It can be used to obtain very narrow bandwidths with good skirt
selectivity and little ringing when the primary ilter is in the peak mode, or to limit
the bandwidth when the primary ilter is in the NOTCH, LOW PASS, or HIGH
PASS mode. NOTE: When using both ilters in the PEAK mode with narrow
bandwidths, the best skirt selectivity and least attenuation will be obtained by
setting both the auxiliary and primary ilter requency controls to the same center
requency. This can easily be done by irst peaking the desired signal with the
auxiliary FREQUENCY control and then peaking it with the FREQUENCY control
on the main ilter.
NOTCH:
NOTCH mode on the auxiliary ilter is set by pushing the FUNCTION button to the
OUT position with the primary ilter in any mode. Set the SELECTIVITY control
about center range and adjust the FREQUENCY control to remove the unwanted
signal. SELECTIVITY and FREQUENCY should be adjusted alternately.
Remember at high selectivity on NOTCH, the NOTCH requency is di icult to
locate and the FREQUENCY control must be tuned very care ully. The auxiliary
notch ilter is used to remove an unwanted signal in the passband o the primary
ilter. It can remove a heterodyne rom SSB or AM signal when the primary ilter is
set to PEAK, LOW PASS, or HIGH PASS mode. It can also be used to notch out
an adjacent CW signal which is too close to be removed by peaking the ilters.

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GENERAL OPERATION
NOISE LIMITING CIRCUITS:
The SSB noise limiter is a peak clipper circuit which ollows the average peak
audio level but clips peaks above the average level. The input signal must be at
least 0.7 volt peak or proper noise limiting.
The CW noise limit is a trough limiter which removes the low level background
noise between the signals. Experimentally, set the audio or RF gain so that the
signal is passed but the noise is blocked. I the audio level into the ilter is set too
low the signals will not get through. I the gain is set too high the noise will also
get through. Setting the audio too high could overload the ilter and distort the
desired signals.
NOTE:When the noise limiter is used (SSB or CW) the audio level will decrease due to
the clipping action o circuits removing the noise.
IMPORTANT NOTES
1. The auxiliary ilter is in series with the primary ilter. I the auxiliary ilter is not
needed, set it to PEAK and turn the SELECTIVITY and FREQUENCY controls to
minimum (counter- clockwise).
2. With high selectivity, the ilter may have a tendency to ring in peak and lowpass
modes. I this occurs, decrease the selectivity.
3. Due to the very high internal ilter gains in the LOW PASS mode with maximum
selectivity and requency, the ilter may OSCILLATE. I this occurs reduce
SELECTIVITY and FREQUENCY controls until it stops.
4. Strong signals can mask a weak signal by causing your receiver AGC (automatic gain
control) to reduce gain. This may cause a weak signal to almost disappear. To
minimize this problem the AGC might be turned o and the RF gain reduced.
Increase audio gain to compensate or the loss o audio.
ALIGNMENT
The MFJ-752D primary ilter is a two-stage ilter which required proper aligned at the
actory. I or any reason a realignment is necessary use the procedure below. UNDER
NO OTHER CIRCUMSTANCES SHOULD THE INTERNAL TRIMMERS (R319 and
R5320) BE ADJUSTED. The ilter will not unction properly once either trimmer is
moved without proper realignment.
An audio signal generator and an OSCILLOSCOPE or an AC VTVM are needed or the
alignment.

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CONTROL SETTINGS:
AUXILIARY FREQUENCY CONTROL: Minimum
AUXILIARY SELECTIVITY CONTROL: Minimum
AUXILIARY FUNCTION SWITCH: In or Peak
POWER: On
PRIMARY FUNCTION: Notch
PRIMARY SELECTIVITY: 3 o'clock
PRIMARY FREQUENCY: Maximum
ALIGNMENT PROCEDURE:
1. Connect scope input to the speaker output or phone output o the ilter.
2. Connect signal generator to input 1 or 2 o the ilter. Set the input switch on the ilter
accordingly.
3. Set signal generator or sine wave o 1V p-p and adjust generator requency
control(s) or a null (minimum amplitude). Null should occur at about 3000 Hz.
4. Switch Primary Function Selector to PEAK.
5. Increase Primary SELECTIVITY control to maximum.
6. Adjust R319 (10K) High Tracking trimmer or maximum amplitude.
7. Set generator requency to 500 Hz.
8. Set primary SELECTIVITY control to 3 o'clock.
9. Set primary ilter to NOTCH and adjust primary FREQUENCY control or a null.
10. Increase primary SELECTIVITY to maximum.
11. Switch primary ilter to PEAK and adjust R320 (500K) Low Tracking trimmer or
maximum amplitude. Reduce signal generator output level i clipping occurs.
12. End o the alignment procedure.

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MF -752D Schematic Diagram

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FULL 12-MONTH WARRANTY
MFJ Enterprises, Inc. warrants to the original owner of this product, if manufactured by MFJ Enterprises,
Inc. and purchased from an authorized dealer or directly from MFJ Enterprises, Inc. to be free from
defects in material and workmanship for a period of 1 months from date of purchase provided the
following terms of this warranty are satisfied.
1. The purchaser must retain the dated proof-of-purchase (bill of sale, canceled check,
credit card or money order receipt, etc.) describing the product to establish the validity of the
warranty claim and submit the original or machine reproduction of such proof of purchase to MFJ
Enterprises, Inc. at the time of warranty service. MFJ Enterprises, Inc. shall have the discretion to
deny warranty without dated proof-of-purchase. Any evidence of alteration, erasure, of forgery
shall be cause to void any and all warranty terms immediately.
2. MFJ Enterprises, Inc. agrees to repair or replace at MFJ's option without charge to the
original owner any defective product provided the product is returned postage prepaid to MFJ
Enterprises, Inc. with a personal check, cashiers check, or money order for $12.00 covering
postage and handling.
3. MFJ Enterprises, Inc. will supply replacement parts free of charge for any MFJ product
under warranty upon request. A dated proof of purchase and a $8.00 personal check, cashiers
check, or money order must be provided to cover postage and handling.
4. This warranty is NOT void for owners who attempt to repair defective units. Technical
consultation is available by calling (66 ) 3 3-5869.
5. This warranty does not apply to kits sold by or manufactured by MFJ Enterprises, Inc.
6. Wired and tested PC board products are covered by this warranty provided only the
wired and tested PC board prod ct is ret rned. Wired and tested PC boards installed in the
owner's cabinet or connected to switches, jacks, or cables, etc. sent to MFJ Enterprises, Inc. will
be returned at the owner's expense un-repaired.
7. Under no circumstances is MFJ Enterprises, Inc. liable for consequential damages to
person or property by the use of any MFJ products.
8. O t-of-Warranty Service: MFJ Enterprises, Inc. will repair any out-of-warranty
product provided the unit is shipped prepaid. All repaired units will be shipped COD to the
owner. Repair charges will be added to the COD fee unless other arrangements are made.
9. This warranty is given in lieu of any other warranty expressed or implied.
10. MFJ Enterprises, Inc. reserves the right to make changes or improvements in design or
manufacture without incurring any obligation to install such changes upon any of the products
previously manufactured.
11. All MFJ products to be serviced in-warranty or out-of-warranty should be addressed to
MFJ Enterprises, Inc., 300 Ind strial Park Rd, Starkville, Mississippi 39759, USA and must
be accompanied by a letter describing the problem in detail along with a copy of your dated proof-
of-purchase and a telephone number.
12. This warranty gives you specific rights, and you may also have other rights, which vary
from state to state.
Rev 04/08/2011