MFJ MFJ-752D Instruction Manual Download Page 5

 

GENERAL OPERATION 

 

NOISE LIMITING CIRCUITS: 

The  SSB  noise  limiter  is  a  peak  clipper  circuit  which  follows  the  average  peak 
audio level but clips peaks above the average level.  The input signal must be at 
least 0.7 volt peak for 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.  If the audio level into the filter is set too 
low the signals will not get through.  If the gain is set too high the noise will also 
get  through.    Setting  the  audio  too  high  could  overload  the  filter  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 of circuits removing the noise. 

 

IMPORTANT NOTES 

 

1.

  The  auxiliary  filter  is  in  series  with  the  primary  filter.    If  the  auxiliary  filter  is  not 

needed,  set  it  to  PEAK  and  turn  the  SELECTIVITY  and  FREQUENCY  controls  to 
minimum (counter- clockwise). 

 

2.

  With  high  selectivity,  the  filter  may  have  a  tendency  to  ring  in  peak  and  lowpass 

modes.  If this occurs, decrease the selectivity. 

 

3.

  Due  to  the  very  high  internal  filter  gains  in  the  LOW  PASS  mode  with  maximum 

selectivity  and  frequency,  the  filter  may  OSCILLATE.    If  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  off  and  the  RF  gain  reduced.  
Increase audio gain to compensate for the loss of audio. 

 
 

ALIGNMENT 

 

The  MFJ-752D  primary  filter  is  a  two-stage  filter  which  required  proper  aligned  at  the 
factory.  If for any reason a realignment is necessary use the procedure below.  

UNDER 

NO  OTHER  CIRCUMSTANCES  SHOULD  THE  INTERNAL  TRIMMERS  (R319  and 
R5320)  BE  ADJUSTED

.    The  filter  will  not  function  properly  once  either  trimmer  is 

moved without proper realignment. 
 
An audio signal generator and an OSCILLOSCOPE or an AC VTVM are needed for the 
alignment. 
 
 

Summary of Contents for MFJ-752D

Page 1: ...UAL CAUTION Read All Instruction Before Operating Equipment MFJ ENTERPRISES INC P O BOX 494 MISSISSIPPI STATE MS 39762 USA 925 0037D 752D REV 1 COPYRIGHT 2010 MFJ ENTERPRISES INC MFJ SIGNAL ENHANCER II Model MFJ 752D ...

Page 2: ...ceiver or receiver to the MFJ 752D Use an RCA phono plug to connect to the filter input There are two inputs on the MFJ 752D allowing connection to two receivers for greater convenience The input is selected by the push button switch on the back of the MFJ 752D 3 Connect a speaker to the filter using shielded wire with a RCA phono plug at the filter speaker output For best results use a 4 to 16 oh...

Page 3: ...he 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 FREQUENC...

Page 4: ...eration It can be used to obtain very narrow bandwidths with good skirt selectivity and little ringing when the primary filter is in the peak mode or to limit the bandwidth when the primary filter is in the NOTCH LOW PASS or HIGH PASS mode NOTE When using both filters in the PEAK mode with narrow bandwidths the best skirt selectivity and least attenuation will be obtained by setting both the auxil...

Page 5: ... and turn the SELECTIVITY and FREQUENCY controls to minimum counter clockwise 2 With high selectivity the filter may have a tendency to ring in peak and lowpass modes If this occurs decrease the selectivity 3 Due to the very high internal filter gains in the LOW PASS mode with maximum selectivity and frequency the filter may OSCILLATE If this occurs reduce SELECTIVITY and FREQUENCY controls until ...

Page 6: ... 1V p p and adjust generator frequency control s for 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 for maximum amplitude 7 Set generator frequency to 500 Hz 8 Set primary SELECTIVITY control to 3 o clock 9 Set primary filter to NOTCH and adjust primary ...

Page 7: ...6 MFJ 752D Schematic Diagram ...

Page 8: ...is warranty is NOT void for owners who attempt to repair defective units Technical consultation is available by calling 662 323 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 product is returned Wired and tested PC boards installed in the owner s...

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