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3/5/06,Dink Stockert,WA5LNL 

REVISED

The following Mods will make the old  MFJ-1270  hear like a KPC3+ !

 

POWER and RFI

Do not use the 12 v power plug! Direct connect [ solder ] your dc power cable. Cut  the 12v trace after the power switch and put  in series a 1N4003, 1 amp pico fuse and a 10uH choke back  to the trace that goes to 7805 regulator. The diode saves  the tnc  from  dc reversal. Next, remount Q3 [ 7805 / 5v reg.]  as it runs too hot.  Drill out the right front screw hole and mount the7805. Run short wires back to the circuit board. Install ( 3 )  .001 uF chip caps on the din socket [ solder side ] on the ptt , rx , mic .Small disc capacitors will also work. Bypass both sides of L2 [choke] with (2) 10 uF  tantalums to ground. Ground can be found on the right side of  R-34 [ 100k ]. Adding several  1 uF tantalums  to bypass the U-22 ,U21, U-23 Vcc will not hurt !

- 5v P.S.

 Install  a 1N4003 diode across C-10 [ cathode to ground ]  to protect  ics from power failure on the  –5v line. Cut out and remove R-1 [ 47 ohm to U-2,LM556 ] and replace it with a 10 uH choke.

To protect  from inductive kick-back from radios with relays, put a 1N4003 across Q10 [ ptt transistor ] collector to ground.

Pull the circuit board out and scrape paint from all of the standoffs. This will help ground the circuit board to the chassis.

Drill a small hole to the left [ looking from the back ] of the power switch and attach  a ground lug to the drilled hole . Then run a wire from the lug  to  ground on the  circuit board. On the outside [ where the gnd. lug is attached ] , connect a low inductance wire and run it to your radio  and earth  ground.

*DCD IMPROVEMENTS*

Remove R-73 [ 470k ]  and replace it with a 150k resistor. R-73 attaches to pin 3 ,U20 and    -5v P.S. . At the –5v end of the resistor solder a 10 uF tantalum and ground . Be sure to watch out on polarity on the tantalums!  Parallel  a .01 uF  and a 180k resistor and  run it from U-20 [ xr-221] pin 3 to U-20, pin 6. Replace C-45 if it is not a .1 uF [ pin 3, U-20 ].

**Rx  Audio Decode Improvement**

Pull and remove IC, U-17  [R] and U-18 [ MF-10].  Replace audio circuit with  that of the MFJ-1274.

 

 

                                            AUDIO PATH                                                     PARTS                                                                                                

                                                                                                                        R1 = 10K

(xr-2211)                                                                                                         R2 = 10K

  U20                                                                                              DIN           R3 = 1K

  Pin2  <----------R1------C2----------C3--------R3------L1-------<  Rx In         C1 = .0015 uF

                       |                            |               |                                                    C2 = .01 uF

                      C1                       R2            C4                                                 C3 = .01 uF

                       |                            |               |                                                    C4 = .01 uF

                    -5v                        gnd           gnd                                                L1 = 10uH

 

 

Be sure to go over your work many times [following circuit traces] ,as you can make a mistake easily.

Not just any audio will do. If using disc. audio [best] , you need to deemphasis [ 6 db per octive]. In  GE DELTAS this is a 130 ohm resistor [ in series with disc.] and a .68 uF  to gnd.

Then adjust R-629 for  .75 v  to 1v PP at 3 khz dev.

Speaker audio  has  deemphasis already.

 

MASTER OSC. CALIBRATION

Check oscillator with a frequency counter  for  4.9152 Mhz  and adjust C-47.

 

1200 BAUD COMPONENT CHECK

C54 = 2200 pf   U19, pins 6/11

R93 = 47.5K      U19, pins 4/13

 

TX CALIBRATION

Put JMP-4 [tx time out]  ON

At the command prompt type ‘calibra’

Type  ‘K’ to key the tnc  and  ‘space bar’ to toggle high /low tones.

Hook your frequency counter  to the mic audio [din plug].

Adj.  R-78 for  1200 ~

Adj.  R-77 for  2200 ~

 

SET DEVIATION

Adj.  R-76 for  3.5 khz  dev.  at  2200 ~ for  GE Deltas  or  3 khz  for less stable radios

Set the deviation clip point in your radio to about  6 khz so that the 2200 ~ tone at 3.5 khz has no chance to clip and distort your tones. While in the calibration mode, press D to see both tones.

While  JMP-4 is off, check to make sure your transmitter drops after about 6-8 sec. (time out timer).

Type  ‘Q’  to stop calibration.

 

RX  CALIBRATION

Jumper  JMP-4 , remove JMP-8

Jumper  JMP-9 , 2-5 [ center two pins]

Type  ‘calset  157’  for a tone of 1685~

Type ‘calibra’ .

Adjust R-79 so that both sta and con LEDS are on.

Type Q  to quit.