3/5/06,Dink Stockert, WA5LNL 


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



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 kickback 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.


Remove R-73 [470k] and replace it with a 150k resistor. R-73 attaches to pin 3, U20 and the -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.



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



C54 = 2200 pf   U19, pins 6/11

R93 = 47.5K      U19, pins 4/13



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 ~



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.



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.



If power drop-outs confuse or locks up your TNC, try replacing the reset capacitors.

Replace C-58 [1 uF] tantalum

Replace C-22 [10 uF] tantalum



 # 5 on     to computer comm port 9600b

 # 7 on     radio baud rate 1200b



JMP 1    OFF            JMP 11, RIGHT

JMP 2, 1 TO 2        JMP 13, ON

JMP 4, OFF            JMP 15, 1 TO 2         

JMP 5, OFF            JMP 18, 1 TO 2

JMP 7, OFF            JMP 26, 1 TO 2

JMP 8, ON              J4, 1-2, 11-12, 13-14 , 17-18

JMP 9, 2 TO 3         JMP 12, 1 TO 2




I have done these mods to over 12 MFJ –1270s (no B or Cs) and it seems I never see the same circuit board. I guess there was a lot of on going changes.



To run U-23 with UIDIGI or tnc V1.19 you must have 32 k of ram U-25 (43256) and JMP-12, 1-2.

Do not use the battery backup on the tnc.

Uidigi PROM chip (27c256) always goes in socket U-23.

Check to make sure U-21, pin 22 (z80/sio) goes to ground.

To talk to a UIDIGI via computer comm port, type ‘esc key’, ‘c’, ‘return key’.



Clock                             duplist

Connect                          btext

Info                                restart

Mheard                           retry

Reset                               led

Test                                 uinly


Sysop / pairs up with your password / netrom password request

Do not forget to program in ‘The New-N Paradigm’ to unclutter needless digipeating!



Cut the trace at pin 1 (300b SW) and run a wire from pin 1 (SW2) to U1 (4040), pin 10. Set the baud switch  #1 to on. 300 baud now becomes 19,200 baud!


Revised: 02/13/10 by KC5PIY, B. B. Stanfield, Webmaster
Copyright 2006 by Intertie, Inc. All Rights Reserved
No use of these symbols or phrases is allowed without the express written permission of Intertie, Inc.