MFJ-1270 TNC MODS FOR APRS / UIDIGI

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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.

Revised: 12/05/04 08:30 PM by N5VRP, Dennis Rogers, Webmaster
Copyright 2006 by Intertie, Inc. All Rights Reserved
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