MFJ-1270 TNC MODS FOR APRS / UIDIGI
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.
*DCD
IMPROVEMENTS*
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.
MASTER
OSC. CALIBRATION
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
TX
CALIBRATION
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 ~
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.
RESET CIRCUIT
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
JMP 10, OFF
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.
SOME COMMANDS:
Clock
duplist
Connect
btext
Info
restart
Mheard
retry
Reset led
Test
uinly
Quit
Sysop / pairs up with your
password / netrom password request
Do
not forget to program in The New-N Paradigm to unclutter
needless digipeating!
19,200
BAUD TO YOUR COMPUTER
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.