
  ADAPTING THE KENWOOD TS-790A FOR USE WITH THE PACCOMM MC-NB96 G3RUH MODEM
                                   12-26-91

 The Kenwood TS-790A makes a wonderful 9600 BPS Packet Radio Transceiver as it
has an excellent front end, powerful transmitter, and 144,440, and 1.2 GHZ
coverage. The CFW-455F filter inside is not too narrow despite what you may
have read and coupled with the low noise front end of the rig enables the
G3RUH modem to reliably decode valid data that barely moves the S-meter.

 The modification discribed below is simple, reversible, and does not impair
normal operation or the appearance of the transceiver. This is a "no holes"
modification as it allows the G3RUH modem to be connected to ACC 4 jack on the
rear panel.

 Place the TS-790A upside down on a soft clean surface so as not to mar the
cabinet. Remove the bottom cover to expose the IF. Board. With the front panel
facing you the work area is the upper left quarter of the IF board. Identify
pin 9 of IC8 (MC3357P). There are two of these chips on the IF board but only
the main demodulator IC is in the work area. RX audio to the modem will be
derived from this point. Next locate J31 a two pin connector near D81 and X2
(10.965 OSC). TX audio from the modem will be supplied to the hot lead on J31.
Note the large aluminum heat sink that spans across the IF board near the rear
of the chassis. A round lug will be mounted on it near the rear of ACC 4 jack.

 Wire an eight pin din plug. Use shielded cable for TX and RX audio. No traces
on the IF board need to be cut. Pins 3 and 5 of ACC 4 are not connected to
anything as the radio comes from the factory. Make sure the Din plug fits the
jack properly before wiring it. 
 
PIN 2 - ground
PIN 3 - TX audio from G3RUH              
PIN 5 - RX audio to G3RUH
PIN 8 - PTT

 Prepare two six inch lenghts of RG-174. Prepare one end of each cable so that
the shield will reach the ground lug mounted on the large aluminum heat sink.
The cables will be routed over the top of the heat sink and will not be crushed
when the bottom cover is reinstalled as there is adequate clearance.

 Solder the center conductor of one cable to pin 3 on the rear of ACC 4 (verify
with ohm meter). Solder the center conductor of the other cable to pin 5 on
the rear of ACC4 (verify with ohm meter). Dress the cable connected to ACC 4
pin 5 over to IC8 pin 9. Cut to size and solder (be careful- use minimum heat
and thin solder). The exposed shield is discarded and heat shrink tubing placed
over the end to prevent any shorts that would otherwise occur from having 
exposed strands of the shield touching something. Dress the other cable 
connected to pin 3 of ACC 4 to J31 and cut to size. Prepare the end in the same
manner as the other cable. Solder the center conductor to pin 2 of J31. This is
accomplished by cutting the lead going to pin 2 about 1/2 inch back, stripping
the insulation back 1/8 inch on the two ends, and then joining the center
conductor of the cable going to pin 3 of ACC 4 and soldering the three exposed
ends together. Use heat shrink tubing to cover the solder joint.

NOTE that the banded end of D81 is not the cathode of the varactor diode !
There is no need to insert any additional capacitance in series with J31 pin 2.
The shields are not connected at both ends of the RG-174 cables in order to
prevent ground loops. The Aluminum heat sink was used as ground as no other
ground was available that was close to ACC 4. Your sight may be better than
mind and you may find one.

 Adjust VR-1 on the MC-NB96 board for 75 mv p-p as measured on pin 2 of J31.
This will give a good initial setting for injected transmit audio level.

 This completes the modification. 9600 BPS Packet can now be enjoyed on each
of the bands installed in your TS-790A. For non-packet operation either unplug
the modem from ACC 4 or turn off power to the modem. For 9600 packet operation
unplug the mic and turn the mic gain to minimum.

 Thanks to Frank Anderson, W7ZTA and Jeff Angus, WA6FWI for their help. Please
report any errors or suggestions to:

Mike Hooper
KF6PU @ WB6YMH.#SOCA





 Auto freq. track for UO-14.
^^^^^^^^^^^^^^^^^^^^^^^^^^^
Auto UO-14 RX frequency tracking for TS-790/FT-736
------------------------------------------------------
 The doppler shift range in overhead pass reaches to 20kHz. It is essential
to tune-in RX frequency for good through put.
 I assembled auto freq. tracking for UO-14 FSK signal reception and achieved
good result.
[Block diagram]
RX discri.out<-- voltage comparater--INV--NOR--switching TR---> Down control
                      (Q1)           (Q2) (Q2)      (Q3)
                                           |
   G3RUH board  DCD -----------------------

[Circuit connection table.]

                GND         Q1:LM324 or TL084  Q2:74HC02  Q3:2SC945 etc.
                 |
              (C:0.01)
                 |    ----      ----      ----              -----
   *a<-(R:1M)--------3 Q1 1----6 Q2  4----3 Q2 1---(R:470)---B Q3 C---> DWN *c
     +12V--(R:8k)--- 2    |   -5     |   -2    |            -E    |
                 |    ----    | -----    | ----             | ----
                 |            |          |                  |
              (VR:10K)       GND         |                 GND
                 |                       |
                GND                      |          GND   +12V  +5V
    *b<-- G3RUH board DCD line------------      Q1   11     4
                                                Q2    7          14

*a: to RX discri out (=G3RUH board RX IN)
*b: to G3RUH board DCD line (U10 pin 13)
*c: to down control pin of mic terminal (TS-790 mic pin 3)

setting of VR:10k
        The discrimiator IC output level is 5.8V+-2V.
        Whitout RX signal, set the VR to Q1 pin 1 level turn HIGH->LOW.

In my case, I assembled the circuit on a small universal PCB and enclosed into
TINY-2 with NB96 board.

In the case of FT-736, DL signal polarity is different from TS-790 so
change as follow
     ------
     3 Q2 1------|>------> DL signal to mic terminal
     2    |    Diode (1S1588 etc)
     -----

73s      sueo asato    JA6FTL


TS790 mods.for G3RUH/TAPR-PSK
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This short note is addressed to Paul DU1POL originaly, but
it may be help of them who has the same problem,I uploaded 
addressed To:All here.
--------------
Hello Paul
 G3RUH 9.6kbps modem connection need mod. to both TX and RX. of 790.
TX..... connect new 1kohm R to the point D81 and R358(1k ohm)
        OR connect to CONNECTOR-31 directly.
RX..... IC8(MC3357P)---for main RX, IC1(MC3357P)---for sub RX
        pin #9 is the point to  draw RX signal.
	Usualy, sub RX is used for satellite communication as
	TX/RX freq. is different. But I drew both main and sub
	signal and switch them with a microrelay.This relay can
	be controled with main panel SW 144ATT.(see 790 manual).

These mods. are essential for G3RUH 9.6kbps, but also usable for
FO-20 PSK.  BUT.....with my experience, there is many trouble
to use this mods. for TAPR PSK modem. TX audio level setting,
RX noise (MC3357P pin#9 has 455khz wave element).....and so on.
 So, I use ACC2 for TAPR PSK modem (also usable for 1200AFSK)
--------------
 ACC2 connection
 4---GND
 9---PTT
 11---TXD audio signal in
 1 --3.3k---\
 3 --3.3k---/  connect together to RX audio.
 two small 1/8w 3.3k regitor mixes main and sub audio, so it is able
to use for cross band full duplex and mono band half duplex operation.
These registor can be mounted in ACC2 13 pin plug.
--------------
 UP/DWN control from TAPR PSK modem to 790
U12 and JP5/6 setting
  U12                 790 mic terminal
pin 8  ----  D  ------ UP   pin 4
pin 5  ----  E  ------ DWN  pin 3
      G ---  B  ------ GND line   
 pin 6 and pin 7 must be jumpered and connect to ground.
* R75 2.7kohm may be changed to 300-500ohm for steady photo couple.
* C32, C34 (10uF) must be changed to 3.3uF or 4.7uF for rapid doppler
  shift tracking.
* R7 in TX audio output circuit must be changed to 2.2Kohm as the
  original have heavy phase distortion. With this change, TX audio connection
  to mic (or ACC2) is no problem (no need dirct varactor feeding).
--------------
Mods. to audio drop problem in 790 computer control.
In computer control from rear ACC1 terminal,frequency setting command
makes 10m sec.of audio drop(mute),and data collapse occur in PACKET
operation. This problem is fixed by killing the blanking pulse to IF amp
circuit. remove Q4(2SC2712) on IF unit and short the collector circuit to
GND (R15 470 ohm).
 This mods. is essential for auto freq. control with KCT/Tuner for low orbit
Packet satellite.

 This is all of my mods. to 790/G3RUH/TAPR-PSK modem

     Best regards.      Sueo Asato   JA6FTL
      
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