TNCs, Kenwood HF Rig's ACC2 Port

And SSB Distortion



 This file describes how to hook either the PK-232mbx or the DSP-2232 to one of the TS-X50S series Kenwood radios and a number of ways to deal with the problem of  distortion on SSB operation when a TNC is  hooked to the ACC2 port  of the TS690S or TS450S ( or similar TS-X50s series radios).  This is a problem which may or may not show up for you - but suggestions on how to fix it are included here at the end in case you run into problems as well!

While every effort has been made to ensure the accuracy of this page, AS ALWAYS  CHECK THE TECHNICAL MANUAL FOR  UPDATES AND ANY WORK ON YOUR EQUIPMENT IS YOUR RESPONSIBILITY!

I have had AFSK and FSK connections hooked up at the same time, and have used both with no problems. I had set the frequency shift set to 170 HZ in the Kenwood menu system for FSK operation. The AFSK frequency shift out of the PK-232MBX is 200 HZ.

I would like to take this opportunity to thank everyone who replied to my question about connecting the PK-232MBX to the TS690S. In particular, I would like to thank Dick (KD5VU) and Erich (KA6AMD) for their input as to how they connected their PK-232s to the 690S and the 450S. The following information has been edited from that which they had sent me earlier.

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Suggested Hookup And Pin Assignments

Dick, KD5VU uses the ACC2 port of the 450S to hook up his PK-232 for AFSK operation. The following list details the Pin Assignments which Dick uses for AFSK Operation:

On the PK-232 J4 is Radio Port 1. J6 is Radio Port 2. Either port may be used.

PK-232MBX (J4)            TS-450s (ACC2 Port)
Pin 5 (PTT)                Pin 9 (PTT)
Pin 4 (GND)                Pin 8 (GND)
Pin 3 (not used)
Pin 2 (AFSK out)           Pin 11 (AFSK in)
Pin 1 (AFSK in)            Pin 3  (AFSK out)

NOTE: In the ACC2 port pins 4 and 8 are connected and go directly to ground

As Dick indicated to me, it is important to ensure that you do not overdrive the rig. Since neither the 690S or the 450S has a Tx monitor you will need to watch the ALC setting after setting the desired power level and AFSK audio output from the PK-232. Please note that it takes very little Mic Gain to overdrive the ALC. Also - while the 450S and the 690S are rated as 100% duty cycle units, I personally felt more comfortable setting the power level to 40 - 60 watts on modes such as RTTY, FEC etc although the 690S manual seems to suggest setting the power level to 100 watts.

Erich, KA6AMD hooked up his 690S to the PK-232 in order to use FSK for digital communications. The hookup he uses works well for him on all the digital modes and is listed below:

PK-232    Function     TS690S/ACC2
J7-4     FSK KEYING          2
J4-1     Rx AUDIO            3
J4-4     AUDIO GROUND        4
J7-2     FSK AUDIO           8
J4-5     PTT                 9
J4-2     Tx AUDIO           11

NOTE for FSK: You will have to set the mark/space frequency deviation in hertz in the Kenwood menu system - 170 Hz is best - although HF packet and pactor require 200. In their case - use of AFSK is recommended since the PK232 sets the frequency shift to 200 in that modulation mode.

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The following information is specific to my current setup - connecting the PK-232MBX ( and my DSP-2232) to the ACC port on the TS690S for AFSK AND FSK OPERATIONS:

AFSK Operation:

TS690s        PK232MBX Function                 (PIN Number)
ACC2 Pin
 3            Receive Audio/AFSK IN                   (1)
 8            Ground: Audio/PTT Common Return         (4)
 9            PTT                                     (5)
11            Transmit Audio/AFSK OUT                 (2)
12            Cable Shield                            NC

Note 1:

I have NOT yet used the option to mute the microphone on data transmission and the squelch option is not hooked up ( PK-232MBX PIN 3).

Note 2:

Ground pins 4 and 8 in ACC Port are directly connected inside the radio and go directly to the rig's ground. PIN 12 on the other hand is connected to ground via a 0.001 capacitor. Source: TS450/690 service manual.

FSK Operations:

I have these connection in place as well so that I can switch between AFSK and FSK operation. You need to watch MARK POLARITY when connection to the FSK jack on the PK-232MBX. If you connect up incorrectly, at least there is a place in the KENWOOD MENU SYSTEM (menu 38) that allows you to change the polarity sense of MARK so that you do not need to resolder the connection. I did find that the MARK POLARITY in my PK-232MBX ( Menu 38 - ON for PK-232 using connection 4) was the REVERSE of that in my DSP-2232 (Menu 38 - OFF for DSP-2232). Check your setup as it may be different than mine here! The manual should tell you the polarity of MARK - if you have trouble finding it let me know and I'll dig the info out for you.

TS690/450 ACC PIN     PK232MBX Function
2                     FSK Keying
4                     FSK GROUND

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 Now the next  step - particularly if you are using the PK-232MBX only for HF operations  -  is setting the transmit audio levels to an appropriate level! The following are some notes as to how I did it before I set up the VHF side of things. After that, I adjusted audio out according to the suggested VHF setup in the manual and then adjusted the mic gain (AFSK) or carrier level (FSK) to keep power output down and ALC to a level indicating minimal or no ALC action ( one - two bars showing on the digital meter!). Do not over deviate or distortion WILL occur.

The big problem is getting the audio set correctly. When I first started I did not have a VHF setup so I had to skip that part. What I did was turn the audio control to the halfway mark on the PK-232MBX.  TEST USING A DUMMY LOAD! I then went into calibrate mode - using LSB and therefore AFSK. I went into the cal mode, typed K and then d ( injects data rather than a tone ) and adjusted the mic gain until the ALC deviation on the ALC digital meter was just showing at the low end of the ALC bar. You should see the SWR meter show power forward ( otherwise - no audio - no SSB output).  Keep the Transmissions short so as not to overdo the transceiver heating. Now look at where the mic setting is. If it is less than half way and full forward power is going out - then the audio setting is probably okay. I set mine so a 40 percent mic rotation from the "off" position results in full power out. You need to do this so - when on AFSK and FEC for example - you can cut the power to 50 percent or so ( the TS-450 / 690S will do 100 percent duty but who needs to chance it? I run mine at 50 - 75 watts at most) by turning the mic gain down. If the mic gain is over 50 percent - turn the audio output from the TNC down some and then retry the above - keeping all transmissions short.

When transmitting - the needle on the SWR meter will be up at the power being put out and just wavering a small amount - you will not see it bounce around like you do on voice!

Now - when you go to FSK ( if you should make this connection) use the FSK button on the front of the radio and adjust the carrier to set the power level to 50 watts or so. The MIC gain is set to OFF!  That should get you started - I did not listen to my adjustments - once you are on the air the people you connect to can advise you if the tones sound too harsh etc.

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Both Erich and Dick indicated that they had distortion when transmitting on SSB when the PK-232 was hooked to their rigs through the ACC2 port. The Kenwood reps were not able to help them with regards to the problem. Erich suggested that I disconnect the PK-232 when using SSB. Dick also had little luck with the reps, but he was able to determine that there are floating grounds in the ACC2 port of the 450S and 690S.

Dick appears to have corrected this problem by adding a separate ground wire from the PK-232MBX  to the TS-450S. As Dick noted (edited :-))(please note that Dick is using AFSK):

 " Yes! I had the SAME problem (I called it distorted xmit audio) and  the GROUNDING fixed the problem. Its history and the rig works now with the PK-232 hooked up via the ACC2 plug. The fine print in the (Ken)wood manual says you have to GROUND the rig. You better ground it as they have some floating grounds.  I grounded the PK-232 to the Kenwood TS-450S/AT two ways:  - Used the single pin 8 in the ACC2 plug to the Gnd connector on the PK-232 (the white molex plug).  - The winner was to ground the PK-232 from the circuit board (used one of the screws that supports the mounting foot nearest the DC input connector) to the Large Ground Lug on the rear of the TS-450S.  Kenwood has some floating grounds in the ACC2 port and will not  admit to the problem.  Without the REAL ground wire PK-232 to TS-450S my SSB audio was  big time screwed up when the ACC2 plug was attached. "

When I originally set my rig up for AFSK operation and I grounded my TS690S to the TNCs as follows:

 I connected a small wire from the screw holding the right rear foot on the bottom of the PK-232MBX - to the rig ground on my TS-690s. This screw is connected to the ground plane of the circuit board in the TNC. I found that the same solution worked for my DSP-2232 as well.


Another solution was published in the November Issue of QST in 1993 on page 82. There, Julian Frost, KC6NSE found that increasing the audio output from his KAM to his TS-850S eliminated the distortion he had on SSB!  He indicates that Kenwood also suggested that by adjusting the TS-850's VR18 trimmer, the distortion on SSB operation can be eliminated.

My Solution:

Once I had done that, I no longer had any  problems with RFI affecting my audio when transmitting on SSB as long as I was using the Kenwood  MC60 desk microphone. However,  when I got a heil boomset/mic set the problem was back. So the ground wire was not a complete cure - depending on the mic in use etc. So another solution was required:

Since one suggestion I had received was to put a SPST switch in the TX audio line from the TNC to rig, I decided to expand on that suggestion and increase the capabilities of my station. I bought a data transfer switch box from a computer supply shop. The box had two input ports and a total of 8 output ports. One input was a female 5 PIN DIN plug that could be switched to 4 separate female 5 pin DIN outputs. The other input was a DB9 plug arrangement that also switched to 4 separate DB9 connectors.  Since the 5 pin DIN is a standard connector on many TNCs, including the DSP-2232, I decided to use the 5 pin I/O on this switch box for connecting my PK-232MBX, and later, my DSP-2232 to my  TS-690S.

I converted the PK-232MBX radio cable to a 5 PIN DIN configuration. I did this by cutting the cable in half. I terminated the cut ends of the shielded cable with male 5 pin DIN plugs. Plug them in to the proper input / output ports of the data switch box and you are away. Initially, I was just connecting one of the radio ports of the PK-232MBX to the TS-690S, so when I was using SSB, I just switched the box to an empty port. Similarly, when I wanted to use the TNC, I just switched the PK-232MBX in-line.  This simple arrangement prevented the audio distortion that is common when a TNC is connected to the ACC2 port. The setup also allowed me to further develop my audio setup in the station.

I now use the DSP-2232 as my primary TNC, with the PK-232MBX serving as a backup system on VHF in KISS mode for TCPIP operations. When I connected the DSP-2232 to the system I wanted to use it for HF, VHF, UHF and satellite purposes. As a result,  I made a number of modifications to the switch box. The DB9 I/O series in the data switch box was used to extend the receive audio capabilities of the TS-690S and allow me to place a W9GR audio DSP filter inline to the DSP-2232 when warrented.

The PK-232MBX is connected to the IC-275 via a MFJ microphone/TNC switch to the mic input on the front of the radio. Interestingly, I have the DSP-2232 hooked up to the 2 digital ports ( AQS for 9k6; ACC1 for 1200 b) on the back of the IC-275 for terrestrial and satellite use at the same time and I appear to have no problems with the setup.

The Benefits:

REMEMBER : Watch the POWER OUTPUT  in FEC or RTTY etc - you do not want to blow the power supply OR Transistors in your FINALS!

I would be happy to answer any questions via  E-MAIL you might have.

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Muting The Microphone on Data Transmission

When using the ACC2 port in the TS-440S or the TS-940S for digital communications, if you only connect to ACC2 pin 13 for PTT, then when the TNC is transmitting the microphone is also activated!

In order to prevent this you must ground pin 9 as well as pin 13 in the ACC2 port. However, if you just join the two, the microphone will not work if a TNC is connected to the ACC2 port.  An elegant solution was published for this problem in the March 1993 issue of QST by Dave Sittner, N0DET. Dave installed a diode (1N914) in the ACC2 plug "which only allows PTT switching outboard of the diode to put the radio into transmit and mute the mike. PTT closures made via the rig's SEND and mike PTT switches put the radio into transmit without muting the mike. A schematic is available from that article which demonstrates Dave's suggestion.

(Note - in my setup I connected to pin 9 rather than pin 13 - it works, but pin 13 is the PTT pin identified in Kenwood's manuals. Gloria, KA5ZTX suggests that in later model radios that only pin 9 is used for PTT. I can confirm that her suggestion works for me in my TS-690S)

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Last edited 22 November 2002 by VY2DCS