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AMTOR - putting one of those old modems back into use

Background

AMTOR is an acronym for AMateur Teleprinting Over Radio and was mostly used on HF. The mode is based on the marine SITOR code that was introduced to amateur radio by Peter, G3PLX. The mode is an enhancement of RTTY with active error correction that copes reasonably well with interference and fading.

There is a good write up of AMTOR in June 1992 QST, written by the late Bill Henry, K9GWT, who was president of HAL Communications. It is available to view or download from the ARRL web site if you are a member. There is a full engineering specification on an ITU web page, it describes the commercial SITOR system in great detail, this link has the document. ITU SITOR Specification

The data throughput of AMTOR is fairly slow under most band conditions, but is generally error free. For normal "ragchew" contacts the data flow will match the typing speed of a reasonable to good typist. The error correction is fairly crude but adequate for most purposes.

Suitable modems can sometimes be acquired for little cost from fellow hams who may have them lying gathering dust in garages and on shelves. eBay is a source for old modems, but beware that prices seem a bit steep for items that are often several years old and not easily repaired.

Available modems

Probably the most popular AMTOR modem was the PK232, in addition there were modems from Kantronics (KAM), ICS (AMT), HAL , SCS and an MFJ offering. Older Pactor modems can also operate on AMTOR. Most of these modems are quite long in the tooth and were often only supplied with DOS software. Currently, as at January 2021, the following new modems can also operate AMTOR:

PK232 DSP from Timewave
PTC-IIIusb (but not the P4 Dragon series) from some SCS dealers, but not from SCS directly as it is no longer manufactured.
KAM-XL from Kantronics

Repairing a KAM Plus

My KAM Plus developed a fault that has also been seen by others and was easy to repair. The issue was the lack of a -7V from pin 5 of U15, an LT1054. First I checked the 10uF 50V aluminium electrolytic capacitors associated with the power supply (C48, C49 and C51), these all had a higher ESR than 20 Ohms and were replaced. That didn't cure the problem. The circuit of the KAM Plus doesn't show a connection to pin 1 of U15, this pin is a shutdown pin which stops the output if the pin goes low, the measured Voltage on that pin fluctuated when measured with a 10 M Ohm input Fluke DVM. One modification I saw elsewhere linked pin 8 (the input) to pin 1 (shutdown), so I tried a 12K resistor between pins 1 and 8 of U15 and it did the trick of producing a -Ve supply every time on switch on, however the fault turned out to be either leakage under the socket or a faulty LT1054 (U15). After removing the socket and thoroughly cleaning the print board, replacing the socket with a new one and fitting a new LT1054, the problem was resolved without needing the 12K resistor.

Software

Many of the older modems used DOS software which can be a problem for modern PCs which often don't even have conventional RS232 serial ports. Some of the modems can be operated from a simple terminal program such as PuTTY, however most users would probably find a dedicated program easier to use, unfortunately, Windows 10 terminal programs written specifically to run a KAM or PK232 do not appear to exist. SCS modem users are able to use the free Alpha program which works very well, although it helps if you can translate the German instructions.

PuTTY is a free terminal program, it is very comprehensive but doesn't specifically cater for amateur radio comms. You can manually enter commands but it becomes tedious to have to remember them.

Terminal BPP is a Windows 10 "donation-ware" program with macros, it is similar to PuTTY but with the benefit of being able to program scripts and macro keys. It doesn't need installing, to install copy the file into a directory and create a short cut, eg. C:\terminal. It works very well with a KAM Plus. I have included a set of macros for a KAM that can be edited in Notepad to replace the dummy callsign of X1ABC and selcall XABC, with your own callsign/selcall. Note it needs to be used in conjunction with a good read of the KAM manual, for example you need to manually set ECHO OFF and to call a station on Amtor you need to be back at the Command Prompt and enter their selcall after the word Amtor, eg. amtor WABC. The macro keys are easily edited and the commands are described in the KAM manual. NOTE, the latest version listed on the link above has a bug in that it doesn't scroll correctly, try this alternative link for a later version: Updated Terminal BPP

This is my KAM Plus Macro file for Terminal BPP, unzip it, edit the callsign, etc. and save it in the same directory as the terminal program, then load it using the "set Macros" button in Terminal. Note the macro buttons for Amtor set the TX delay to 20mS and for GTor set it to 80mS. Edit those settings to suit your radio, these timings have been tested with an Icom IC-7300 over a 200 mile path. The "50bd WX" macro sets the KAM to 425Hz shift, 50 baud and inverted with unshift on space off, this is to work with the commercial RTTY weather transmissions.

Specific settings needed for the KAM Plus are Echo off, XMitecho on and FLow off. Otherwise anything you type in Terminal BPP (or PuTTY) will stop the incoming text stream, or mix your typing with that stream. Note in order to use the XMitecho command you will need to change the INtface to Terminal from Newuser. If you want to use 1400/1600 tones, the shift needs setting to modem (command shift modem), this makes setting the receiver frequency a little easier. In version 8.2 firmware Amtor standby will ofen result in spurious messages similar to <FREESIG 20 GWxxx>, to stop this set FREequal to 0.

For a screen shot of Terminal BPP decoding commercial RTTY from Germany, click here.

Alpha for SCS modems is free from SCS.

Simple32 Gold is shareware for SCS modems from Seaglemail.

The log program Logger32 supports KAM, PK - 232 and MFJ 1278.

In addition, there is one Linux ( KPTC ) offering but unfortunately it only works with SCS modems - and needs compiling for the particular version of Linux used.

Suitable radios

The requirements are fast transmit/receive change over and a reasonable level of frequency stability. A dedicated modem socket (or line socket) is desirable to avoid needing to change microphone and speaker leads. Most modern HF transceivers are suitable for use on AMTOR, however it is worth checking the specification and ARRL reviews for "suitable for AMTOR" or similar wording. I used to use an Elecraft K3 and an Icom IC7300, both of which work without modification on AMTOR. My current Elecraft K4D needs a hold on circuit to prevent the PTT line dropping before data has left the transmitter, hopefully that will be resolved with a firmware modification by Elecraft, who are aware of the issue. Some other DSP radios may suffer from the same problem. I have built a simple "hold on" circuit and the diagram is available from my Radio Mods web page. In the KAM Plus settings there is a Post Key parameter to cater for this additional delay, unfortunately SCS modems do not have such a parameter.

I previously used a Kenwood TS-480sat and that was fine too. Some much earlier transceivers may need modifying in order to change over quickly enough and some later transceivers, especially SDR types, may not switch fast enough, nor be easily modified. There is a slight advantage in using transceivers with diode antenna change over to avoid the endless "click click" of the antenna relay when operating AMTOR, although it is doubtful that relay wear will be significant unless you want to operate AMTOR 24 hours a day.

Mostly SSB is used with audio tones on AMTOR, although FSK can be used with some modems as AMTOR is based on RTTY. Note that modems which include Pactor will only use audio tones as FSK (direct frequency shift keying) is not suitable for use on Pactor.

Hooking it up and getting going

Wiring modems to radios and computers is covered in the user manual, all that I have either wired or seen, have been comprehensively covered in the manual. In some cases running a ground wire from the radio to the modem or passing the connecting cable through a large ferrite ring is useful to avoid RF hanging up the modem, it depends on how much RF you have in the shack as to how much of a problem it might be, again refer to the user manual for any grounding or filtering information.

Beware of how you connect the screening and ground connections between the modem and the radio, in general you should use multicore screened cable with the braid/shield connected to the shell of the plugs, the ground pins should connect to a wire within the screened cable and not to the shell of the plug. These images show the connection to an IC-7300 13 pin accessory plug (below left) and to a KAM Plus (below right). Note that I have not used the moulded plastic cover for the KAM end DIN plug as with the older KAM Plus the DIN socket is recessed in the rear panel and the plastic cover of most plugs prevents the plug fitting properly. Using a piece of heatshrink works well and allows a normal sized DIN plug to fit into the KAM Plus. Also note that I have cut off the unwanted pins in the 13 pin DIN plug on the IC-7300 end of the cable to to make soldering the plug much easier.

Tuning

Depending on the tone pairs used, you will have to pick USB or LSB, working out the actual carrier frequency for the mark and space tones may require a little mental arithmatic in order to meet up with someone on a certain frequency.

AMTOR uses 170 Hz shift, the same as for RTTY. Some modems will only accommodate a 200 Hz shift, which is near enough for all practical purposes. Some modems include a feature to specify the mark and space frequencies, traditionally for RTTY these were 2125 and 2295 Hz. The more modern modems, which include Pactor, often default to 1400 and 1600 Hz tones, which are the initial tones for a Pactor call. In any case, your transceiver needs to be able to work with whatever tone pairs your modem is set for. With an Elecraft K3 these tones are best set around a centre frequency of 1500 Hz as this is fixed at that frequency when operating in synchronous data modes such as AMTOR and Pactor.

The basic parameters necessary on both the radio and modem are as follows, these are based on using an SCS modem with an Elecraft K3, other modems and radios will have similar parameters - although not all radios have a specific data mode:

Radio settings

Mode - DATA A (not the alternative side band where "REV" is shown in the display)
Bandwidth - 300 Hz
AGC - fast
Synchronous data on (-S symbol displayed)
PTT RLS 12mS (some radios will have this parameter fixed, it allows time for the audio to exit the TX DSP filtering before returning to receive)
Microphone input - Line
By using the synchronous data mode and line input, the transmit/receive turnaround time is minimised and the transmit audio equaliser is bypassed.

Modem settings

ARX 1 (enables AMTOR, ARX 0 would prevent AMTOR operation)
BC 1 (AMTOR FEC enabled)
MARk 1585 (mark tone frequency in Hz)
SPAce 1415 (space tone frequency in Hz)
TOnes 2 (allows for free choice of mark and space tones)
MYSelc (enter your four character selective call)
TR 0 (TX and RX shift normal)
TXDelay 4 (20mS delay before audio is sent on transmit, allows time for the transceiver to change to transmit).

There are dozens of modems commands, these are just the most basic ones. In addition, the tone amplitudes may need adjusting depending on the sensitivity of the line input to your radio. With the tone frequency pairs above, the radio operates in USB even though it's in a "data" mode.

Note the settings for a KAM include a TX to RX delay setting, POStkey sets a time delay to allow sufficient time for data to exit the radio before returning to RX. The Elecraft PTT RLS setting is another way to allow for this.

If your radio covers 490 and 518 KHz, you can test your Amtor receive by listening to Navtex, which is basically the same as Amtor broadcasts. There are many navigation and weather bulletins broadcast on these frequencies in Europe, although there are none in Australia.

Finding other stations to work
BEACON or Call CQ on 14109 CF (14107.50 USB DIAL), 10146.50 CF (10145 USB DIAL), 7047.90 CF (7046.40 USB DIAL) or 3609.50 CF (3608 USB DIAL). There are active Pactor and Amtor users on the Groups.io Pactor group, it is well worth joining that group. See https://groups.io/g/pactor

Format for contacts

Normally a station would call CQ in AMTOR mode B (FEC) in a similar manner to this:

CQ CQ CQ DE G4AON G4AON (GAON)
CQ CQ CQ DE G4AON G4AON (GAON)

PLEASE CALL IN ARQ K

The four letters in brackets are my selcall, it is helpful if stations list their selcall during a CQ as a few do not follow the normal arrangement for creating a selcall and confuse those of us who try and guess what they used! Normally for two letter suffix calls you would repeat the first letter, W1AB would use WWAB. Equally WA1BCD would use WBCD to make a 4 character selcall. You can use what you like provided that if you deviate from the expected selcall you make it clear in your CQ.

The caller would selectively call the station calling CQ and give their callsigns as the AMTOR exchange (unlike Pactor) does not include a callsign. It is useful if the modem is configured to auto reply, when called, with your callsign and name. The change over command is +? and can be manually typed or done via one of the pre-configured buttons in most terminal software.

CW ID may be necessary depending on your licence conditions, again most modems are configurable to auto send CW IDs periodically and at the end of a QSO.

All the above is usually covered in the user manual for the modem.

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