Pactor modem / ICOM 706 Ham radio problem

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shamrock

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Location
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www.tydewi.co.uk
Request for help from the real radio experts out there....

I'm posting on behalf of a friend out here in the Caribbean who has an Icom 706Mk2G ham radio set with a Pactor IIusb with PIII protocol. Using Winlink via Airmail software. Has an ATU (not ICOM) and 5m whip antenna.

His set works fine at low power but of course rarely makes a connection and then at low speeds. As soon as we ramp up the RF power or PSK amplitude, the radio switches off as soon as we try to initiate a connection.

First suspect was voltage at the set but the cables are adequate and our attempts at measuring sugest that 12.5v is maintained as the radio switches off. There is some RF radiation symptoms (nearby VHF speaker buzzes) when transmitting at lower power. He suspects the ground might not be very good (no grounding plate but 1m2 of copper inside engine room).

Voice comms are fine all the way to Europe and beyond.

My set (IOM802 with same modem) performs perfectly from our boat a few yards away.

What do we check next? Thanks.
 
I would agree that this is almost certainly an RF ground issue. Not easy to diagnose at a distance, you must (of course) minimise distance between ATU and ground and use good wide copper tape for this. Ensure ground plate is flat against the hull. The set should be connected to the same ground.

Also you don't need, in fact should not use, full power for pactor. Best results vary with setup but 30% (ie 30W) should be plenty. Remember that voice at full power is actually putting out much less power averaged over a few seconds, than pactor at 30%.
 
ok - it's not high SWR switching the radio off the output protection circuits on the Icom (and most others) simply reduce the RF output, not switch the radio off.

However it could be either voltage (the 706 doest like low voltages at all it's quite fussy) -try connecting it (firmly with thick short cables) direct to a fully charged battery or simerlar 13.8v source - a suitable power supply for such a radio would be a good idea for testing at 20A+

As to the don't use full power - absolutely -unless you want a large repair bill for the Icom - its NOT rated at a 100% duty cycle - the 100W rating is for voice SSB about 30W as stated should be regarded as about a maximum for a data mode.

RF feedback can also switch off the 706 - the RF earthing (totally different to DC earth) on your system does seem limited - have you tried it using a simple dipole strung up in the air (dipoles don't rely on an RF earth in the same way) and would allow you to discount one of the possibilities.

you're welcome to pm me if you would like
 
Thanks to all for these replies, I'll pass them on and keep you informed of any progress. We certainly weren't trying to use full power, just kept inching up the output until (at well below 30%) the cut-out problem happens. It's only drawing about 5A when this happens. By comparison, my set draws up to 20A during Pactor connections.

Apparently it happened yesterday during a voice call after all, so it's not a Pactor problem really, will certainly investigate the grounding further.

May well be time for a focussed intervention from a good local professional, but it's always best to get as much on the symptoms and possible causes before doing that especially as many of the 'professionals' around here seem capable of running up a lot of hours to discover very little if you're not careful.

thanks again, Nick
 
Hi Nick, sound like rf getting back to the PT Modem...
Common problem of inadequate ground..

Just connect the rf ground to an isolated bronze through hull.. link any heavy metal bits by 0.1 uF disc ceramic capacitors...

Also, try to use a trnasformer (Bux comm or similar ) or opto's in the TX to PTM link..

Have fun, the joys of RF..

I suspect the 706 is shutting down with a combo of ALC issues AND low voltage.
 
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