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Icom: Icom IC-720a not receiving

Joined
Feb 23, 2014
Messages
563
#21
Can you receive anything in the other bands-- other than that HF net you reported? ... like the AM broadcast frequencies ? I don't know about your antenna, but I'd stick a simple 10 foot piece of wire in the antenna jack and use that for your tests- you should hear at least the AM broadcasts if the 720 is working at all.
To receive AM broadcast frequencies (below 1600kc), there is a separate "Low Band Antenna" jack on the rear panel.
A quick glance at the schematic looks like the signal is then injected into the RF Unit at the 1st Mixer stage thus bypassing some of the front end
circuitry including the TR relay. Also, be aware that the Low Band Antenna input jack is dual functional and if someone has made the
appropriate jumper changes inside the set it can also serve as a relay output to key up a linear amp for example.

So, to sum it up:
1. Sticking a wire in the PL-259 main antenna connector will not be useful for AM broadcast reception
(although the strongest local stations may bleed through).
2. If an antenna were plugged into the Low Band jack and AM broadcast stations were heard the front end is still suspect
as it appears to have been bypassed.
3. If the Low Band Antenna jack was internally jumpered out you would hear no AM stations (except maybe strong locals bleeding
through).
 
Joined
Jun 13, 2018
Messages
335
Location
Colorado, New Mexico- and now in Washington DC
#22
Hmmmmm- I may have a different model.

Mine will receive- and transmit - at least as low as 1500 KHz. That low frequency input/bypass on mine takes the receiver down below ~500 KHz to 30 (doesn't do badly for WWVB at 60 KHz.) Unfortunately my 720 is a long way from me right now, DC-Colorado; or I'd look at it more carefully.
I agree, the previous owner(s) may have done some mods and that low frequency port may have been internally wired as a linear amp control. The front end on the 720 stinks for frequencies above ~20 MHz, and I have inserted, in the past, a pre-amp between those ports.

Barring a simple wiring issue, the front end may be blown, King. I hold in suspect the previous owner's admonition that it "receives poorly" He was aware of a problem- it maybe easy to fix-- but you'll have to find someone with experience. The 720 is an old war horse and may not be worth it.

BTW- My 720 doesn't like a power source much above 12.5 volts- there are some voltage regulators just beneath that little door on the top- where you can adjust several Pots- that get really warm if the supply voltage is much higher- warm enuff I put a heat sink around one.
Also, if you are so inclined, and it hasn't been done in the past (highly doubtful it hasn't) you can clip that turquoise wire, the only one in there of that colour, and it will open up the radio to xmt. on all frequencies ;) .

I obtained my 720 yeras ago- it was my government issued radio to take out in the field. It was previously used by others, but well maintained and carried in its own Poly case. It still looks new. Mine has worked flawly in many countries- from the the tropics to the arc/antarctic poles. Years ago it must have fallen off the inventory lists, but they know where to come for if that list ever re-emerges. :)

Lauri :sneaky:
 
Last edited:
Joined
Feb 23, 2014
Messages
563
#23
Not sure about the antenna jacks on the straight 720.
The OP says he has a 720A.
Per the 720A Instruction Manual, the radio automatically switches to the Low Band Antenna jack
when tuned below 1600kc.
 
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