Icom: Icom IC-720a not receiving

[King18471]

I was given an old IC-720a. The previous owner told me that it transmits fine but won’t receive well. I’ve hooked it up in my shack and have also had no results receiving. It powers on and seems to function well otherwise. I do not have my general license so I can’t test the transmit capabilities. I would greatly appreciate any suggestions to repair the receiver on this radio. Even a starting point. I am a novice with radios so anything would help. Thank you.


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[ShawnInPaso]

A few things come to mind.................

Have you checked the RF gain?

The of the radio may have a separate connection for RX (in which case the antenna may not really be connected).

And most likely, with many of these older rigs a relay could have gone bad or the relay needs cleaning. Sometimes a momentary transmit will bring it to life (by cycling the TR relay).

Good luck.

 

[jwt873]

You can transmit into a dummy load.. MFJ makes a cheap one that will take brief transmissions... Enough to do a quick power measurement They're bout 40 bucks, but if you shop you can probably do better. (This is a tool that all hams should have). MFJ Enterprises Inc.

 

[King18471]

A few things come to mind.................

Have you checked the RF gain?

The of the radio may have a separate connection for RX (in which case the antenna may not really be connected).

And most likely, with many of these older rigs a relay could have gone bad or the relay needs cleaning. Sometimes a momentary transmit will bring it to life (by cycling the TR relay).

Good luck.

Ok. I will look into this. Thank you.


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[King18471]

You can transmit into a dummy load.. MFJ makes a cheap one that will take brief transmissions... Enough to do a quick power measurement They're bout 40 bucks, but if you shop you can probably do better. (This is a tool that all hams should have). MFJ Enterprises Inc.

You’re right, I’ll work on buying one of these. Thank you.


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[WA8ZTZ]

Check the positions of all the controls... AF Gain, RF Gain, Attenuator, TX/RX, etc.
Is the speaker bad ?... try using headphones.
Make sure on the rear panel there is a jumper installed between the Receive Antenna "In" and "Out" jacks (RCA type).
The TR relay is a sealed unit. If it is bad, it will have to be replaced.
If you don't already have them, get a copy of the Instruction Manual and the Maintenance Manual.

 

[cmdrwill]

Do you even get background noise? Is the speaker even working?

 

[iMONITOR]

I was given an old IC-720a. I’ve hooked it up in my shack and have also had no results receiving.

What are you using for an antenna. Your typical scanner likely will not work.

 

[King18471]

Do you even get background noise? Is the speaker even working?

Yes the speaker is working

 

[King18471]

What are you using for an antenna. Your typical scanner likely will not work.

It’s a long wire. Mfj I think.

 

[iMONITOR]

I found a lot of related info at the links below:
Icom IC-720a problems

https://www.eham.net/reviews/detail/549

 

[WA8ZTZ]

Do you hear background noise in the speaker?
Are you receiving anything?
Do you hear any signals at all, even very weak ones?
What do you have for test equipment, do you have a calibrated signal generator available?

 

[King18471]

Do you hear background noise in the speaker?
Are you receiving anything?
Do you hear any signals at all, even very weak ones?
What do you have for test equipment, do you have a calibrated signal generator available?

The speaker is working and I hear static. I recently picked up a weak signal with very poor audio of a HF net. That’s more than I’ve gotten out of it so far. I have no test equipment at all. I may need to take it to a professional to have it checked out.

 

[King18471]

I found a lot of related info at the links below:
Icom IC-720a problems

https://www.eham.net/reviews/detail/549

Some good info here. Thank you.

 

[WA8ZTZ]

Per my post #6, did you check that there is a jumper installed between the receive antenna jacks on the rear panel ?

 

[King18471]

Per my post #6, did you check that there is a jumper installed between the receive antenna jacks on the rear panel ?

Yes I did. It is installed in the correct configuration.

 

[WA8ZTZ]

If you have checked out all the simple stuff like controls and connections, it's time to go inside.
It may be something as simple as the TR relay. (Hopefully not a problem with the rotary
bandswitching relay).

You will need a copy of the operating instruction manual to guide you. The TR (transmit-receive) relay
is located on the Filter Unit (sometimes referred to as the LPF Unit) PC board on the very corner.
It is a small sealed plastic relay. Try tuning in a weak signal and tap on the relay case with an insulated
tool and see (hear) if the signal improves.

If you don't feel comfortable and qualified doing this, get some help from a knowledgeable professional.

 

[King18471]

If you have checked out all the simple stuff like controls and connections, it's time to go inside.
It may be something as simple as the TR relay. (Hopefully not a problem with the rotary
bandswitching relay).

You will need a copy of the operating instruction manual to guide you. The TR (transmit-receive) relay
is located on the Filter Unit (sometimes referred to as the LPF Unit) PC board on the very corner.
It is a small sealed plastic relay. Try tuning in a weak signal and tap on the relay case with an insulated
tool and see (hear) if the signal improves.

If you don't feel comfortable or qualified doing this, get some help from a knowledgeable local ham or radio club.

I can hear a loud relay switching when I change bands which I’ve read is normal operation for this radio. I will try the relay tip you suggested.


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[WA8ZTZ]

I can hear a loud relay switching when I change bands which I’ve read is normal operation for this radio. I will try the relay tip you suggested.

The loud switching you hear is the rotary band switch relay. That noise is, as you say, normal.
Hopefully, that relay is OK because supposedly no new replacements are available.
The TR relay is a small sealed plastic unit that is only about 1/2" high.

 

[Lauri-CoyoteFrostbite]

I have a 720, King- and that stepping relay can awaken the dead as you switch bands (when you get it working, I'd strongly suggest you, or someone else, place a 5 Ohm 2 watt resistor in series with its coil- it will quiet the beast and take some of the strain off that rotary switch- tho that switch's contacts may need looking at- get some help there, its not for the feint of heart. If the problem lies in that stepping switch, there are no replacements- handle with care !)

In the mean time, I am assuming you have set all the controls properly- like turning off the attenuator. That little piece of jumper cable with the two phono connectors is suspect- check to see if its good--- and does it jump the correct jacks on the back ?

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.

Good luck; keep letting us know how its going---

Lauri