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Thread: Icom IC-720A

  1. #1
    Forum Addict KA9MOT's Avatar
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    Icom IC-720A

    I don't know if we could call it a boatanchor but it is old so here goes.....

    I was given a non-working Icom IC-720A. I was able to get it to work for about 20 seconds on 40M and then back to no RX or TX. A quick google search lead me to check the band switch and I think that is where the problem lies.

    Anywho, I've deoxed the switch and still no happiness. I did find a mod online which uses relays and a few IC's to resolve the problem. I also found a mod for the mod.

    The Mod:

    http://www.hayseed.net/~jpk5lad/IC-7...y%20Switch.htm I am unable to find anything from anybody who has actually done this and says it will work.

    The Mod for the Mod:

    http://zaverio.com/~asbesto/ic720amod/ The 2 links on the bottom right of the page have very good detailed pictures, but there is little text explaining what has been done.

    I could disassemble the switch and clean it as is shown at http://zaverio.com/~asbesto/ic720amo...switch-images/ but I think I'd rather do the mod.

    Could you guys take a look at the mod and help me decide if this will actually work and be worth the expense (14 relays aint gonna be cheap, and I am thinking about using Solid State Relays) and I have no clue if that board is available or what it would cost to have it reproduced. I am going to email F5IKI and see if he has any boards left, assuming the only email I was able to find is still good.

    Sorry for being so needy guys.
    "One man with courage makes a majority." ~ Andrew Jackson



    Steve KA9MOT
    Macomb, IL

  2. #2
    Administrator N8YX's Avatar
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    First, you want 2A reed relays - not solid-state.

    Next, this is the same type of circuit which Kenwood, Cubic and a number of other manufacturers have employed to switch the bandpass or lowpass filters in their HF transceivers. I would have to look at the '720 mod info but it should be fairly close to the other designs: That is, a decoder (3-to-7 or 7-to-7, depending on the band control word), buffering logic and a set of relay drivers.

    It's not that big a deal to construct - the hardest part will be finding the correct relays. I'll examine the info during the next couple days and offer additional comments as needed.
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

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    "Island Vampire" KB3LAZ's Avatar
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    Quote Originally Posted by N8YX View Post
    First, you want 2A reed relays - not solid-state.

    Next, this is the same type of circuit which Kenwood, Cubic and a number of other manufacturers have employed to switch the bandpass or lowpass filters in their HF transceivers. I would have to look at the '720 mod info but it should be fairly close to the other designs: That is, a decoder (3-to-7 or 7-to-7, depending on the band control word), buffering logic and a set of relay drivers.

    It's not that big a deal to construct - the hardest part will be finding the correct relays. I'll examine the info during the next couple days and offer additional comments as needed.
    Is that in the same era as the 271? If so I wonder if it too may have eprom issues. Both my 271a and 271h needed new eproms when I got them. Simple fix, decent price. Sadly I dont think the man is still in business that makes the after market replacements.

    "A night sky full of cries. Hearts filled with lies. The contract: is it worth the price?"

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    Forum Addict KA9MOT's Avatar
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    Quote Originally Posted by N8YX View Post
    First, you want 2A reed relays - not solid-state.

    Next, this is the same type of circuit which Kenwood, Cubic and a number of other manufacturers have employed to switch the bandpass or lowpass filters in their HF transceivers. I would have to look at the '720 mod info but it should be fairly close to the other designs: That is, a decoder (3-to-7 or 7-to-7, depending on the band control word), buffering logic and a set of relay drivers.

    It's not that big a deal to construct - the hardest part will be finding the correct relays. I'll examine the info during the next couple days and offer additional comments as needed.
    See, that is why I asked the experts! Thanks!



    Quote Originally Posted by KB3LAZ View Post
    Is that in the same era as the 271? If so I wonder if it too may have eprom issues. Both my 271a and 271h needed new eproms when I got them. Simple fix, decent price. Sadly I dont think the man is still in business that makes the after market replacements.
    No eprom issues with this one and I sent an email to the guy who makes them and he still has a few available. He is the one who told me there was no back-up battery for the eprom.
    I don't know if this rig will actually work when I get the bandswitch issues resolved, but I am crossing my fingers and hoping it doesn't cost allot to find out. The rig came with a Heil Hand Mic that looks like a CB mic. It looks pretty crappy. I plan to build a pre-amp (probably copy the Astatic Pre-amp that comes with a D-104) and use something else. Probably the same computer mic and footswitch I used with my TS-480SAT. I really miss that radio.
    "One man with courage makes a majority." ~ Andrew Jackson



    Steve KA9MOT
    Macomb, IL

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    Volcano Tamer W2NAP's Avatar
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    I AM THE VOICE OF THE VOICELESS!

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    Silent Key Member 5-25-2015 W1GUH's Avatar
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    I've got a 720A. There's a yahoo group on them with lots of information. You'll probably get all the info you want there. That's a great radio -- it's a "vintage rice box classic." Lots of information on the infamous rotary relay.
    Last edited by W1GUH; 01-21-2012 at 12:05 AM.
    If it's a war on drugs, then free the POW's.

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    Administrator N8YX's Avatar
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    That '720 is missing a couple things:

    501353431_tp.jpg

    u127lrg.jpg

    The EX-7072 allows one to slave an IC-720(A) to an R-70, which incorporates a number of interference-fighting features that the transceiver does not.

    Icom's omnipresent Molex plug is used to daisy-chain the control facilities of a number of their accessories...the IC-2KL amplifier, AT100/500 tuners, EX-1 expansion accessory and EX-627 antenna switch to name but a few.

    I have an R-70 and an EX-7072 in the shack. I also have an IC-745, which does not offer the data I/O on its Molex connector that is needed to slave it to the R-70. Thus, I'm going to design and build an I/O converter...
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

  8. #8
    Forum Addict KA9MOT's Avatar
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    Quote Originally Posted by W2IBC View Post
    Thank You! I needed the Service Manual!



    Quote Originally Posted by W1GUH View Post
    I've got a 720A. There's a yahoo group on them with lots of information. You'll probably get all the info you want there. That's a great radio -- it's a "vintage rice box classic." Lots of information on the infamous rotary relay.
    Thanks Paul! I'll check them out!




    Quote Originally Posted by N8YX View Post
    That '720 is missing a couple things:

    501353431_tp.jpg

    u127lrg.jpg

    The EX-7072 allows one to slave an IC-720(A) to an R-70, which incorporates a number of interference-fighting features that the transceiver does not.

    Icom's omnipresent Molex plug is used to daisy-chain the control facilities of a number of their accessories...the IC-2KL amplifier, AT100/500 tuners, EX-1 expansion accessory and EX-627 antenna switch to name but a few.

    I have an R-70 and an EX-7072 in the shack. I also have an IC-745, which does not offer the data I/O on its Molex connector that is needed to slave it to the R-70. Thus, I'm going to design and build an I/O converter...
    That is cool. I'll bet it is not easy to find one of them. I'll have to see what other accessories were available for this rig. I'm not a huge Icom fan... I had an IC-730 that I really liked but it had the dirty relay problem for the pre-amp. I contacted Icom and they had a relay, but they wanted $85 for it. I found a numbers matching relay at RF Parts for $9.99.....That turned me away from Icom. I don't mind people making a dollar or 10 from me, but I refuse to be raped. lol
    "One man with courage makes a majority." ~ Andrew Jackson



    Steve KA9MOT
    Macomb, IL

  9. #9
    "Island Vampire" KB3LAZ's Avatar
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    Quote Originally Posted by N8YX View Post
    That '720 is missing a couple things:

    501353431_tp.jpg

    u127lrg.jpg

    The EX-7072 allows one to slave an IC-720(A) to an R-70, which incorporates a number of interference-fighting features that the transceiver does not.

    Icom's omnipresent Molex plug is used to daisy-chain the control facilities of a number of their accessories...the IC-2KL amplifier, AT100/500 tuners, EX-1 expansion accessory and EX-627 antenna switch to name but a few.

    I have an R-70 and an EX-7072 in the shack. I also have an IC-745, which does not offer the data I/O on its Molex connector that is needed to slave it to the R-70. Thus, I'm going to design and build an I/O converter...
    You have an addiction! :P Seeing your thoughts on a few things of this matter. I still think it would be neat for them to come out with a modern day twin set.

    "A night sky full of cries. Hearts filled with lies. The contract: is it worth the price?"

  10. #10
    Administrator N8YX's Avatar
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    Quote Originally Posted by KB3LAZ View Post
    Seeing your thoughts on a few things of this matter. I still think it would be neat for them to come out with a modern day twin set.
    I would love for Icom to come out with a transceiver which matches the R-9500 and offers all-mode capability on everything through 1.2GHz...inclusive of 222 and 902MHz. Yes, I would buy one.
    "Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."

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