eHam
eHam Forums => Elmers => Topic started by: W4AJF on March 17, 2023, 04:37:50 PM
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Hi all, need a little advice please: I received in an IC-751 (plain) and find it is dead on 18mhz band and 21mhz band, no Xmit or receive. The other bands are "OK" but a couple have a little fade in and out at times. Ramboard battery is 3.20VDC and it seems OK, no crazy frequency numbers etc so what board would take out the 18mhz and 21mhz band? LPF or RF unit maybe?
Any help appreciated. Hoping someone is familiar specifically with the IC-751.
Adam Farber W4AJF
Cooper City, FL
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I had a 751A years ago...great rig, IMO better than a lot of the new SDR radios including my IC7300...
Many if not all will eventually need to be recapped as mine was. Again years ago it wasn't that expensive back then and I found a fellow back East who had a shop and routinely did this repair. He knew exactly which capacitors needed replacing and did them all. He also did a realignment and the radio worked great again. I don't remember the shop name.
So, I'm guessing your 751A is in need of recapping, but I'm not an expert here. Do a search for ICOM repairs shops, the good techs will know what is wrong with your rig...however, I doubt ICOM factory would be willing to help. One last thought, have you searched YouTube for advice?
GL/73
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From the sound of it, I'd suggest checking whether the VCO
is locking on the problem bands. It may be that some other
bands are marginal as well.
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From the sound of it, I'd suggest checking whether the VCO
is locking on the problem bands. It may be that some other
bands are marginal as well.
Exactly. I have an Icom IC-745 which is of similar design. I think that the IC-751 uses (4) separate VCO oscillators in order to achieve the full coverage. You will need a service manual. Likely your problem lies with one or more of those oscillators. They are located on the PLL Unit.
My IC-745 had similar problems but problem was erratic and about drove me nuts until I located the problem. Turned out to be an intermittent solder connection on the underside of the logic board.
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Here is a link to information which may be helpful:
https://www.qsl.net/icom/oldicomfaq/ic-751_51a.html (https://www.qsl.net/icom/oldicomfaq/ic-751_51a.html)
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Thanks good people! Adam W4AJF
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Almost guarantee the fault lies with the VCO trimmer caps. Specifically, C71 (going from memory - don't quote me).
One of my 751As is in the bench queue for this very thing, and a couple R-71As (same basic VCO circuit) are right behind it.
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Yes, thanks, the trimmers needing adjusting and realistically replaced. Thanks to all. Adam