eHam

eHam Forums => Elmers => Topic started by: N4JTE on August 28, 2009, 04:32:27 PM



Title: Icom 751A Help
Post by: N4JTE on August 28, 2009, 04:32:27 PM
Well my favorite radio is basically bonkers.  While trying to tune up a rearrainged wire experiment I encountered the following;
1; I could not get a previously resonant antenna to load but the radio meter was showing full power out, while tuner was acting like no antenna was present, no dummy load here except me.
2; Disconnected all antennas and feedline from radio, and on lsb, cw, rtty when mic was keyed the radio meter still showed full power out, with rf and mic gain at zero, weird.
3; I tried to check monitor on rtty and lsb usually helpful but nothing heard key down.
4; To see a meter full power out on the ic751a in the lsb mode without an antenna connected tells me me I got a potential door stop.
Any thoughts would be appreciated.
Bob


Title: Icom 751A Help
Post by: WB2WIK on August 28, 2009, 04:44:32 PM
It needs service.

I would never operate with no antenna or dummy load connected, even with the RF output level turned all the way down.  This rig, along with the IC-720, 740, 745 and others of that family had finnicky power amplifiers that were prone to oscillation, then failure.  Some got lucky and nothing ever went wrong, others were not so lucky.

I used my IC-751A with the "receiver patch cable" removed from the rear panel, so I could use a separate RX antenna (very common for 80m/160m ops), and connecting a Beverage antenna to the RX jack while transmitting with a kilowatt to a vertical about 150 feet away blew up several parts of the rig.  That was fun.  ;-)

There's a transistor Icom used to short out the front end when the rig transmitted.  It was "Q5" on one of the boards, from the schematic.  If that popped, and it was only a 60v transistor, so it popped easily if exposed to too much RF from the RX antenna, the whole rig went crazy, the PA would oscillate, all sorts of neat stuff.

I never would have known what was happening without a spectrum analyzer to look at the output via a directional coupler and attenuator.  The output meter indicated "full scale," but all the output wasn't on 80 meters, it was up on VHF someplace.

I like the 751/751A, they were very fine rigs in terms of performance, but they were finnicky for me.  You might solicit the help of a local Icom expert if you have one nearby to de-bug and see what's going on.  The rig can likely be saved, it's probably just a failed component or two.

WB2WIK/6


Title: Icom 751A Help
Post by: WB2WIK on August 28, 2009, 04:48:04 PM
Looks like you're in NY.

You're in between the Icom service centers in Michigan and South Carolina:

http://www.hamstation.com/icomsupport.htm

You might call one of them and describe the symptoms as specifically as possible, see if they have any ideas.


Title: Icom 751A Help
Post by: N4JTE on August 29, 2009, 04:54:34 PM
Thank you very much for the insights and info, love the radio, my 3rd, even with the little glitches, will contact the service guys and take it from there.
Tnx,
Bob


Title: Icom 751A Help
Post by: N4JTE on August 29, 2009, 04:54:53 PM
Thank you very much for the insights and info, love the radio, my 3rd, even with the little glitches, will contact the service guys and take it from there.
Tnx,
Bob


Title: Icom 751A Help
Post by: N4JTE on August 29, 2009, 04:55:27 PM
Thank you very much for the insights and info, love the radio, my 3rd, even with the little glitches, will contact the service guys and take it from there.
Tnx,
Bob


Title: Icom 751A Help
Post by: N4JTE on August 29, 2009, 04:59:16 PM
Thank you very much for the insights and info, love the radio, my 3rd, even with the little glitches, will contact the service guys and take it from there.
Tnx,
Bob


Title: Icom 751A Help
Post by: K1WGU on September 01, 2009, 07:30:17 AM
You might contact Mike Nadeau, N1EQ who has an outstanding record for repair of Icom rigs. Sent my IC-735 to him and had it back in 2 weeks. His charges are very reasonable. His web site is www.n1eq.com and you will see that his e-ham reviews are give him a 5 level rating.  Hope this helps you out.
  Bob