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Author Topic: Icom 718 Wild SWR on 10m through 17m  (Read 27106 times)
KB3MDT
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Posts: 269




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« Reply #15 on: December 28, 2014, 08:14:29 PM »

Hi,
   Check out this thread.   http://www.eham.net/ehamforum/smf/index.php/topic,72334.0.html    It talks about the same problem and the possibility of the Driver MOSFET going bad.  I have not tried replacing it.   Also check out the Yahoo ICOM 718 newsgroup at https://groups.yahoo.com/neo/groups/ICOM718/conversations/topics.  Search on "Erratic SWR".   No surefire solutions, but more folks describing the same problem.

   If you send the rig back to ICOM, plead, beg, scream, yell or demand they test the rig on a real antenna and not a dummy load.   I could never recreate the problem on a dummy load, even when I put the dummy load up on the roof.   Others have had the same experience.

   Let us all know if you solve the problem.   Good Luck.

73
Ken
KB3MDT
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WB2HTO
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Posts: 62


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« Reply #16 on: December 21, 2015, 04:53:17 AM »

Wow, just saw that my post on the SWR fix was referenced in this thread.  Glad to hear that it worked for you.  I am now experiencing the same issue, except on the lower bands: 30, 40, and 80 meters. 
« Last Edit: December 21, 2015, 04:59:14 AM by WB2HTO » Logged

-30-
NM3G
Member

Posts: 75




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« Reply #17 on: December 21, 2015, 02:51:14 PM »

Works fine into a dummy load, oscillates on an antenna. Problem usually occurs on the higher HF bands. Same symptoms I just resolved on my 746Pro.

This is a problem found on the 746 and Pro, 756 and Pro models, among others.

Connect the rig to the antenna, turn the RF power down, go to CW mode, and key. If you get full power output and can't control it with the power level knob unkey. If you don't have any abnormal operation, turn the power level knob up in small increments and repeat. Once you observe the oscillations, stop and be sure to unkey.

Next, enable the receive attenuator (don't just turn the preamp(s) off), and ensure "ATT" is showing on the display.

Key the radio (again in CW), and if your problem is no longer apparent, you can be assured you don't have a problem with your PA or driver transistors.

What you probably have is an issue with the receive muting section between the antenna T/R relay and the output of the attenuator. Without this muting circuit working, there is enough of a feedback loop to create oscillations that run up to (and sometimes over) 300 MHz.

I was able to demonstrate this on the bench:

Connect a directional coupler between the transceiver and the 50 ohm dummy load, and connect the low-level directional coupler output to the spectrum analyzer.

Connect a paddle (I used a pair of wires, since I still don't own a set of paddles) to the front panel keyer input, and in CW key dits at about 15-20 WPM while watching the spectrum analyzer with the attenuator disabled. I used full break-in but this isn't absolutely necessary. You will, depending on the sweep speed of the spectrum analyzer, see transients from around 90 MHz up to almost 300 MHz. Big, FAT transient oscillations.

Again, enable the attenuator, and you should not see any oscillations.

At this point, you get to look at the circuitry and trace out the NPN transistor used to mute the receive line during transmit. My problem transistor was a $0.40 part available from DigiKey (and others) ... and once replaced, resolved the oscillation issue.

Ok ... I can hear the mumbling from the crowd ... "I don't HAVE a spectrum analyzer". That's ok, because I am writing the bench test portion up for those who repair these radios.

To simplify troubleshooting on the bench, I highly recommend a bandpass filter for 17, 15, 12, 10, and 6 meters rather than trying to test on an antenna. This will cause any transient oscillations to reflect back to the radio, forcing power foldback (and actually encouraging the oscillations). Select the appropriate bandpass filter for the band you're testing.


Use your DVM's diode junction test position and check the transistor and the PIN diodes (if so equipped) in the suspect area. Replace anything that doesn't look correct (open NPN transistor, leaky PIN diode). Repair time depends on your troubleshooting and soldering skills ... for the average bench technician, this is about a 20 minute repair.

Why does this happen? (again, the crowd is getting restless)

An antenna system arcing condition, a close lightning strike, or operating in close proximity to other transceivers (such as field day) without adequate receiver overload protection can cause the transistor and/or PIN Diodes to fail. This is not a normal operating condition, and you should provide overload protection to your receive path if you are operating under the conditions mentioned above.

I hope this helps.

73,

Rick
NM3G




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