IC-718 problem auto oscillation or so
Dear Hams,My Icom IC-718 has been working for over 5 years without problems but recently a problem mainly happens when working on 15 and 17 meter bands. I noticed on 30, 20 and 12 meter the same problem but not so clear. For example, when transmiting on PSK31 mode sometimes RF output power disapears and then go back. Sometimes rig cannot put power up or measurment on cross needle wattmeter shows needles exactly crossed in the middle of scales and SWR is 3:1 or 5:1 and rig seems not to be transmiting although there is such indication. On a dummy load no problems, full power 100W is OK. Any clue, pse? TNX73s Isaac EA8BNP
I would run this problem by the 718 yahoo users group if not already done. What is changing to make the swr move about like it does? Is it the antenna/line, the relays and output parts in the 718, heat? Failed swr detect diode? What?
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If your swr goes above 1.6:1 or so, the the power fold back circuit will start kicking in, on the 718. It does this to protect the finals. If it works ok with a dummy load, that would indicate a tuner/line/antenna problem.
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Yes the power does fold back when you have a high SWR. If the antenna is unchanged and you suddenly have this problem it might be a spurious signal sneaking out. This signal will not be on your operating frequency and it is picked up by the SWR detectors in the IC-718. That would cause the unit to cutback power because the frequency of the spurious emission is the one with the high SWR. The IC-718 doesn't distinguish between either your desired operating frequency or the spurious emission that is outside the the acceptable range to produce a SWR indication. When you put it on a dummy load every signal is seeing a 50 ohm load and therefore the SWR will be low but the spurious signal may still be there. It's just seeing a load that will not produce a high SWR reading.
Okay, to test if this is happening either use an antenna analyzer and check your antennas. If the antennas are okay then you probably have a spurious emission. The other way to do this is to have another amateur bring their rig over and connect it to your antennas. If that rig doesn't have a problem then again you may be facing a spurious emission.
There is another method to test this and it would require the use of a spectrum analyzer. Those are hard to find and expensive. They are a very effective troubleshooting tool in cases like this.
One more thing to check and maybe you should have done this first is to make sure your power supply is functioning properly. Check the voltage coming into the IC-718 at the back panel power connection. I have seen strange things happen to rigs that do not have the proper voltage getting to them. Check the voltage at receive and then, using the dummy load, key the rig at full power on CW. If the voltage drops to about 10.5VDC or less you could have a problem.
Hope this helps
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Hello guys, tnx for replies. I got Science Workshop's Poor Man's Spectrum Analyzer some years go. I checked the carriers from 17 to 10 meter bands. I think there are not spurious emissions, at least when output power is 1W the other signals I noticed are high as the noise level or just a little over. The carrier is clear. At higher power other signals appear but are less of 25% of carrier. Also I think two problems I have: The first one is a high SWR on the multiband vertical antenna and 40/80 dipole due to a lack of maintenance. The second one is the worst. I thought it was common to the first one but I don't think so. It is when operating a homebrew magnetic loop antenna. SWR is keeped low but when tuning at 1W and mainly when key is up, needles go crossed. Here SWR is very high and I found out a carrier on 18.8MHz or so. I noticed it when tuning on 15m. band. On 17m. the same problem. So this second problem what I cannot understand at all. I think in a self-oscillation on loop due to a bad and cheap building but not sure. When operating at 40W power fold back probably due to the high SWR.
That's a problem a few of the IC-718's have had before. They sometimes put out spurious signals outside the band they are supposed to tuned up on. I would be a bit concerned with other signals showing up at higher power. A signal that is 25% of your full power out is equal to 25 watts. That amount of signal will definitely be picked up by the SWR circuits in the IC-718 and will cause a fold back of power. Spurious signals should be suppressed to greater than 50db below the full power level. That's about 1 milliwatt and it doesn't look like your unit is doing that. The other problem could be your spectrum analyzer isn't calibrated for a high degree of accuracy.
A common problem that mostly showed up in the IC-706 series were improper grounding returns in the final amplifier section. This was causing the IC-706 to break in to oscillation. Adding grounding eliminated the problem. You might want to try that. It seems ICOM is still having a bit of difficulty but there are thousands of IC-718 units out there and very few have reported this problem.
I also have a magnetic loop. Mine is made for the lower bands and tunes 80/75, 60 and 40 meters. It does okay. Since the magnetic loop is just a single turn tuned circuit it would be difficult for it to be a cause of your problem. It is remotely possible there is some RF feedback but that would be very unusual.
Your tests need to be into a non-reactive load. A dummy load does this very well. If you don't have one then using a jar (perferred to be glass) with a lid you can make one. What you do is to mount a SO-239 to the lid and dropping down from the lid solder one wire to the center of the SO-239 connector. Solder another wire about the same length on to the shield connection of the SO-239. The wires need to be bare copper and long enough to reach about half way into the jar. Fill the jar with water and measure the resistance between the center of the SO-239 to the shield. Add just a pinch of salt to the water and stir the water until the salt is completely dissolved. Measure the resistance. If it reads 50 ohms or close to it them you're ready to apply power to the dummy load. If it doesn't read 50 ohms then continue adding little bits of salt until you have the proper resistance. The jar you use should be about a 500ml or a bit larger. When you use this type of dummy load make sure you monitor the water temperature and allow it to cool between tests.
Hope you resolve your problem.