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Old 02-12-2008, 08:20   #1
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ICOM M802 Tuning Problem

I have an M802 SSB with an At-140 tuner which I installed myself about 3 yrs ago. It has worked fine, but now the radio is not tuning to frequency. Specifically, when I push the "tune" button the word "tune" appears for a short time, then it reverts back to "thru". It does appear to be sending a tune signal, because both the forward and reflected needles jump on my SWR meter when I push the tune button.

Anyone see this same problem? I had this problem once before, but too many brain cells have died since then I can't remember what I did to fix it. I will check the connections on the control cable between radio and tuner, but that's kind of a pain to access right now.

I'll appreciate any help you can provide.
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Old 03-12-2008, 05:27   #2
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I had similar problem caused by being over enthusiastic with self amalgamating tape to waterproof connection. The tuning lead plug & socket nearest tuner had parted when I wrapped the tape around, thus hiding the fact.

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Old 03-12-2008, 07:57   #3
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The problem may be where the wire is attachehed to your rigging as well. Check both the connection to the tuner and to the rigging. The wire can also go bad if water gets into it. When you reconnect the wire to the rigging make sure you form a "drip loop" so that the end of the wire is pointing down not up. If your tuning problem is limited to certain bands, you may have a bad coil in the tuner - I had this happen a few years ago. By varying the length of the antenna this problem can be dx'd as different bands will "tune" as the antenna lenth is changed.
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Old 03-12-2008, 09:48   #4
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Thanks Richard & Special D. I'll check those connections and put a drip loop in the wire to my antenna (hope i still have some of that special wire (GTO?) leftover. I don't think there are bad coils in the tuner since it won't tune on any freqs right now. The hard part of the job will be moving all the Christmas presents off the guest berth to get access to the tuner. Such is life when living aboard.
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Old 21-12-2008, 17:23   #5
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I crawled back under the starboard berth and disconnected all the leads to the tuner; i.e., coax from radio to tuner, GTO 15 wire to antenna, RF ground connection to dyna plate and control wire plug. I didn't see any obvious corrosion, but I cleaned evertything with a bronze brush and sprayed some WD-40 on terminals before putting it all back together. After I was done, the radio "tuned" immediately and I'm getting good radio checks. I don't know which connection was the culprit, but the lesson learned maybe -- good, clean connections to the tuner are a necessity.
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Old 17-01-2009, 00:16   #6
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Greg,

The reason your tuner appeared to be shutting down without tuning is that it detected a very high SWR that it could not tune. This being caused by the poor connections you found. The tuner does this to protect the servos from excessive heat cause by continuous tuning. Glad you found the problem.
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Old 17-01-2009, 06:46   #7
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Hal, thanks for the explanation. Radio is working really well now and I'm talking to friends in the Exuma, Bahamas regularly. One of the other things I did when cleaning connections is to replace wing nut with a hex nut on the connection to the antenna. I was able put a little more torque on the nut with a socket wrench and that may have helped also.
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Old 17-01-2009, 06:55   #8
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one other thing you may want to do is put some washers under the nuts. Use the kind that can get a bite on the metal, not the flat ones. Good luck.
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Old 17-01-2009, 07:50   #9
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Glad you were able to fix the problem on your own, good job. Just to clarify, there are no servo's in the AT-140 tuner. Relays are used to switch in/out the capacitors/inductors. When in tune mode, the 802 only outputs about 10 watts. This is further reduced in the tuner by an attenuator circuit that is switched in during tuning so that only about 300 milliwatts goes thru the relays and to the antenna. There is a timer in the tuner that stops the tuning process if it doesn't successfully tune within around 15 seconds. All this is to eliminate any possible damage to the tuner/radio during tuning as well as to reduce on-air interference because of the very low milliwatt level output.

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Old 17-01-2009, 08:32   #10
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Eric,

Point well taken. I was thinking of some older tuners I've worked on (Collins type) and still haven't had my morning coffee. Thanks for bringing me back into the solid state era.
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Old 17-01-2009, 10:37   #11
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Eric & Hal, Thanks for the education. I was always hesitant to push the tune button during a radio net, but the low power should not create the squeel that interferes with traffic. Its also good to hear that protective circuits prevent the system from self destructing when something isn't right.

Hal, I did add washers to improve contacts on my connections. Hopefully, I won't need to do this again for awhile.
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Old 17-01-2009, 10:43   #12
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Greg, you should always tune away from the net a short distance (to an unused area of the band) before pressing tune. Even though you are putting out a weak signal it could still interfere with someone's reception. You are hearing other people tuning when you hear the squeals you mention.
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Old 17-01-2009, 12:34   #13
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Yes, I absolutely agree with DeepFrz on that. People tune up on top of a conversation in progress (QSO in ham terms) often on the ham bands and it's very annoying and considered very bad practice. Find a clear frequency just a few Khz away to tune up on and you won't have to worry about high swr when you go back on frequency. While the low power output during tuning does help prevent interference, it doesn't eliminate it. Lot's of hams communicate over great distances with "flea power" of under one watt with the right propagation conditions.

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Old 17-01-2009, 18:51   #14
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I have made contacts using as little as 25 milliwatts with good signal reports. Always a good idea to tune off frequecy to tune the radio and then come back. 5KHz is not to far to go and that will clear the frequency with traffic.

Sorry about the mis-information on the tuner. Glad there are enough people around keeping an eye on us old guys. I have five tuners that use servos with variable inductors and variable capacitors. Three of them came off of military ships and the other two came off of transport aircraft. The ship tuners are capable of handling 10KW and are quite large.
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