icom AH-4 Tuner, Good Tune Light then High SWR

[R7000]

Looking for advice on this Icom AH-4 tuner that I have had for about 12 yrs.

I'm using it with a 70' longwire antenna and numerous elevated radials, and it has always worked well.

In the last year or so when I push TUNER on my Icom transceiver, I'll get a good tune indicator but then high SWR. Sometimes the SWR is not too bad but the radio cuts back to 50W. In this case I can occasionally tune elsewhere on the same band and the SWR goes down, so the match is best on a frequency other than what I wanted.

Other times with a good tune indicator the SWR is out of sight. On some bands it works just as advertised with no problems.

I'm wondering if the SWR calculator is malfunctioning.

I looked inside and it looks like new in there. No obvious burnt components.

Anybody?

Tom

 

[AB4BF]

Check your cables first. Could be water in the cable or a squirrel has had its way with it. Good luck!

 

[900mhz]

a way to check your inside stuff is to disconnect the antenna and hook up a dummy load to the antenna port on the tuner. Try it again. A 1:1 match would eliminate your rig to tuner coax and the tuner. If all is good, hook up the coax for the antenna back up to the tuner output, and disconnect the coax at the antenna and hook that up to your dummy load. Retune. If all is good, you should again have a 1:1 match. If not, I would suspect a feedline/feedline connector issue.

 

[R7000]

Thanks 900Mhz, the dummy load procedure is an excellent suggestion. I'll do that and see if I can isolate the issue. 73

 

[R7000]

Well I checked the cables with a dummy load and they were fine. I’ve since acquired an antenna analyzer and when I run the tuner on 10M and then plug the coax into the analyzer, it’s showing swr dip on undesired frequency, for example; tune on 28074 kHz, resulting dip is at 31,000 kHz. On other bands the tuner gets the right solution but on 10M it gets the wrong answer. With the resulting tune solution I can still make 50 watts but this is a head scratcher.

 

[prcguy]

Could be there is a bad relay eliminating some tuning solutions for a specific frequency and Xl/Xc. Or there are just not enough tuning solutions to land on your particular frequency. You could try tuning out of band a little ways to see if the tuner can learn another solution. Unfortunately it would be out of the amateur band but if you could transmit at say 27.500 or 27.800 maybe the tuner would find a solution that is better on 28.074 than the one at 31.000.

Well I checked the cables with a dummy load and they were fine. I’ve since acquired an antenna analyzer and when I run the tuner on 10M and then plug the coax into the analyzer, it’s showing swr dip on undesired frequency, for example; tune on 28074 kHz, resulting dip is at 31,000 kHz. On other bands the tuner gets the right solution but on 10M it gets the wrong answer. With the resulting tune solution I can still make 50 watts but this is a head scratcher.

 

[900mhz]

I looked at the manual for that tuner and found an interesting tidbit about undesirable antenna lengths. I ran the calculation for 28.074 and it is close to an undesirable length. It basically states about avoiding multiples of 1/2 wavelengths. I calculated 70 feet (antenna length) as 21.336 meters. Per the instruction sheet 300/21.336 equals 14.061. Multiply by 1/2 equals 7.03. A 4X multiple of this is 28.122...darn close to 28.074.
This calculation is based on the instruction sheet from Icom for this tuner.

 

[prcguy]

That's typical for most auto tuners since 1/2 wavelength is really high impedance. You can clip on a few feet of wire and see if the tuner will do its thing on your freq then do a permanent addition if it fixes the problem.

I looked at the manual for that tuner and found an interesting tidbit about undesirable antenna lengths. I ran the calculation for 28.074 and it is close to an undesirable length. It basically states about avoiding multiples of 1/2 wavelengths. I calculated 70 feet (antenna length) as 21.336 meters. Per the instruction sheet 300/21.336 equals 14.061. Multiply by 1/2 equals 7.03. A 4X multiple of this is 28.122...darn close to 28.074.
This calculation is based on the instruction sheet from Icom for this tuner.

 

[R7000]

If I run the tuner at 28.9 Mhz I get a pretty low SWR at 28.074 Mhz so that is what I'll do until I figure this out. I plan to change out the wire here soon. It's been up for 6-7 years and may be chaffing on a tree limb or something causing odd behavior.

 

[900mhz]

Glad you found a workaround for the time being. Like @prcguy said, when you replace the wire, add a few feet to it, if possible. That should hopefully get you away from your tuning issue. Since you now have an antenna analyzer, it makes life a lot easier to figure out the right length.

 

[2HowardFine]

I have found on my two AH-4 tuners that the center conductor on the internal PL-259, where is connects to a little "stub" coming out of the circuit board, was very lightly soldered and had broke, causing an intermittent connection and similar symptoms.

 

[R7000]

Good suggestion. I’ll have to go in and take a look. Ion 6 meters I need to move 2 MHz to get a good tune and then dial back where I want to be. Odd.

 

[R7000]

Well I went into the tuner this morning to examine the coax connection inside. The solder connection is solid. I undid and redid the coax cable inside, counterpoise connection and antenna connection. Put it all back together. Still does the same thing. It gives the wrong tune solutions on 80, 15, 10 and 6. This limits power on the IC-746 to 50 watts. The radio puts out 100w on all bands into a dummy load FB. The condition of the boards in the tuner was like new. No apparent burnt components or anything. So my workaround to tune at a different frequency than the desired frequency will have to do for now. I know it did not behave like this when it was new . It would tune anything I threw at it with no problems at all. Hmmm. I guess compared to manual tuning back in the day this is a minor problem. It works solid on 60, 20, 17 and 12M. I've tried adding wire (actually tunes on 160M), subtracting wire, etc. Same issue, problem on same bands. I was a bit concerned that the workaround tuning might cause some unintended consequences to the components inside but nothing is apparent. I guess I'll keep using it until it quits.

 

[AK9R]

I was a bit concerned that the workaround tuning might cause some unintended consequences to the components inside but nothing is apparent.

Have you contacted Icom tech support to see if they have any ideas?

 

[prcguy]

I suspect you have a bad relay on one of the small value L or C components causing the tuner to go to the next tuning solution. I downloaded the service manual and there is no self test for relays in this unit but there are some general test points and a few alignment adjustments. You might go through those to see if there is any improvement. Otherwise its not easy finding a bad relay unless you can pull the plastic top off each relay and measure its contact resistance while manually changing it.