ic718 swings backwards

[darklife]

100% modulation should never swing either direction. It should stay steady on an average power RF meter.
Positive or negative shift is also known as carrier shift when using old terminology and is acceptable to a point in the positive direction up to 125%+ modulation before it becomes reduced carrier modulation which is what some of you seem to want with these swing kits.

When it swings in the opposite direction that is almost always a sign of a weak power supply or overdriving the amplifier, bad drain capacitors on the PA stage if naturally using a piss weak supply, or issues in the loading of the antenna system.
Those would be the first places I would look in order.

Negative swing is almost always caused by a power supply feeding an RF linear that just can not handle peaks of full modulation for extended periods OR is the cause of overdriving an amplifier final where it just can not support the full positive peaks.
This is not always a downfall though as voice peaks are more rare than the peaks of a sine wave tone input which is why some old AM rigs got away with tiny power supplies but still put out most of the power. It is only when you run a clean sine wave into a transmitter that you find out real quick if it was built for full duty service or short peaks of vocal loudness

Sorry if I am off discussion but I think this was on point?

 

[longhaireddwb]

Im on the road right now but when i get home im going to go to his house and check things out. Ive been just going by what hes been telling me. I did pass along the forum thread addy so he can read it himself.
I thank you all for the info. And more so to the owners of other 718's giving the info they see and have experianced on there rigs.

Last night he told me he did a mod to the factory mic a couple days ago so that it will give the radio more punch. I bet now hes overdriving and the alc is working hard to get it under control and that is why his power supply is dropping on the amp meter.

Ill go over and check/set his carrier to 15 and test with the gain properly set and see what happens to his swing. If that dont work then ill work my way up and also make sure he hasnt turned the rig up so it does more power and be sure hes not running the compressor. You know these guys think they need more power all the time.

 

[KK3Q]

I talked to him this morning and he said his carrier is 30watts. I told him that he might want to turn it down some and see if it helps.

To start with, the 718 is limited to 40W on AM (http://www.icomamerica.com/en/products/amateur/hf/718/IC718_QSTReview.pdf) so if he is looking for 100 watts I don't see it happening.

Since it's a used rig you might suggest he take/send it to some one good with radios to make sure the "screwdriver technicians" didn't do something to it they shouldn't have. Try another power supply, and don't forget to look at his rig/antenna grounding as well.

 

[longhaireddwb]

I know hes never going to become a ham. He just tells me he will someday just so ill quit complaining at him about running a hf radio on cb bands. It dont stop me but i kbow hes just one of those and wont quit.

Im going to see if i can get him to sell the icom and get another radio. I know hes been looking at some modified 10 meter rigs. If all goes well ill check out what hes doing with the icom, check the adjustments and settings and push him to sell.

Ill still keep you all informed on whats going on in a day or two when i get home long enough to kick him in the backside

 

[longhaireddwb]

Got the 718 in my shack. I've set the carrier to 10w, 15w, 20w, etc...
I've adjusted the mic gain down to 10 and 20. 20 gives me about 85% modulation. Compressor is off.

Now with any of these settings even my power supply that does over 50amps shows the power supply amp draw dropping during modulation. Also no mater what I set the carrier to the AVG power reading drops about half the total avg during modulation then goes back up when you quit modulating. As far as the pep, It seems to swing up nicely. At a carrier of 30w I see a pep reading of 75-80w depending on which meter I use.

I even turned the carrier down to 5w and the mic gain down to 5 out of 100 and the AVG still dropped during modulation. Couldn't drop much but it did.

The audio sounds good even with the meter swinging backwards during modulation and it don't seem to be getting too hot.

The rig will do more than a 40w carrier but I'm not going to turn it up that high. 15-20w sounds good to me.

Of course this testing is done into a dummy load.

Weird! Any comments?

 

[Shockwave]

The problem is the way Icom designed the radio to operate on AM. Power control is determined by the ALC. This method works good in all modes except the one where audio changes the power from the carrier setting (AM).

Now if you set the carrier for 15 watts, that's also where the ALC begins to fold back. When your positive modulation peaks go higher, the ALC clamps the power to the lower level and ruins any ability to reproduce strong positive peaks. The negative peaks will still drive the carrier lower.

The only success I've seen with AM on Icoms was when the ALC was disabled on AM. Depending on the model of radio this may also disable the power control and require you to add a carrier control for AM. That is easy to do by controlling the voltage to a driver stage.

To sum things up, your radio is working as it was designed to. It's just that little thought was given to AM performance.

 

[Mudfoot]

Especially in their entry level units.

 

[longhaireddwb]

Thanks for the reply. I think I follow what your saying. I'm not going to mod this radio for my buddy and try to do what you are talking about as it seems to work fine as far as the audio and power output.

This all started when he bought a meter and found the backwards AVG swing. He was happy with it before he seen that. I tried to talk him into selling it and getting a RCI or another kind of "Big Radio" but he likes his HF rig. He tells me hes still going to get his "Ticket". I told him I"m not going to hold my breath.

And to the guy that sent the PM. PM back to you.

 

[W5LZ]

Skimming through this thread what comes to mind is that too much is expected from too little, just unrealistic expectations. The other biggy is a lack of knowledge about how something behaves and why.
Everything has limits. Sure, you can 'push' things a bit and get away with it. But there are always consequences. You have to know what those limits are (have a reasonable guess anyway), and don't expect miracles. 'Pushing' is a normal human reaction. The 'catch' to it is knowing when to quit, and sometimes, you just can't get there from here without going someplace else first.
- 'Doc

 

[w9cll]

Skimming through this thread what comes to mind is that too much is expected from too little, just unrealistic expectations. The other biggy is a lack of knowledge about how something behaves and why.
Everything has limits. Sure, you can 'push' things a bit and get away with it. But there are always consequences. You have to know what those limits are (have a reasonable guess anyway), and don't expect miracles. 'Pushing' is a normal human reaction. The 'catch' to it is knowing when to quit, and sometimes, you just can't get there from here without going someplace else first.
- 'Doc

Well said Doc

 

[Onthecoast]

If it sounds good and works he should be happy with it regardless of what the meter says

 

[longhaireddwb]

I agree with the last few posts.
I was just trying to figure out an issue. Or to find out if it was an issue. I've told him to stop by tonight and pick it up. I told him it checks out fine and to quit messing with it. Run it and be happy. It works just as it should on AM. Sounds good and has plenty of power.

I checked into the mod he did on his mic and found it was causing a lot of overdriving of the ALC. I replaced the parts he changed out and did a slight mod to the mic myself and it sounds clearer than stock now without overdriving the audio.

Now to get him to leave it alone!!!

 

[KM3F]

This radio and many other act the same way on AM.
My Kenwood TS480 does the same thing.
You have to understand these radios have an immense amount of circuit control in the audio chain and the output detector feedback chain that cut the peaks back but slow enough to be seen on an external meter.
On the built in output meter you may not see it........for obvious reasons.
If you look on page 5-6 of this units service manual you will see the modulation percentage max at 90%.
In the KW it's 85%.
This means you cannot over drive the AM modulation but can flood the audio chain with outlandish mike drive that will be cut back before it causes splatter etc but sound awful otherwise as a legal signal but just with distortion within the normal operation.
There is no carrier insertion front panel control like on the old tube sets.
It's set in service menu setting.
40 watt carrier x 4 = 160 watt peaks x .9= 144 watt peaks at the most to be correct.
The circuits cut this quickly to be sure the final stage is kept within clean limits.
It must be remembered that the AM mode is obtained by unbalancing the SSB balanced modulator to get a carrier to impress audio on.
Unlimited audio cannot be used to drive this type of circuit so has to be under tight control.
In an AM CB there is no balanced modulator to get the mode in this way so there are no tight controls as such.
.
The bigger problem is two fold.
This owner has the CB mentality of 'power meter addiction' coupled with lack of any knowledge about how things work outside of the CB set world.
You all know how difficult it is to explain to that type of person once they have been exposed to the CB world where everybody else just has to be wrong.
He will 'NEVER' be happy with seeing the power meter go backward no matter what is explained to him because he's addicted to the swing like the rest so it can't be right if it goes 'wrong' (backwards) but still sounds very good on the air.
This is called fooling one's self by lack of understanding..
This is perhaps the biggest difference between CB and Ham radio.
Aw, what to heck does a Ham know anyway?
Good luck.

 

[longhaireddwb]

You said it brother!

 

[4ZERO99]

I recently purchased one of these jewels and found the same backward swing problem thinking my radio was junk and it is compared to some of these rigs, but affordable for me anyways I looked around and found this website and it was dead on for a fix for me, and with the mic gain set at 50% I have had great reports on am and sideband with the RF set all the way up,

The only difference is I used a 103J 100v mylar capacitor and a 2k 1/4w resistor.

The above info took care of this backward swing problem for me and now have from the reports great modulation.
73's and I hope this helps someone else.