Icom-706: What type of AM?

[W2PHL]

I’m not sure I understand what’s happening here.

          I have a 706 that I modded for direct audio to the balanced modulator. Unexpectedly when modulating, the average RF output easily doubles. I’m measuring with a Bird 43 (not in PEP mode). The envelope looks right on a YO-101 station monitor (although approaching 200%) and the recovered audio sounds FB as well. Why is the avg power increasing?

           In contrast I have a Yaesu FT-757GX that also uses a low level balanced modulator. On the Yaesu the avg power does not move when modulating despite 120% or more positive peaks.
           
           From what I have learned (so far) about amplitude modulation, the Yaesu is behaving as expected the ICOM is not.

           I un-modded the 706 and even in stock configuration the avg power still swings forward big time.

Thanks!

[Jim KF2SY]

Hello Phil,
what is the resting carrier level (unmodulated) for each radio.  ?
if its set low enough and alc either not kicking in or disabled 200% is within realm of possibility.

[W2PHL]

Hi Jim,

          What I don't get is why the average (not PEP) power is swinging foward on the Icom. If I set the carrier at 10 watts on both radios the Yaseu will continue to show 10 watts average while modulating and 40 to 50 PEP on the Bird 43. The average power does not change on the Yaesu. The Icom's average power will swing from 10 to 20 or more with the same amount of modulation. From what I understand, the average power reading (on the Bird Wattmeter) of the AM carrier should stay the same while modulating. So is the Icom making a different type of AM modulation?

[N3DRB The Derb]

maybe it's a controlled carrier deal of some kind.

[W2PHL]

Thanks for the comments,
           I have no experience with controlled carrier rigs but from what I'm reading the Icom does seem to behave like one. I guess the modulator chip is designed differently from the ones used in the older Yaesu and Kenwood radios.

[Pete, WA2CWA]

It sounds like the internal ALC is not working properly in the AM position. Typical carrier output in the AM position should be set to about 25 watts. I would find some way not to stress the finals. Depending on the age and version of 706 (706, 706 MKII, 706 MKIIG), replacement finals may no longer be available. Icom had to make a redesign of the final section of the 706 MKIIG several years after it was introduced because the original final transistors were discontinued by the manufacturer.

[flintstone mop]

My "plastic radio" front panel power meter reads 25 watts carrier and will shwang to 125 watts on voice. The Kenwood's meter is sort of peak reading.
My ALC was disabled by Dave Calhoun, W2WV, when he modded for hi-fi audio. Best decision I ever made.

Fred

[W2PHL]

I'm going to try disabling the ALC.

      I always thought the ALC was only responsible for folding back the power. The ALC on my FT-757GX started to fold back at around 10 watts which caused the carrier to decrease with modulation. I had to disable the ALC circuit on that rig to make it modulate properly on AM.
 
    I don't see any ALC action on the meter or power decrease on the Icom 706. Maybe the ALC is increasing and decreasing the carrier as it's being modulated? I'm not sure why it would do that. But if I can find an easy way to disable the ALC I'll give it a try. It is a very difficult rig to work on!

     

   

[Pete, WA2CWA]

Which 706 are you using?

[W2PHL]

Pete,

       It’s the early model 706. The one with the out of production finals. I am proceeding with caution!
         
          I don’t want to put the rig on the air as is. The upward swinging carrier is bound to cause problems during weak signal or QSB conditions. I am also unable to use my cute little Boonton 8210 modulation meter. It can’t lock on since the signal is more like SSB than AM.  Wow maybe that’s it. Maybe the injected carrier (into the balanced modulator) is too weak. Too much sideband energy and not enough carrier? I'm going to see if that could be the problem.

       

Phil

[Pete, WA2CWA]

In my opinion, the original 706 was not one of Icom's glory rigs. The web abounds with a number of "yellow wire" and "green wire" mods and numerous other diddles to make those 706's perform well. Seems I remember (I had an original 706 when they first came out) that there was a carrier insertion or carrier balance adjustment (don't remember what they called it) for the AM position. That, and checking the ALC setting, might/should keep the carrier from the wild swinging.

[flintstone mop]

Dave warned me when he disabled my ALC to make sure the finals were working into an almost perfect load. You are taking away the foldback protection in case of excessive SWR. Transistor finals do not like high SWR.
YUP the transistorized radios will make a lot of modulation with no strain. There are circuits designed to protect them, though.

Fred

[ka3zlr]

Howdy,

 Well I think at this point it would be wise to address just how much Audio we're supplying that balanced modulator, looks like there's more than ample drive There..

Before I'd start changing Circuit parameters.

[W2PHL]

        Well things turned out OK with the Icom 706.

        For whatever reason, Icom specs this little rig at 40 watts AM even though the PA is rated at 100W PEP. That fact coupled with the swinging carrier lead me to wonder if Icom was doing something different with way the rig creates AM.

        Long story short I retuned the transmitter (not following the service manual) and now I'm able to get a clean 15 watts of average power that doesn't increase with modulation. Plenty of head room for positive peaks.

         I added a transistor switch to bypass the crystal filter on AM transmit. In the receiver I removed a cap in the demodulator stage to increase high frequency response.
 
        Now I can get on 6 with some FB audio! I attached a couple of audio clips of the results.
       
        Thanks for the comments and suggestions.

[flintstone mop]

Very nice audio. Very clean and balanced and will get through the not-so-great conditions we have.
You're definitely in the ballpark for good sounding AM OM

fred

[W2VW]

Dave warned me when he disabled my ALC to make sure the finals were working into an almost perfect load. You are taking away the foldback protection in case of excessive SWR. Transistor finals do not like high SWR.
YUP the transistorized radios will make a lot of modulation with no strain. There are circuits designed to protect them, though.

Fred

Hi Fred,

    Your ALC is only potted down. It is still there but will not be used to keep average power up as that is now done in your external audio processing gear.

You can dial the rig up to about 130 watts out on FM and watch the ALC meter start to read. This level should only be used for a brief test.

The SWR foldback protection is still there protecting your 2SC2879s.

The reason to play the TS-440 into a 50 ohm non-reactive load is to keep you out of the ALC. It will act on the peak power sooner with any SWR other than 1:1.

73,
Dave W2VW.

[wb1ead]

Evening all..Phil i used to own an Icom 706MK11G..never seemed to do what yours did initially..was able to get out more than 55watts AM and very little kick-up past 70 or so..i realise it wasn't the same model but you got it squared away so more power to ya..Fred have the same rig as you do and already did the throttling back of the ALC circuit..the Kenwoodie does not like ALC on AM one bit!!..prefers a flat as possible SWR too..not adding anything that ya don't already know..like Dave W2VW said she'll kick in (the ALC) when ya start pushing it near 100+..run mine on 40 to 50 carrier..kicks up to about 70watts on peaks..absolutely no ALC..IMHO ALC is a bad thing when running AM on a modern "ricebox"......usually..AMEN         73 de DAVE

[KD6VXI]

Evening all..Phil i used to own an Icom 706MK11G..never seemed to do what yours did initially..was able to get out more than 55watts AM and very little kick-up past 70 or so..i realise it wasn't the same model but you got it squared away so more power to ya..Fred have the same rig as you do and already did the throttling back of the ALC circuit..the Kenwoodie does not like ALC on AM one bit!!..prefers a flat as possible SWR too..not adding anything that ya don't already know..like Dave W2VW said she'll kick in (the ALC) when ya start pushing it near 100+..run mine on 40 to 50 carrier..kicks up to about 70watts on peaks..absolutely no ALC..IMHO ALC is a bad thing when running AM on a modern "ricebox"......usually..AMEN         73 de DAVE

I'm confused:

You run 40 to 50 watts of carrier , and PEP at 70?

That's what, less than 50 percent modulation?


--Shane

[KX5JT]

I think he is referring to average power.

[Opcom]

In my opinion, the original 706 was not one of Icom's glory rigs. The web abounds with a number of "yellow wire" and "green wire" mods and numerous other diddles to make those 706's perform well. Seems I remember (I had an original 706 when they first came out) that there was a carrier insertion or carrier balance adjustment (don't remember what they called it) for the AM position. That, and checking the ALC setting, might/should keep the carrier from the wild swinging.

Those mods are numerous and will keep one busy. In the VHF RX filter section, there is a choice of two mods to do and that makes confusion because you can't do both. Read twice, do once! I did the 'easy' one that re-used the SMT components and it works FB. Both mine are the oldie 706's. The AM RX audio is poor compared to the Alinco DX-70 (non-TH). fiddling with alot of these ss radios has made me prefer Alinco for ease of operation and at least I think it sounds better on AM. Their VHF and UHF rigs DR0135, DR-435 also do not overheat or go down in power when run out of (ham) band. The rep explained that they are sold all over the world for different sub-bands and the PA is wide so they need only make one design of PA. Not to hijack.

[Opcom]

Dave warned me when he disabled my ALC to make sure the finals were working into an almost perfect load. You are taking away the foldback protection in case of excessive SWR. Transistor finals do not like high SWR.
YUP the transistorized radios will make a lot of modulation with no strain. There are circuits designed to protect them, though.

Fred

I did not know that.. unobtanium finals. ALC intact here.