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Icoms on AM, external ALC mod




 
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Author Topic: Icoms on AM, external ALC mod  (Read 29153 times)
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ke7trp
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« on: September 26, 2010, 02:04:50 AM »

I am not sure if this has been covered in the past on AMfone but a local AMer mentioned this mod.  A friend came over and we built two of these ALC boxes.  They work quite well.

We used cheap radio shack plastic project boxes and 1 meg pots.  I slapped resistor across the pot to lower it to 500K.  We used 9 volt batterys.  These things take 10 minutes to build.  Remember that you are applying NEGATIVE voltage to the ALC jack on the Icom.  The Center pin of the RCA is the negative lead and the outside of the RCA gets the positive lead.

You set the RF power full up.  Then adjust the carrier using the pot on your new alc box. I chose 25 watts.  Now you can turn up your audio and the radio never pulls back the power.  The audio doubles out of the rig. I can hit 100% mod now now problem from 25 watts.

Pretty cool little project and no need to open radio up.  We have tried this on 751, 735, 725 and 756Pro. 





* ALC%20carrier.gif (0.58 KB, 214x91 - viewed 856 times.)
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Ralph W3GL
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« Reply #1 on: September 26, 2010, 02:11:27 AM »



   Just curious, didn't that mod box also apply to the Kenwood TS940 as well?

   I could be wrong but I seem to remember something along that line being
   published here some time ago.

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73,  Ralph  W3GL 

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ke7trp
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« Reply #2 on: September 26, 2010, 02:40:04 AM »

It may have been posted here. I have been here a few years and cant remember it. Its new to me so I posted it. It should work on any solid state transciever that pulls back the carrier when modulated on AM.  My FT450 for example does not do this so its not needed. My FT1000D does pull back a bit but carefull setting of the knobs and you do not need it.  Most icoms are horrible on AM and really yank the power back if you try to run any audio at all.

C
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Pete, WA2CWA
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« Reply #3 on: September 26, 2010, 03:34:54 AM »

Nothing new. As Ralph points, this info has been floating around for almost 30 years. Kenwood first introduced this circuit concept for external applying of an ALC voltage to their TS-820 and TS-830 for reducing power output on SSB when driving linears.

It's also been discussed and pointed out in several threads here in the AMfone's Online AM Handbook > MODERN RIGS. The last time was in June 2010, http://amfone.net/Amforum/index.php?topic=24495.0

Actually most current (within the last 10 to 15 years) Icoms produce very decent AM. A good read and insight on Icom DSP type rigs on AM can be found here. More on AM in Icom DSP-based transceivers

The same circuit you attached (right down to "P1") is also shown here under Icom IC-756: http://www.ipass.net/wb4iuy/radio/wb4iuy2l.htm
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ke7trp
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« Reply #4 on: September 26, 2010, 11:26:44 AM »

So far this is really working nicely.  It really fixes the icoms.  I got some emails from a guy who just tried it on his Kenwood and he can now hot 100% mod in the AM mode.

Good deal!

C
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W1AEX
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« Reply #5 on: September 26, 2010, 12:27:26 PM »

I played around with this when using my Pro III on AM last spring. It's worth doing as it does remove the gritty artifacts that the internal ALC seems to produce. I could never modulate mine much beyond 90% though. Adjusting the R2229 pot (AM Modulation) indicated that it simply runs out of headroom when you attempt to go higher.

http://www.youtube.com/watch?v=CqQl_Ks8qnM

For better or worse, I have a few notes on using the Pro III on AM at the link below. I have pretty much abandoned using it on AM simply because of the bandwidth limitations imposed by the DSP on AM (2.9kc + 2.9kc = 5.8kc total = often difficult experience for the guy on the other end) and have assigned it to digital and cw duties where it really does excel.  

http://www.w1aex.com/756AM/756AM.html


* ALC2.jpg (22.99 KB, 400x279 - viewed 709 times.)
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ke7trp
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« Reply #6 on: September 26, 2010, 01:18:32 PM »

The videos of the watt meter really tell the tale of this little device.  Where you using the Icom power supply in your test?  It wont allow the radio to go to 100% mod in the AM mode with external ALC.  switched to the Astron 35M and it easily got there.

C
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W1AEX
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« Reply #7 on: September 26, 2010, 01:32:41 PM »

I also use an Astron 35M with mine. Some users I have talked to have found their Pro series rigs seem to be able to reach 100% while others don't reach much beyond 90% which indicates that other factors are probably at work. It could be variations in alignment settings or production runs. That being said, I don't recall getting complaints about the audio level being too low once I started using the simple external ALC circuit. It really is a very simple, cheap, and effective project for anyone who plans to use one of these rigs on AM.
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ke7trp
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« Reply #8 on: September 26, 2010, 01:34:57 PM »

Mine is an old 756Pro.   So maybe that is why.  Anything past 80 is gravy anyways.  I dont plan on using the Icom on AM much. But its sure nice to be able to turn it on and ask a question or talk to a Friend real fast while the big rig is down for service.

C
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WB2EMS
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« Reply #9 on: September 26, 2010, 11:36:00 PM »

I've considered trying this on my 706MKIIG. Not so much for the AM modulation, though it will probably help that rig also because the ALC mucks with the AM on that rig too, but to fix the key up overshoot. In various modes, including AM, even if you have the power output set lower to drive an amp, it seems to initially key up at near full output and then taper down. Not good when you only need 5-10 watts and more will abuse the grids of the tubes in the amp. Anyone using this circuit observed if it helped with that?

I'll have to give it a try.
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73 de Kevin, WB2EMS
ke7trp
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« Reply #10 on: September 26, 2010, 11:39:23 PM »

I dont see any overshoot on my rig.  Give it a shot.  Its a 10 minute build.  Just get an old RCA cable, Cut it and wire it up to a 9 volt and 500K or what ever pot you have. You will need -1 to -2 volts for most radios.

C
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flintstone mop
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« Reply #11 on: September 27, 2010, 11:15:46 AM »

Even though the mod is 'old hat' I found it to be quite interesting that they use ALC approach to control carrier.
A former TS440 was modified for hi-fi AM by disabling the power limit control to allow more pos peaks. Schwang the munky!! It was huge audio BTW!!
Still had to set the carrier power to 25W.

So will the ALC approach give the same results of big positive peaks.........like 125%?

FRED
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Fred KC4MOP
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« Reply #12 on: September 27, 2010, 12:07:23 PM »

Anyone using this circuit observed if it helped with that?

Kevin,

I have a little Yaesu FT-897D that exhibits the same overshoot problem you described. With that rig, the external ALC resolves the issue very nicely. Give it a shot!

Rob W1AEX
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ke7trp
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« Reply #13 on: September 27, 2010, 12:16:16 PM »

We did on the air testing last night.  The audio doubles with this in line.  The 735 will make the full 100 watts from a 25 carrier. 

C
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WD8BIL
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« Reply #14 on: September 27, 2010, 03:46:39 PM »

Yup, that worked on my IC718. With audio into the balanced modulator input I had no trouble doing 100% mod with a trilite 40 amp power suckply!
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ke7trp
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« Reply #15 on: September 27, 2010, 05:54:03 PM »

Spent more time on the air last night with my friends 735 and this box.  He sounded really good with it.

C
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K4IMZ
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« Reply #16 on: October 01, 2010, 07:34:24 PM »

Interesting topic...  will this work on the IC-761 and it's DSB "simulated" AM ?
at least I think I remember that is has some kind of AM workaround.

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w3jn
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« Reply #17 on: October 02, 2010, 07:26:03 AM »

DSB "simulated" AM



Huh

DSB+carrier=AM (in fact even SSB is AM).  Doesn't matter how it's arrived at - plate/collector modulation, balanced modulator, or DSP - it's not simulated at all  Grin
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W3SQE
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« Reply #18 on: January 31, 2011, 06:58:10 PM »

Hi Everyone:


        I built one today at work. I used a 6 AA holder and a 500K pot. I verified the proper polarity,center negative, and preset it to 2V as someone recommended. When I plug it into my 735 and transmit however the RF out goes away all together. My meter will read 40 watts out on max RF power but  when I plug in "the mod" and key up it sits at zero and another radio I have then receives no carrier just some noise. I tried other voltages and the same result: no power out.

         Any thoughts? Oh by the way I find that I can get the Icom to work well on AM by turning off the compressor , reducing the mic gain to about 25 percent and watching the power meter during transmission. If I avoid over modulation the power meter moves forward as I speak. If the gain is too high and I speak loudly you can see the meter deflect backwards as the ALC or something kicks in.


                                                               Earl Thomas
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K5WLF
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« Reply #19 on: January 31, 2011, 09:14:36 PM »

Anyone using this circuit observed if it helped with that?

Kevin,

I have a little Yaesu FT-897D that exhibits the same overshoot problem you described. With that rig, the external ALC resolves the issue very nicely. Give it a shot!

Rob W1AEX

Looks like time for me to build one of those gadgets, since I too have an FT-897D.

ldb
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KL7OF
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« Reply #20 on: January 31, 2011, 09:24:00 PM »

Does the Yaesu FT 102 respond to the alc voltage mod?  I am using mine to drive an amp on AM and I can't seem to use more than about  175ma drive before the audio make the needle go the wrong way...My rig has the overshoot mod done, but something has changed as the overshoot on keyup has returned...
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Steve - K4HX
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« Reply #21 on: January 31, 2011, 10:46:11 PM »

There is an internal ALC mod for AM. I can't remember or dig up the specifics, but it involved changing one resistor to reduce the ALC action. Or you would probably just be better off removing the ALC. It's not needed on AM and on SSB it can cause splatter.
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« Reply #22 on: February 01, 2011, 01:29:08 AM »

In any case the dongle works well and kept me from getting rid of the 735's. Thanks!
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« Reply #23 on: February 01, 2011, 10:57:01 AM »

.
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KD6VXI
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« Reply #24 on: February 02, 2011, 03:01:11 PM »

I've built the same circuit here, and use it on my Yahoo 857 at the house.

Works fine. 

The thing to watch is polarity...  Yeasu is negative, other radios (may) be positive going?  I saw a thread on another reflector with someone having an ALC issue hooking up their amp (was neg going) to a positive ALC type radio.  Could be, just needed the battery terminals reduced.

I HAVE noticed on my 857 that if I apply TOO much negative to the ALC tip, I get odd SWR readings...  Both on the internal idiot meter and the external meter.  Nothing major, but goes to show the ALC system is probably tied into the SWR as well (which would make sense, high swr shutdown).

Anywho, yeah, this circuit will work on pretty much all the radios I've tried it on...  Just beware of what YOUR radio wants for ALC voltage... + or - 2 to 4 volts???



--Shane
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