The AM Forum

When I first joined this forum, I promised myself that I would never get on AM with my Icom IC-765 (or IC-751A). However, I recently broke my own rule, and would appreciate some advice about a problem that I ran into doing that.

The other day on 40 meters, I noticed something really odd. I cut the drive on the IC-765 back to roughly 20 watts, and drove my SB-200 (two 572Bs) with it for perhaps 200 watts output. I ran it for about a minute or two into the dummy load, and everything seemed fine. The plates were not showing color, and the meter readings looked fine. I switched to the transmitting antenna and got on the air with it.

However, after awhile, I noticed something really odd. Although the power output meter on the IC-765 showed forward movement on modulation peaks, the wattmeter between the SB-200 and the antenna did not move. At that point, the other AM station said I sounded distorted. (Before that, others said I sounded good, even though I didn't ask for audio quality reports.)

I thought I might have destroyed something, but everything seems OK now. But I don't want to try AM again with that until I have an idea of what happened.

Any ideas?

I have been thinking about building a 160-10 meter plate- or cathode-modulated AM transmitter using a pair of paralleled 833As. I doubt whether I'll ever find a modulation transformer that I can afford, and the design at http://www.montagar.com/~patj/cathmod01.htm looks quite interesting. It uses a power transformer from a solid-state audio amplifier, and I happen to have one of those out in the barn. However, I'm not sure how such a transmitter with two paralleled 833As would work on 10 meters (double the interelectrode capacitance of a single tube), and I like 10 meter AM.

Generally speaking, most modern rice boxes will make approximately 40 watts of AM on peaks.  Tune your amp with full output from your rice box and increase the loading control slighly (about a 10% drop in output).  If your rig makes 100% positive peak modulation, then set your output drive to 10 watts.  This should yield APPROXIMATELY 100 watts of carrier from your amp.  I believe you were running out of head room on your amp due to it being over driven by your exciter.  Do you have an oscilloscope?

Thank you! The next time I try this, I'll have a scope connected. :-)

FWIW, the Icoms (the ones I own) don't 'peak' on am, they limit.....  I run my '746 at 20 Watts;  my voice characteristics do not overdrive the output. 10 Watts will give you plenty of headroom fer the Heath.......  JG gave you gud advise

klc

Silly rule, glad you broke it.  ;) There's plenty of room for all types of AM signals. The days of the few but loud AM snobs who needed to seem superior by running down anyone not running plate modulated rigs has long passed. The important thing going forward is to embrace AM operation, regardless of equipment choice. It's been that way for some time, actually.

And Jay has nailed your problem. The only thing I'd add is to be sure to shut off your ricebox's processor, if it has one. Next to improper tuning, this is the leading cause of distorted audio for SS transceivers. You may need to increase the audio gain a bit after doing so to modulate your carrier properly, but it will sound much cleaner with the processor off.

The fact that you say that it was okay at first and for a time, then seemed to develop trouble makes me think you might have gotten into some sort of heat related problem.

There are all sorts of suspects:  modern ham gear unmodified is not always up to the demand of a long AM transmission.  I don't know much about the 200, but the stock 220 with the voltage doubler p.s. and stock cooling that does not move much air through the p.s. can get into trouble with a long geezermission on AM.   Do you have passive components in your feedline path?  Maybe a low pass filter, ferrite choke, tuner with fixed value padding caps...these can heat up and change value and result in a line Z change giving the amp a different load or, the amp's pi network may have fixed value caps paralleled with the load air variable that are not up to the AM continuous circulating current and change value (although this is more likely to be an issue on 160 m. where the dielectric current handling ability is less).

Since I have started operating AM I have fried:  a low pass filter (threw it away; didn't need it), old CRL doorknobs, MFJ RF current amp meters,   ferrite chokes, RF disc ceramic load padder caps IOW everything listed above that was obviously designed for slopbucket.    Running a vintage rig is a worthy goal in my opinion, but that's a discussion for another time.

Just a tweak.  If the rig on CW key down is showing 100W maximum, you will never get anymore power out than that.  That is the upper limit.  Divide that by 4 and that gives you your maximum unmodulated carrier output power you can run.  In this example that is 25W.  Now when you modulate 100% that will raise the peak power to 100W.  The 4 to 1 ratio holds true.  Many contemporary rigs due to ALC issues will not allow you to achieve 100% however.  

Have fun and enjoy AM!

Joe, W3GMS      

I didn't know that. I figured that I would not be welcome on AM with my Icom; that is very reassuring. Thank you for that. :-)


The processor does not even work on AM. But it's shut off anyway.
Funny you should mention this. http://www.w0btu.com/files/misc/Icom/IC-765/IC-765%20Mods%20&%20Service%20Info/IC765_notes_VE3HUR.pdf suggests that the processor should be used on AM. So, I tried it, but found that it did nothing in the AM mode.

I have a couple of scopes. For now, I may also put one between the IC-765 and the amplifier, besides the one I used to have link-coupled to the antenna. Since this only happened after I had been talking for a minute or so, something tells me that this problem is in the IC-765 and not the amplifier. (How COULD it be in the external amp?) Perhaps the Icom's  "power output" meter is not actually in the output of the PA.

Maybe it was a fluke. It may be something obvious that I just was not paying attention to.

What kind of SWR do you have between the Icom-7XX and the SSB-220?

Before I put a low power (<300 Watt) matching unit between my Icom and the Henry, it used to cut back due to SWR.


On my Icom 706, I turn the mike gain down just enough to modulate about 90% on the scope, with no processing and the tranceiver ouput power on steady state carrier is 20 Watts or below.

Phil - AC0OB

 

I doubt anything in the amp is causing this, unless the plate blocking cap is heating up.

Heat problems in the xcvr driver or final section are more likely culprits. I often use my TS-950SD on AM and the LK-500ZC only requires 20W for 350W+ carrier.

Carl

                           

i say that it almost has to be that the amp is running out of headroom, which is why i never run anymore than 150 watts with my amp (a hunter 2000b-quad 572B's), combined with the fact that the micas in the output can't take the heat of any transmission longer than 3 minutes. it will do 180-200 watts easily, but the micas in the output can't take it, so i drop the power back to 100-120 watts.
shelby

Put 10 to 15 watts carrier into the SB-200 (which was tuned for max in CW mode).  That should make about 100 to 125 watts of carrier with enough headroom for peaks.  That's what I do with my TS-570 --> SB-200.... scope always helps!

Thanks for all the help. :-)

The SWR between the rig and amp was very low.
I'll run less power next time.
I'm trying to find my old oscilloscope tuner so I can monitor the signal next time I try this.

The solution to your problem is found here:

http://amfone.net/Amforum/index.php?topic=25323.0


Build the simple little box and plug it into the ICom.
Turn the Icom RF power FULL open.
Use the box to set the carrier level.
Turn the audio up to 100% while watching the scope.
You will sound as good as the bandwidth limitation of the icom and or the microphone you use.

This mod is very very popular, Its posted all over the net.  Mine, has a switch on it so it can be disabled for SSB use.  However, I only use the icom on AM if my Other rigs are down or its 120F outside and I cant stand the thought of firing up 4-400s and 304TLS to FURTHER heat the room.

My buddys old Icom,ALC box, with the audio fed Into the back ACC connector (balanced modulator input), A cheap Behringer mixer and a decent sterling audio mic, is damn nice.  Really..

C

Thanks! This looks great. As soon as I get the scope hooked up, I will build this. :-)

Let me know if you need any help. I can take a video of mine or a photo or two.

C

Thanks.

I guess I'm going to have to build a new scope tuner. I haven't seen the old one in years, and I've looked everywhere I can think of.

The old one used a simple link-coupled, parallel-tuned circuit, with the each end of the inductor going to the vertical deflection plates of an old 10 MHz Heath IO-18 scope. I have a 100 MHz Tektronics 465B, but I'd rather not tie that one up.

I found a suitable variable cap, I just need to find (or wind) a coil now.

The DC input to the ALC will follow after that.

This afternoon, I'm working on an antenna design in EZNEC. A modified ZS6BKW multiband dipole that ought to let me get on 10 meter AM, among other places. 10 meters is full of AM QSOs right now!

Mike,

I kind of feel the same way. I'm a newbie and have made two AM contacts on 75m using my ricebox, but have been keeping my eyes open for just the right vintage plate modulated transmitter.

Glad to see your post just now as I rounded up the parts today to make that circuit for controlling ALC output, which I found on another web site a while back. I'm planning to build mine inside a round electrical junction box since that's what I have on hand. Hope to find a few minutes in the next day or two to put it together and test it.

I have enjoyed reading through the various threads on amfone.net, including the humor. There's so much to learn. The information I've found here combined with reading a 1951 ARRL Handbook and lots of help from a local AMer as well as listening to some of the AM QSOs have helped tremendously.

73,
Brad

Brad,

Welcome to the AM Forum.  Your mug shot looks a little familiar, can't place it yet but I'll figure it out.

You can learn a lot here, and after a few hundred posts you'll be as confused as everyone else.

Fred

Looking back over my "log notes" (scraps of paper) over the last several weeks, out of 47 AM contacts, only 3 were using vintage boatanchor equipment. The rest were using solid-state transceivers, class E rigs, Elecraft, and Flex rigs.

I noticed a lot more "modern solid state transceivers" on 10 meter AM than the other bands.  I really feel GREAT making my contacts with the Viking II on 10 meters.  It's so much funner to me than the Kenwood.  Oh and I seem to get a lot more QSO's with it whether vintage or modern on the other end.

you will find that a lot of others have had the same problem with similar icom rigs.. the problem is an over agressive alc.  google alc mods for your rig.  i believe the tric on yours is to slow it down with an added resistor

Fred,

Thank you.

It's actor Pierce Brosnan as Professor Kessler in the movie "Mars Attacks!" - that movie's good for some laughs. In that getup he kinda resembles Fred MacMurray in "My Three Sons".

73,
Brad

And also Bob! 

(http://static.libsyn.com/p/assets/1/c/f/1/1cf11f85a08935c6/subgenius.jpg)

Worked WB1EAD, Dave on 29.010 just now.  It was marginal condx but sounding great for a couple rounds when free and clear of the noise floor.  It's quite a different thing getting to know people on forums and then finally working them on the air.  I think it's a new experience somewhat.  The norm used to be working hams on the air and then meeting them at hamfests to put the voice to a face.  Occasionally I had met people at hamfests and them worked them on the air.  But the future is here and now we can meet "dx" personalities online first.  Then the random experience of working them on the air is pretty neat!  I posted this here because he mentioned just reading my earlier post in this thread before working me.  ;D

Forums and email are very impersonal.  Its hard to tell if someone is being a jerk or not and often times arguments insue.  The meek can hide behind the keyboard and act like a lion. 

I worked a person on this forum that is very tough on people here.  Short one liners. I know more then you ect ect.  Then when I worked him on the air, I realized that he was a pretty nice normal guy. 

It's true that intent and humor are often misjudged on forums.  Glad you figured out I was an okay guy Clark.  ;D

Another thing to think about using Transceivers in AM mode is the resting carrier output and modulation.
ALL 100 W PEP SSB transceivers will need to be adjusted to 25 watt carrier in AM. And your microphone level has to be adjusted so that there is NO ALC activity. ANY ALC in the AM mode and your positive peaks are gone and flattened out.
Being that you broke your rule not to use your radio on AM, do not try to get into the electronics to disable the ALC or the SWR foldback circuits.
This was done to an old work horse transceiver of mine a TS440 and some mods to the filter networks. It had B'cast quality audio and easily made 130% pos peaks,,,,thank you Dave,,,W2WV.
With careful adjustments in regards to carrier setting and audio, a lot of the transceivers in use really have very nice TX audio. Some are unbelievable.
Fred

Thanks, Fred.  :)

This is one question that I never could reconcile.   My Icom 718 runs 40W of carrier which seems high, but the rule of thumb is that the PEP power of a double sideband AM signal is 4 times the carrier power.    What if the ricebox is SSB with an unbalanced modulator with the SSB filter still in the circuit, would that mean you can run the carrier at one-half the PEP power.   Is that what Icom does when it sets the AM carrier to 40W.    They seem to work OK at that level as long as you keep the mike gain below the ALC tripping level.   I never did quite figure this out but have never had problems running the 40W of carrier.   Lowering the carrier power doesn't seem to improve anything.   This has been covered before but I never saw a good explanation.

At a 40 watt carrier the Icom cannot fully modulate the signal.  If this is ok for you or band conditions allow it, Then great.

The ALC is to agressive and will not allow 100% modulation from a low carrier of 25 watts.  The most mine will produce is about 50 watts from a 25 watt carrier.

You can open your icom and adjust/modify your rig.  However,  I do not want to modify the rig.  The solution is to use the external ALC device and simply plug in the unit to the ALC jack. No mods and the problem is solved.

You can then run Any carrier level you wish, With producing the full 100 watts output when modulated on AM.

This device really shines when using a typical Ham amplifier with Tentrodes or high power Triodes that does not want much drive.  My amp only wishes to have about 15 watts of AM carrier as Drive.  I can then modulate the rig and adjust the mic gain until I have 100 % modulation while watching my station O scope. This is not possible with out the ALC device.  From 15 watts carrier my icom will not reach anywhere near 100%. 

On air testing produces from my friends IC756Pro 2/with 4cx800 x2 amp, shows a very strong increase in audio and clarity.  This is expected since his rig goes from 750 watts pep to 1500 with the device inline and his Carrier is fully modulated. 

C

What if the Icom was only transmitting one sideband?

Somewhere recently, I read that a certain Icom transceiver only transmitted ONE sideband on AM. (I don't know if that's really the case or not.) If that is true, is the four-times-carrier rule still true?

I cant speak for all Icoms. My 756 Pro uses both sidebands.  I can see them on the transmitting Spec scope. At 40 watts, Carrier. It can only modulate to the 50% range or so.

C

The ALC box would be a good addition, but I only use the Icom on AM for emergency use, i.e. before the tubes warm up on the AM rigs.

Following up, I built the circuit referenced in this thread and connected it to my Yaesu FT-920 transceiver.  I modulated the carrier using voice then a 1000 Hz tone.

Judging from the transceiver's ALC meter, the ALC was unaffected by the voltage output of the circuit (and there was no change in the waveform on the scope) until the circuit output reached approximately 4 VDC (about the level of the internal ALC voltage).  At that point, the ALC meter pegged and the transceiver power output went to zero, with a corresponding disappearance of carrier waveform on the scope.  Several tests produced the same result.

I'm wondering if similar results were obtained by others who have tried the circuit and where they went from there. I'm reluctant to test further for fear of damaging the transceiver's ALC.

Brad K4RT

My ricebox is now older than my Johnson 500 was when I used it. 

Not sure if that circuit works on Yaesu radios.

It is reported to work on several Icoms.

I would read these threads:
http://amfone.net/Amforum/index.php?topic=25323.0
http://amfone.net/Amforum/index.php?topic=24495.0
And AM in Icom DSP rigs: http://www.ab4oj.com/icom/dsp_am.html

Pete,

I had reviewed the two amfone.net threads, but the AB4OJ link is new to me. I'll check it out.

73,
Brad K4RT

One thing about the meters ...

It could be the meter you were using that showed no movement may have been an averaging meter, and not a true peak reading meter. On AM you would not see any movement on the average meter.

Do I have that right? I think so.

John

Depends on what kind of rig you were running on AM.

I've talked to quite a few guys running FT-101s. Some of the early ones would be 40 years old now! Even the ones made at the end of the line (not the ZD models which were more like a 901 than a 101) are now 30+ years old.

a 101EE was my first amateur rig back in the early 80s. Wonder where it is now?

Before the Flex came along, the 101 was probably the best sounding if not most prolific ricebox found on AM. An incredible amount of good audio with minimal effort. Hearing N3DRB booming into VT with his at a whopping 25 watts a few years back still amazes me. I was sure he was on a big plate modulated rig.

I see a little wiggle on mine. 

Bill, W3DUQ was the first one I every heard using a 101 on AM. His was modded for hi-fi and lots of positive peaks. This was in the late 70's or early 80's.


http://amwindow.org/tech/htm/ft101.htm

KC6MCW has those mods done to his FT101.  He runs outboard studio mic with some audio proc gear.  Its better sounding then most big BC transmitters I have heard.  The only issue with doing this is that they will be a good 25KC wide and will not have any internal limiting. 

Even stock with a Shure 444 mic they are just fantastic.  Most of them will do 150 positive if not 175 so the use of a scope is a must.   There is no ALC action on AM.  It will fly.   

Excellent point. This is what really lets the positive peaks soar.

Speaking of Bill's (W3DUQ) FT101ZD, I have that rig sitting on a shelf in my shack.

Back a year or two ago when my TS440 died and the TS940 was out of town, Dave
(K3ZRF sk) donated it to the WFD cause.  With the TS440 back in operation and the TS940 back in my possession, the 101 was retired in ready reserve, so to
speak...

Yeah, it produces good peaks of 125% and more with little effort on AM and the receive is not too shabby as well...

I have DUQ's FT-101F.