[Boatanchors] Downward Modulation

Sun Apr 24 12:00:06 EDT 2022

With older vintage SSB/AM transmitters and transceivers, what you
describe is typically correct. 

However, the original poster is using an Icom IC-7300. a modern DSP-type
transceiver, which defines the circuitry of various modes in entirely
different ways. In these newer rigs, the older methods of adjustment and
diddling with various external controls to control modulation and/or
distortion, generally will not apply. In these newer rigs, the modulation
"levels" are mainly controlled by the ALC circuitry. Depending on the
"big 3" manufacturers, there are variations in design on how this is
implemented in the transceiver and it may not apply to all transceivers
on the market today.

I keep forgetting that this a text based reflector and that copy and
pasting a particular link may not appear correctly when posted. I noticed
that with the link I posted in my previous post.
SO, if you want to read and learn more about "reverse carrier control"
that being used in many of today's modern transceivers when operating in
the AM mode, GOOGLE ( amplitude modulation ("AM") using natural
asymmetrical voice ) without the outside ( ) and scroll down to near the
bottom for WA3WDR's explanation.

Pete, wa2cwa

On Sun, 24 Apr 2022 14:51:59 +0000 K2GKK D C_Mac_ Macdonald
<k2gkk at hotmail.com> writes:
> It is very easy got get "downward modulation" when
> using an SSB transmitter on AM.  It is simply a case
> of excessive audio applied to a carrier that is reduced
> to 25% of the usual SSB PEP output to keep the AM
> PEP equal to that of the SSB signal.
> 
> That 25% (25 Watts in most SSB rigs) will have its
> PEP peaks at 100 Watts when fully modulated.  Any
> more audio will attempt to drive the signal below the
> actual output and that will cause distortion/trash on
> the output signal.
> 
> Obey the manufacturer's instruction manual.  There
> should be only very slight increases on the output
> meter if you are operating correctly.  Easily seen if
> you have a scope monitoring your transmitted signal.
> 
> 73 de Mac, K2GKK/5
> Licensed 30 Nov 1953
> Oklahoma City, OK
> USAF (Retired) 61-81
> FAA (Retired) 94-10
> 
> ________________________________
> From: boatanchors-bounces at mailman.qth.net 
> <boatanchors-bounces at mailman.qth.net> on behalf of 
> manualman at juno.com <manualman at juno.com>
> Sent: Sunday, April 24, 2022 00:46
> To: wa3mck at gmail.com <wa3mck at gmail.com>
> Cc: boatanchors at mailman.qth.net <boatanchors at mailman.qth.net>
> Subject: Re: [Boatanchors] Downward Modulation
> 
> Actually it's not the same "downward modulation" that can be 
> experienced
> with boatanchor vacuum tube-type transmitters.
> It's found in many Icom and Yaesu transceivers and some Kenwood rigs 
> over
> the last 20 to 30 years. Years ago, WA3WDR, coined the phrase 
> "Reverse
> Carrier Control" to describe what was happening.
> For a detailed description, go to this link and scroll down to 
> "Comments
> from Bob, 'Bacon', WA3WDR":
>
32%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik
1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=4od8xtg1IGgLOsf26tml%2BP1V
FO12KTX0sZPtJN1xEng%3D&amp;reserved=0
> 
> It's actually a good thing. With your IC-7300 you can drive your
>  to its maximum allowable drive and not worry that your voice
> peaks on AM are going to exceed the plate dissipation of the 
> amplifier
> tubes. SO, you can drive your amp if capable, to a 1500 watt 
> carrier,
> modulate the carrier on AM, and not exceed the 1500 PEP limit. It 
> also
> protects the RF output and driver transistors in the transceiver 
> from
> being driven beyond their typical specifications.
> 
> Operators get upset when they have a forward reading power meter in 
> the
> feed line and when modulating on AM, the see the meter moving in a
> downward direction. Put a scope in the feed line and see what's 
> really
> happening.
> 
> Also on AB4OJ/VA7Oj's web site, which is loaded with tons of Icom 
> product
> information, he discussed this years ago, and also ran some tests on 
> the
> IC-756 series which performs the same way.
> 
> You want to defeat it; it's simple. Actually, Kenwood published this 
> info
> for a different reason back in the 70's, but it works equally well 
> in
> this application.
> You need to apply a variable small negative voltage to the ALC line 
> of
> the transceiver; or you can go into the ALC circuitry of the rig 
> and
> diddle with the ALC pot in the AM circuitry. Personally, I've never 
> found
> any advantage with the diddling. I've tried it several times on my 
> IC-756
> PRO II without telling the amateurs I was working and no one seemed 
> to
> notice any difference in my audio quality or power. And after 22 
> years of
> running my 756 on AM, the rig is still running fine without any 
> internal
> diddling.
> 
> Pete, wa2cwa
> 
> 
> On Sat, 23 Apr 2022 20:27:21 -0400 Submarine Radioman 
> <wa3mck at gmail.com>
> writes:
> > Can anyone point me to a clear and concise explanation of 
> downward
> > Modulation? I?m experiencing it with my IC-7300 running about 
> 20
> > watts of
> > carrier. I remedy it by lowering my carrier slightly and playing
> > with my
> > antenna matching unit. I remember having the same problem back in 
> my
> > old
> > boat anchor days?
> > But I don?t remember the cause or explanation.
> >
> Tnx de Howie Wa3mck