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FT-900 Negative Swing on AM ??

jwhawg

Member
Oct 2, 2009
25
0
11
My brand new Kenwood TS-480SAT does the same, On the 6 different watt meters I have, which range from my El-Cheapo Midland vintage, to my Bird 4430 with Peak reading Kit. Best I can tell it sounds ok. As for signal strength on receive it seems to do the same. It also draws LESS current on voice peaks. Now this I'm sure can be minimized, and probably doesn't sound too bad, however, it sure as hell isn't going to sound better then my Old EFJ Valiant as some on the forums seem to think.

BTW, It makes absolutely no difference, 5 watt minimum, or 25 watt max, Antenna, Dummy Load, or band, does the same thing, Now SSB, thats a different story...

One of these days I'll get into the service menu and try some different settings, from what I've read it will help some, just not a priority.

Hi Mr Fatty, sounds exactly like what this rig is doing. I'm going to have the rig checked out and see what is really going on. It may just have a button that says AM. It really does nothing of the sort. I'll be posting, and thanks!
 

jwhawg

Member
Oct 2, 2009
25
0
11
Sounds like a classic case of low level modulation being overmodulated. Does the radio have an ALC menu function? Sounds like the ALC is detecting and overmodulated signal and is attacking/overshooting your audio. Try adjusting the ALC and play with diff. mike gain settings. None of these low level modulated solid state rigs use true AM. They actually detract the intelegence from the carrier instead of add to it. An oscilloscope will show you this. In other words if you look at the carrier on a scope and it shows 10v pkpk. The modulated signal peaks will still be 10v pkpk but the "lows" will be 2.5v pkpk(asumeing a 4to1 100% modulated signal) So what you have efectively done is lower your avg output when the inteligence is applied.

Now that said, there is a way around this. Again by adjusting these basic radio settings
ALC
Mic gain
Carrier power level

There other setting that you could look into also(not sure that your rig has these opotoins)

Am freq roll off- this adjusts the transmitted audio response giving base or trebble sound

AM mic gain- this is independent of total mic gain

Using these settings I have achieved "true"(I use this term loosely here) 100% modulated AM on both my yaesu ft857 and my old Icom 746 Both about 20 watt carrier with around 85-100 watt peaks. And these peaks are seen on my LP-100A watt meter. My AVG talk power is about 45-60 watts depending on mic and drive.

I hope this helps. The best thing to do is get a watt meter and dummy load and just play. A scope would even be better

Hey thanks dxhound. I will try these settings but up til now NO adjustments have helped. I'll post my findings and thanks again !
 

dxhound

Active Member
Nov 17, 2006
783
58
38
I think you will find reasonable results without having to go inside the radio. These rigs would require ALOT ALOT ALOT of modification to produce high level modulation, and that would likely result in compromising SSB fidelity.

As said above in reference to my earlier post 80 watts peak with a 20 watt carrier is 100% modulation( hence my reference to 4 to 1) But when done properly, overmodulating the signal(also known as cliping) can add a punch to the audio(effectively the same as an AM speech processor) Again, a scope is needed to do this correctly and monitor the flat toping.
 

ButtFuzz

Anti-BS Advocate ● WO0WOO ● Reverend Doctor
Aug 11, 2005
584
398
173
Sunny Salem, VA
robco.ath.cx
You are experiencing the effects of modern low level IF modulation.

Ding! Ding! Ding! Ding! Ding! Ding!

And almost ALL "modern" Ham rigs do this very thing.

Even some Export CB radios (such as the President Jackson) apply the mod low level instead of "high" level (such as a "vintage" Cobra 148 GTL and others, of course) and that is why people have been driven insane trying to get certain radios to "sound good" on AM. Or at least sound like a "big radio", in the CB parlance.


Winner! Winner! Chicken Dinner!
 

Shockwave

Sr. Member
Sep 19, 2009
3,778
3,269
273
Just as a side note, low level modulation is not the problem. Low level modulation can be made to sound excellent without the need for any additional audio power that would be related to a high level modulator. In fact it is much easier and efficient to Hi-Fi low level modulation and reproduce extreme audio peaks.

The real problems center around the characteristics of the balanced modulator and the crystal filters that follow. The audio level used to feed the balanced modulator is of sufficient power to modulate an RF stage in many HF rigs. I usually go for the first RF stage after the last TX mixer.
 

W5LZ

Crotchety Old Bastard
Apr 8, 2005
6,832
820
173
Oklahoma
jwhawg,
No, I'm not saying you did anything to your radio. I'm saying you may have some unrealistic ideas of what it will do. That 'low level modulation' thingy pointed out in later posts, is another way of saying the same thing.
So why don't they build radios like they used to do, that 'high level' modulation thing? Cuz it's too expensive for only producing one modulation mode that's very seldom used or needed. Sort of like why they don't make steam engined cars. Sure, you can, but what's the point? Don't know about you, but I got a 'problem' with any $1K plus radio. Buying the same $1K radio for a gob more than that just because it does AM better, is just not gonna happen. Would you want that same radio for twice the price? That's not as much exaggeration as you might think.
- 'Doc
 

Alcahuete

Member
Oct 27, 2009
2
0
11
43
California
as the radio is putting out 130w on other modes chances are they are misaligned too,as it should only be doing 100w,it reeks of a cb'er or amateur with lack of knowledge trying to screw extra wattage from it with no concept of the consequences of doing so,if radio is opened for wideband tx it was almost definately a cb'er.

I assume you're referring to my radio, since it's the one doing 130w, and not jwhawg's, but in any event, I guess you didn't read my entire post. Allow me to clarify for you:

I bought this radio BRAND NEW from Yaesu itself back when they first came out. It was never messed with; in fact, I've never opened it up. After it was having an issue, I took it back to YAESU ITSELF in Cerritos to have it repaired. Their techs. are apparently the ones who did this, whatever it is they did. Their written reports showed everything operating within specs. after the repair. I can't imagine the Yaesu guys would mess something up, but then again, their quality control seems to be hit-and-miss. This was years and years ago though.

The other comments are interesting as well, regarding it just not being a great AM radio. Agreed that most amateur radios are not spectacular on AM, but it was working great before the Yaesu guys screwed with it. In fact it worked quite well brand new from the factory. I'm particularly liking the comments about the deadkey being set too high. That's definitely something to look at. It's only supposed to be running at 25w according to factory specs.

[EDIT]

Okay, some more information... jwhawg, I pulled the radio out of my car a few minutes ago (no, that didn't look shady at 1am :p) and ran a few tests. It is indeed swinging backwards on the s-meter of the other radio. Figuring that's absolutely bizarre because what the wattmeter is showing is an ADDITION of power output.

The test equipment tonight is my beloved LP-100A, a borrowed Bird 43P, fed into a Vectronics 1500w dummy load.

Low Power:
Bird - 2.5w deadkey, 11w modulated
LP100 - 2.6w deadkey, 11w modulated

This is fairly universal across all bands and modes

High Power:
Bird - 64w deadkey, 130w modulated
LP100 - 63.8w deadkey, 131-ish w modulated

Fairly consistent across all bands and modes. Some dropped to around 120w, 160m showed 140-144w


So, the wattmeters are completely contradicting what the S-meter and ammeter are showing. At low power, there is about a 1 amp drop, and 2-3 amps at high power. ALC is within normal, acceptable limits, at least according to the ALC meter on the radio.
 
Last edited:

dxhound

Active Member
Nov 17, 2006
783
58
38
The drop in current has to do with what I was describing. The carrier will actually drop when audio is appied instead of the inteligence being added to the carrier. As mentioned, duty cycle comes into play here also. The amp draw drop is "normal" As for your radio meter, I dont know what to tell you there other then alighnment.

And DONT run the carrier that high, you will cook the finals. Id stay below a MAX of 30 watts25 to be safe!
 

W5LZ

Crotchety Old Bastard
Apr 8, 2005
6,832
820
173
Oklahoma
"The drop in current has to do with what I was describing. The carrier will actually drop when audio is appied instead of the inteligence being added to the carrier. As mentioned, duty cycle comes into play here also. The amp draw drop is "normal" As for your radio meter, I dont know what to tell you there other then alighnment."

Change a couple of words in that and you'd be closer to being right about what happens. As it stands it isn't correct.
The carrier's power level should not change at all, it stays constant. If it does, something isn't right. That's with the current/voltage applied at the input to the carrier 'generator' (Oscillator).
The output power of the modulation stage changes with the audio applied to that stage. No input audio, no output power from the modulation stage (circuit). Audio is not constant, it fluctuates with the frequency of the audio applied.
When you combine these two signals you produce an amplitude modulated signal, which when not over driven by the modulation will produce a signal who's power level fluctuates between the sum of the powers of both signals to zero power level at 100 percent modulation.
The power supplied to the two stages will only be as low as the max power of the carrier. That's the power drawn from the power supply/battery. The power when measured at the output of the transmitter will be a max of the sum of the two stages, carrier and modulator. That's normal, to be expected. That input power is never the total drawn from the power supply, there are the circuits themselves which consume power, along with any other circuits of the transmitter. You can't use the measured input power to the transmitter as an indication of output power with out accounting for efficiency. Power in will always be more than power out. Nothing is 100% efficient.

The other 'end' of it, what you see on an 'S' meter.
The minimum reading on that 'S' meter is band noise with no received signal. That level may or may not be constant, but figure on it being constant at whatever level you see indicated with no signal present. When a signal is received, that 'S' meter will read higher and gives you an indication of how 'strong' that signal is (the reason for having an 'S' meter to start with). If that 'S' meter is accurately calibrated it can give you a fairly accurate estimate of the strength/power output level of that signal is you know what the 'other' factors in that calculation are (distance between you and that station, ERP of both antennas, and more). That calculation can certainly get close, but is never entirely accurate (one of those "at least this much" guesses). Knowing enough to do that calculating almost never happens, aside from the fact that 'S' meters are almost never calibrated accurately. So, what you have is ONLY an indication of a received signal's strength above band noise, and no two stations are ever the same. All of which is sort of beside the subject of 'reverse swing', but you should know so you can understand what you're seeing.
If a signal has 'reverse swing' it is an indication of less power output on modulation being applied to that carrier (on AM, no such thing as 'reverse swing' without that carrier). Which means that the modulation and carrier powers are 'canceling' each other. Which means that the applied modulation's power level is higher than the carrier's power level, which produces a 'negative' power which only means it's sucking power out of the ether instead of putting power into it. Which! If you know anything at all about this stuff is impossible. (But a neat thought though, ain't it?) What's actually happening is that there is a time period (microseconds) when no power is being transmitted. That causes the 'S' meter needle to drop back or 'swing in reverse'.
All this is the long way around of getting to the fact that 'reverse swing' is an indication of over modulation, which results in less or no transmitted power. The root problem is the over modulation part. All the rest of this is just leading you by the hand through the whole process so you can see what/why it's happening.
ALC has to be adjusted properly for that ALC meter to tell you anything. It sounds like it isn't adjusted properly. Why? I have no idea. If you get the knowledge, understand it, and remember it, you don't have to put up with all this crap of someone telling you. It's your own fault.
- 'Doc

Some times it's easier to understand if you think of it in terms of CW and SSB combined which yields AM mode. (How'z that for complicating things??)
 

dxhound

Active Member
Nov 17, 2006
783
58
38
The carrier's power level should not change at all, it stays constant. If it does, something isn't right. That's with the current/voltage applied at the input to the carrier 'generator' (Oscillator).
The output power of the modulation stage changes with the audio applied to that stage. No input audio, no output power from the modulation stage (circuit). Audio is not constant, it fluctuates with the frequency of the audio applied.
When you combine these two signals you produce an amplitude modulated signal, which when not over driven by the modulation will produce a signal who's power level fluctuates between the sum of the powers of both signals to zero power level at 100 percent modulation.
The power supplied to the two stages will only be as low as the max power of the carrier. That's the power drawn from the power supply/battery. The power when measured at the output of the transmitter will be a max of the sum of the two stages, carrier and modulator. That's normal, to be expected. That input power is never the total drawn from the power supply, there are the circuits themselves which consume power, along with any other circuits of the transmitter. You can't use the measured input power to the transmitter as an indication of output power with out accounting for efficiency. Power in will always be more than power out. Nothing is 100% efficient.



What you say is true, for a plate modulated rig. But these radios do not produce "true" AM. When you look on an ossciloscope you will see a loss of pkpk voltage on the carrier when modulation is applied (Im not talking about just probing the output of the final RF stage). This becomes real evident when looking at it through a RF demodulator.
 

kc4eoe

Member
Jun 18, 2018
12
12
13
74
Very interesting...new member and happened to find this site after a Yahoo search for something totally unrelated. What do you know, this thread was right near the top.

I have had a very similar issue with my radio (FT-900AT) for quite a few years. The funny thing is that this happened right after a visit to the Yaesu factory (in Cerritos) for some warranty work. The radio for whatever reason blew a resistor and capacitor (I believe, though I don't have the exact report on-hand right now), which ultimately ended up with nothing but RF and no audio at all. After getting the radio back, it appeared to be backswinging on AM as well.

I haven't tested it side-by-side with another radio in years, so I don't remember if it was actually swinging backwards on another radio's S-Meter. The ammeter on my power supply shows a good drop in amperage when using the radio, from deadkey to modulation, but after hooking it up to a watt meter, I got some interesting results. My peak-reading watt meter showed that it would deadkey around 40 watts and swing up to about 130 watts when talking. The radio itself puts out about 130 watts on all other modes as well.

So I guess I'll sit it side-by-side with another radio one of these days and see what the S-meter is saying.

Interesting thread. Nice to finally come across a discussion on this. :)
If your FT-900 is putting out that kind of power on AM mode, then someone has definitely put the golden screwdriver to it.
 

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