Question about modifications to the FT101 AM Modulator to get real high fidelity

[steve_qix]

If you've ever modified an FT101, how far did you take the modifications?

I'm working on an FT101E for a friend (I do not own an FT101 myself).  He wanted a negative peak limiter.

So, the absolute easiest and probably best thing to do first is to completely bypass the internal audio.  Ok, easy.  Use the phone patch connector; connect it to the modulator input; add a switch to allow either the internal audio system or the external input to be used.  Done!

Next, I tested the modulator for linearity.  Ok, hookup the triangle wave generator and have a look.  Ooops.  Bad.  Reasonably linear from about 85% negative to about 105% positive.  Looking at the design, the modulator is incapable of linear modulation in the negative direction above a certain percentage.  Ok, at least fix the positive peaks.  The modulator operating point is wrong.  This is easily corrected by removing R8 (a 2.2k resistor) and putting in a 5k pot.  This allows you to set the operating point.  Ok, now we get 200% positive with fairly good linearity.

It's pretty good, but the non-linearity still exists on the negative peaks.  This does not appear to be correctable without actually adding another device (MOSFET or transistor) and then applying modulation to the power supply line going to the existing modulator rather than using the base modulation scheme they're using now.

Has anyone here solved this problem?

Ok, the non-linearity actually enhances the positive peaks because it "compresses" the negative peaks, but it does add a certain amount of distortion, both harmonic and intermodulation distortion and this can definitely be heard when you compare it to a transmitter that does not have this distortion.

Oh, I did do the negative peak limiter.  I'll publish the circuit shortly.  It can be used with any good quality transmitter that has *excellent* VERY LOW FREQUENCY response (down to 1 cycle per second) such as a Flex, etc.

[N2DTS]

Did not w3duq come up with a DC to light modulator for the ft102?
Should work with the 101 as well, no?

http://amfone.net/Amforum/index.php?topic=32949.0

You could also do these mods while in there..

http://amfone.net/Amforum/index.php?topic=32896.0

[K1JJ]

Unfortunately, the NE-602 is a balanced modulator based modification and will generate the dreaded wavelets that Steve is tring to avoid.  The FT-101 uses an amplitude modulation scheme that will generate conventional 100% cutoff and large positive peaks without wavelets.  

T

[kb3ouk]

I have a pair of 101s and a pair of 901s, and after messing around with them I can honestly say the easiest solution would be to just get rid of the entire audio section and build an external modulator. It looks to me like they are base and collector modulating one of the buffers to get the modulation. if it were me, I would bypass the entire audio chain and just build a class A series modulator connected to the collector of the buffer. After that, mess around with the final amplifier and try to get it to act a little more linear (the 101s weren't as bad there as what my 901s are).

[W2VW]

Brett wins the prize. Not sure what the prize was though.

If another transistor is added to the original FT-101 modulator it will still require some form of amplitude compression to take care of cutoff. The optional FT-102 AM board would hold the clue to Yaesu's fix.

I'd rather have the occasional wavelet. They are more forgiving spectrum wise of poor or mis-adjusted compressors. I've spent many years using balanced modulators and cheap mis-adjusted compressors.

Somebody needs to tell the current crop of 101 users heard here in 2 land that they don't use the balanced modulator on AM. These guys have done the cookbook mods and still call it a balanced modulator

That negative going modulation is important to perceived loudness and should be as close to 100% as possible without making the waveform harmonic rich by clipping without low-pass.  

A good experiment would be to utilize the existing balanced modulator for AM.

[W2NBC]

"That negative going modulation is important to perceived loudness and should be as close to 100% as possible without making the waveform harmonic rich by clipping without low-pass."

Bingo Dave! (may I call you "Dave"?)  Negative close to 100% is the ticket.. We all know the positives about "positive" asymmetrical et al .. But negative peaks close to 100% with 100% positive (or more thank the man vocal gods) is LOUD.. As others have said "Running reduced carrier DSB never really rang my chimes" and the caveat is: with lopsided positive peaks without Mr. Negative..

Signed, Mr. Negative  

[steve_qix]

The 101 uses a real modulated stage however it suffers from a certain degree of non linearity.  I have the idea of adding another device as a modulator, but I'm trying to find out if anyone has done this before me and if so, what were/are the results?

It looks quite do-able.  Of course the 2nd question is will anyone actually build it (besides me, that is!).

As mentioned in the first post of this thread, I have bypassed all of the internal audio and feed audio directly into the balanced modulator.  Any negative peak limiting and/or filtering takes place outside of the modulator itself, and if the modulator is very clean there will be no additional spurious components generated.  This is why I would like to explore improvements to the existing FT101 modulation scheme, which is somewhat nonlinear as the negative modulation gets high.  But, it is a real modulator, so no wavelets, etc.

Currently the negative peak limiter I have designed for the FT101 is in fact followed by a filter to keep the bandwidth in line.  This is the same filter as is used in the class E rigs, with the corner at 5.7kHz and should be sufficient.

The same negative peak limiter should work with any transmitter that has excellent low frequency response down to a few cycles per second.  This is necessary because most transmitters are A.C. coupled, and what will happen is when the negative peak limiter is working, there is a momentary DC shift.  If the low frequency response is not good, the DC shift will be averaged out quickly, and the entire waveform will shift down very quickly (at an audio rate).  Also, waveform "tilt" will result due to the differentiation affect of the audio system's poor low frequency response on the clipped waveform.  If the frequency response is very good in the low end, it will take time for the waveform to shift - hopefully enough time that the waveform shift will be very small before the next non-limited cycles go through the system which essentially resets everything (DC-wise).

The class E rigs and most tube PWM transmitters are DC coupled, which is why this waveform shift/tilt is a non-issue in these transmitters.  Same holds true for the 3 diode negative peak limiter that is used in some transformer coupled tube rigs.  This scheme is a DC coupled implementation as well.

The idea is to get very near to 100% negative modulation, without ever exceeding it.  The idea is to utilize every possible dB of energy available - and go well above 100% positive (cleanly) at the same time.

[Steve - K4HX]

DUQ had some mods to increase the positive peak modulation. I'll have see if I can dig them up.

[N2DTS]

If running a lower power rig, I always ran the modulation very close to 100% negative, even going over on peaks from time to time.
Not a big deal with lower power rigs, with high power rigs, pushing the limits is not needed, and would cause serious splatter.

I remember Bill (DUQ) waving his hand in front of the microphone and it would modulate the rig.
What is that, 10 Hz?

Whatever the modulation setup, I think its good to compress/limit in the low level audio stages, not in the transmitter.

I used to run the 3 diode limiter gently in a few rigs and got reports I was very wide.
It might work well with vacuum diodes, but it did not work well with solid state bricks, switching times or something.

I scrapped it and just use a compressor and watch the mod monitor closely.

[steve_qix]

I used to run the 3 diode limiter gently in a few rigs and got reports I was very wide.
It might work well with vacuum diodes, but it did not work well with solid state bricks, switching times or something.

Yeah, I don't know about those solid state bricks - they may be very slow.  I use fast diodes in the 3 diode limiters I build.  They are available in high voltages and fairly high currents, too.

[K1JJ]

DUQ had some mods to increase the positive peak modulation. I'll have see if I can dig them up.

Steve, you just reminded me of an experimental circuit you designed and I built about 20 years ago.  We wanted to artificially amplify the positive peaks more than the negative and then run it through a regular AM rig.

It was two op amps fed side by side and then combined with the gain adjustable so that the positive peak amplifier was 30% to 70% higher than the negative - artificially.  It actually worked with tones, but produced a noticeable distortion on voice. Big peaks - yellowy audio.  After that test it never saw air time again...  

T

[kb3ouk]

DUQ had some mods to increase the positive peak modulation. I'll have see if I can dig them up.

Steve, you just reminded me of an experimental circuit you designed and I built about 20 years ago.  We wanted to artificially amplify the positive peaks more than the negative and then run it through a regular AM rig.

It was two op amps fed side by side and then combined with the gain adjustable so that the positive peak amplifier was 30% to 70% higher than the negative - artificially.  It actually worked with tones, but produced a noticeable distortion on voice. Big peaks - yellowy audio.  After that test it never saw air time again...  

T

Sounds like a homebrew version of the Kahn Non-Symmetra-Mod.

[Steve - K4HX]

A JS design indeed!

[N2DTS]

Past some point, about 130% positive, running the positive modulation up just sounds like crap on a normal receiver.
Why the worry about those extreme positive peaks?

A good rig should do 130% or more at 90% negative modulation.
And if it does 110% positive at 90% negative, it does not sound much different.

I just got done playing with my 32V3, I scrapped the audio stages and put in some KT90 tubes, driven with an 8 ohm to 5000 ohm transformer, and get over 133% positive at 90% negative, not sure exactly why.
I also get at least 130% with the 813 rig which has a seperate modulator power supply, while the 2x4x150 and 3x4D32 rig gets about 110% positive at 90% negative as it has a common supply.

I do not hear any difference between them, and doubt anyone at the far end could tell any difference.

It seems to me, if you want to sound loud, you need a multi band compressor limiter like radio stations use.

Looking at the sdr display, which gives a VERY detailed display of audio density/bandwidth, I can see people who run good gear, they are the ones with a lot of audio on the carrier, and its dense over the audio spectrum.
I see other people with a mic plugged into the dx100 and the modulation is low over 50% of the spectrum and overall density is very low, with 100% modulation in a very small band of frequencies.

I also see prople with very odd EQ or problems with the setup, lots of low and mids, not much upper mids, and a bunch of audio very high, like 8 or 10 KHz.

I also know of someone who has a lot of audio up to about 2000 Hz, then almost nothing, but it goes out well past 10 KHz on the extreme highs. That is a muffled sounding but very wide signal...

The people that sound good and loud have a nice spectrial density over the passband of audio.
Lots of people (most) have very low average modulation, almost everyone who runs a vintage rig with a microphone does, although they can sound very good.
It seems 90% of people do not monitor or measure their modulation at all.

I have an odd issue with one rig, if I test the response with a signal generator, its flat over a wide range, but using the microphone, it seems to roll off badly at about 2500 Hz.

Its not in the RF deck, so I need to check other things.
 
 

[steve_qix]

An update:  I built a "real" modulator for the FT101 consisting of a couple of 2n2222 transistors modulating the collector of the existing FT101 buffer/AM modulator transistor.  It works very well !  Quite linear, and sounds a whole lot better, too.  The modification is fairly simple, although it does involve removing a couple of components from the modulator board and adding a few others.  There is room on the board to do it.

Still experimenting, but it is a viable and reasonably straight-forward improvement.

Regards,  Steve

[WD5JKO]

Steve,

   We all look forward to that circuit you are developing.

Perhaps some remember the video made in 1986 by west coast hams called something like, 'The Story of YEAM'.  In it was a section showing Norm WB6TRQ going through the trials and tribulations of re-designing an early Yaesu AM modulator circuit. Norm pointed out many of the problems, and highlighted the fixes he was trying. That video was most recently hosted by our buddy Brian, but no longer. Maybe you have a copy?

Jim
Wd5JKO

[W2VW]

Is a balanced modulator imaginary?

[steve_qix]

Is a balanced modulator imaginary?

The 101 doesn't use a balanced modulator 

[K1JJ]

An update:  I built a "real" modulator for the FT101 consisting of a couple of 2n2222 transistors modulating the collector of the existing FT101 buffer/AM modulator transistor.  It works very well !  Quite linear, and sounds a whole lot better, too.  The modification is fairly simple, although it does involve removing a couple of components from the modulator board and adding a few others.  There is room on the board to do it.

Still experimenting, but it is a viable and reasonably straight-forward improvement.

Regards,  Steve

Ya know, a stand-alone kit AM modulator with an NPL  would be quite a nice module to insert into a ricebox, or even as a low level driver for a homebrew linear chain.  It would need custom input/ output matching for the particular I.F. used, just like when inserting an NE-602 into a rig.  Or could even be configured to put out 3.8 Mhz, etc., if the devices will do it.

The FT-101 mod is a good stepping stone for experimentation.  I was gonna build up a replica of your NPL from the PDM generator board design for my 4X1 rig low level audio.  But hopefully your new NPL board will do this for me in a kit.

Will your NPL be using the same 6 KHz low pass filter or is it even needed when not having PDM artifacts?

T

[steve_qix]

Is a balanced modulator imaginary?

I say "real" (and absolutely, a balanced modulator is certainly real)  only because the FT101 applies audio to the base of the buffer stage transistor.  This achieves modulation, but it is not overly linear and there isn't an actual, separate modulator.  It is very simple.

So, the so-called "real" modulator is actually another transistor added to the circuit that modulates the buffer stage's collector voltage.  The buffer's operating point is changed to run the buffer stage at saturation (or as near to it as I can get it).

This works a whole lot better!  Did the other one work? Yes, absolutely and in reality it was probably "good enough" for most Amateur purposes.  It is distorted - no doubt about it, but it was still pretty good.  My suspicion is that only a minority of people would really notice the distortion at a remote receiver.  I could hear it in the mod monitor headphones for sure.

Ok, it is definitely being somewhat of a perfectionist with the addition of the additional audio stage to accomplish modulation (there are probably support groups to help with this neurosis !).  But, seriously, it is a whole lot less distorted.  If I then were to go on and redesign the buffer stage to move the device further into saturation, I could probably take it to the Nth degree of perfection, but at this point we have truly reached the point of diminishing returns, and probably the additional work would be for not.

[W2VW]

Is a balanced modulator imaginary?

The 101 doesn't use a balanced modulator 

Sure it does  just not on AM.

AM transmitter rally winners in 2010 and 2011 both used balanced modulators exclusively.

I thought they must be imaginary because you referred to the other modulator as real.

On a more serious note my HP sig gen's AM modulator falls apart way before 100% positive. A 3 diode circuit made it play OK for the PW net but it would be nice to get a real square law mixer working.

[steve_qix]

On a more serious note my HP sig gen's AM modulator falls apart way before 100% positive. A 3 diode circuit made it play OK for the PW net but it would be nice to get a real square law mixer working.

I've heard some sig gens on the air as low power AM generators, and at least some of them sound unbelievable!  I don't remember what models those guys were using.  I think there is a guy in Springfield, MA using one, but the call escapes me at the moment.

If you put a triangle into your signal generator, does it modulate the waveform cleanly?  Maybe you could fix it if not.  Usually HP makes good stuff.

[w1vtp]

On a more serious note my HP sig gen's AM modulator falls apart way before 100% positive. A 3 diode circuit made it play OK for the PW net but it would be nice to get a real square law mixer working.

I've heard some sig gens on the air as low power AM generators, and at least some of them sound unbelievable!  I don't remember what models those guys were using.  I think there is a guy in Springfield, MA using one, but the call escapes me at the moment.

If you put a triangle into your signal generator, does it modulate the waveform cleanly?  Maybe you could fix it if not.  Usually HP makes good stuff.

Herb, W1XU

[W2VW]

On a more serious note my HP sig gen's AM modulator falls apart way before 100% positive. A 3 diode circuit made it play OK for the PW net but it would be nice to get a real square law mixer working.

I've heard some sig gens on the air as low power AM generators, and at least some of them sound unbelievable!  I don't remember what models those guys were using.  I think there is a guy in Springfield, MA using one, but the call escapes me at the moment.

If you put a triangle into your signal generator, does it modulate the waveform cleanly?  Maybe you could fix it if not.  Usually HP makes good stuff.

I'll try that when I can walk again. Should be a few more weeks. Years back there was a guy who used a sig gen into a Harris HFL-1000. That worked.

Last few times Herb was on he had bandwidth problems. Happens to everyone.