Question on NE602 modulator on FT-102

[KG4DAG]

I've built up on of these AM modulators for my FT-102 and have read all of the threads about it.
I'm just not clear on one of the details, where does the carrier injection that goes to the
1meg pot get hooked to?   Is it from LO2 or is it to the 8.2 mHz IF?  Thanks!

[K1JJ]

Hi!

There's just two inputs to the board. (And +12V, of course)  One is the audio and the other is the 8.2 Mhz I.F.  The I.F. gets injected at the pot which goes to pin 6 of the chip.    The "carrier pot" has no input and just sets the ratio between AM carrier and audio peaks.  IE, you may want the FT-102 to do 10 watts of carrier and 100 watts pep -  or maybe 20 watts of carrier and 100 watts pep, etc.

I'm not sure what you mean by the "LO" or local oscillator.  Maybe you thought there was an input to the carrier pot. The label, "carrier inject" is probably misleading.

Here's the schematic I am using:

http://www.amwindow.org/tech/htm/602mod.htm


I've built up four of these and they work FB. You will be pleased once you get it working well. The input levels and tuning of the output coil are somewhat critical to getting perfect audio and flawless modulation, so play around with it.  The output coil match needs to be correct and will have a big effect on form and quality. Adjust the turns to get the highest and cleanest I.F. output peaks when using a sine wave audio source at the input of the chip.

73,

Tom, K1JJ

[KG4DAG]

Ok. Thanks Tom, now it makes sense to me!   Yeah... It was the "carrier inject" that
stumped me... He He....

[KG4DAG]

I finally got a day off and managed to get the modulator stuffed in the rig.
At first, the output was weak with the 220pF cap so I took it out and
tried a few others. When you hit the sweet spot you'll know it! There
is really quite a sharp peak where the output jumps up dramatically.
150pF worked for me.

After a few adjustments, I think I have it working pretty well. Here's
a few scope pics on the output of the modulator...

X10 probe, about .3V 3kHz sine wave from my generator, around 100% modulation


a bit more from the function generator, over 100%  


2 kHz triangle wave in at about 100%


more in and over 100% again



Anyway..... I think it's working as it's supposed to. I wish I had a spectrum analyzer, but I think
the waveforms look pretty good.
Next project is to build a balanced tuner and put up an open wire fed dipole. Maybe someday I'll get on
AM HF in my lifetime...

[K1JJ]

Good job!

I'd say it's working FB.  The key is you found the sweet spot of the tuned circuit. This does help  it to work properly.

The turns ratio can also have an effect. If you have the patience, adding or subtracting some turns may make it even better. You're using an adjustable slug-tuned core, right?

I have not used a digital scope as you have there - so the sine wave white crossover points look strange to me. But I think it may be the way digital scopes work vs: analog scopes.    

Your triangle waveform seems to have the basic desired shape, though they appear to have some slope? Shud be straight, unless it's the function generator doing it.    

If you haven't already, build a simple off-air AM diode monitor and listen closely to your own signal. There are a few designs on this BB.  You will need this to set up your audio tailored to your particular voice. The settings are different for most of us.   Get an audio EQ unit to suck out that mid-bass effect and you shud sound great with that NE-602 mod.


T

[KG4DAG]

Oh, it's an analog scope, Tek 7704A. Just quick pictures with a digital camera.
The crossover point just washes out in the pics. It seems to look fine to me. The
 triangle shape may be due to the audio response not being flat all the way to DC.
It's a good test wave that shows things that are hard to see with sinewaves.

I just want to button up the 102 and move on to something else... I did a lot of work on it.
All the relays...PS caps... Schottky diodes in the detectors and the noise blanker. Cyan LED's in the meters
Cleaned all the pots and some of the trimpots. Aligned it...HiFi audio mods, All sorts of stuff... Must have had that audio board out like 10 times...

It seems to put out like 180W on 40, but the ALC pulls it right back down to 100 or so. Probably like it's supposed to.
I saw the one resistor ALC mod, but I'm gonna wait 'til I get it on the air to mess with that.
Time to button it up and move on!  

[K1JJ]

OK on the analog scope and digital camera.


The ALC mod is critical to clean operation on AM. You do not want the ALC shifting the carrier level on peaks. It will create an undesirable pumping action to your audio.

T

[K1JJ]

BTW, Frank / WA1GFZ suggested we add an emitter resistor to the transistor for better linearity and stability.  You might experiment with this for an optimum value.

T