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AM for the FT-102, Even If You Don't Have an FM/AM Board!




 
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Author Topic: AM for the FT-102, Even If You Don't Have an FM/AM Board!  (Read 34086 times)
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Steve - WB3HUZ
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« on: June 12, 2005, 06:31:31 PM »

This is the NE602 modulator info. The info posted is specific to the using the modulator in the FT-102, but the basic circuit could be modified to work in other radios or even as part of a homebrew TX.

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


Some Test Results

The pix below are of one I built on a small perf board (2" x 2" or so). Better component placement would yield a smaller board, but the one I built fit easily in the space where the Yaesu FM/AM Board would go (see second photo - board is in lower right corner of open access panel on bottom of the FT-102).






FT-102 driving a pair of 3-500Z in GG linear service

200 watts out - clean 180-200% modulation
300 watts out - clean 150% modulation
400 watts out - clean 100 % modulation

This equates to roughly 20, 30 and 40 watts output from the 102 barefoot.

Frequency response 10 Hz - 186 kHz (-3 dB). Yea that's right, 186 kHz on the high end. How ridiculous is that!?! I could be 400 kHz wide if I wanted! KAYMOAN.

Ultimately, the distortion products are defined by the IMD of the 102's final amp: around -40 dB. The modulator itself is much better. Measurements with a spectrum analyzer showed harmonics (by harmonics I mean if I'm modulating with 1 kHz, on the spectrum analyzer I see the "legitimate" sidebands at +/- 1 kHz, and then some other sidebands at +/- 2, 3 and so on) at least -55 dBc (dB down from the carrier level) at 100 % modulation. Most were more than -60 dbc. In fact I could only see the 2nd, 3rd and just barely the fourth on the analyzer since I was up against its dynamic range/noise floor.

At 150 % modulation no harmonic was greater than -50 dBc. Similar results were obtained at 200% modulation. Higher harmonics (4, 5, 6) were close to -70 dBc. Very clean!

Things weren't quite as clean when going through the 102 final amp. But I tweaked the NE602 carrier injection to achieve at least -35 dBc even at 150% modulation. It was -40 dBc at 100% modulation. This is as good or better than almost all SSB setups on the air.

This last point is somewhat critical for users of this modulator. You can produce some pretty ugly looking spectrums if the carrier injection control is not adjusted properly. I can't think of a good/easy way to do this without a spectrum analyzer. I suppose you could do it with a receiver tuned to one of the harmonics (preferably the 2nd or 3rd) on one of the sidebands and then adjust the carrier injection for minimum signal. This would get you pretty close. The reality is that it's a trade-off between the 2nd harmonic and the higher order harmonics, most especially the 3rd (most of the rest are usually so far down the aren't worth worrying about).
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K1JJ
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« Reply #1 on: June 12, 2005, 06:49:10 PM »

Yes, a nice mod for the FT-102!

I might add if you do this mod, there is no key up
overshoot and no reason to do the MAL overshoot
mod.

I have four FT-102's here, all with this NE-602 mod.
I can attest that once done, the rig produces perfect
AM hi-fi audio. Well worth the effort.

73.
Tom, K1JJ
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« Reply #2 on: June 14, 2005, 07:35:31 AM »

Thanks for posting this.

The only HF I've been able to do for the past few years has been mobile from my truck via an IC-706 (blah).  I've just moved to a new QTH where I can have some antennas and now slowly building the new shack.

One of the rigs I've been considering for my base rig is the FT-102 and this thread pretty much confirms that, with a little work, it'd be a decent lil rig on AM.

Now to go out and find one...
 
73's

JT
W6FO/4
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K1JJ
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« Reply #3 on: June 14, 2005, 01:15:01 PM »

JT,

Put a "WANTED" ad on the QTH.com classifieds in the HF transceiver section.  

You will probably get many responses and be able to pick and choose.

Axe each seller, "what is there about this FT-102 that does not make it perfect electrically, cosmetically and operationally?"  They will usually spill their guts thinking you are a perfectionist and wanting to avboid trouble later.

Axe them if the relays were replaced. You do not need an FM board... it will go way cheaper w/o it. Axe if the VFO dial binds slightly as it turns. This is a common mech problem from shipping damage. Have them pack it so NO weight  gets put on the VFO dial or it will happen to you.

There's other questions to axethat maybe some others can chime in...

I acquired three of my FT-102's this way in a short period of time.

73,
Tom, K1JJ
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #4 on: June 14, 2005, 02:06:54 PM »

Thanks for the tips Tom.

There is currently one on ebay that I am looking at bidding on once the end time is a bit closer.  The case isn't as perfect as I'd like it to be but it appears to be in good working order.

73

JT, W6FO
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KI4NR
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« Reply #5 on: April 30, 2008, 07:45:42 PM »

The use of the NE-602 will function as AM Modulator if you unbalance it to create a carrier. The down side of doing this is you now have a four quadrant multiplier modulator and the unbalancing setting is very critical. It will produce great positive audio, but if modulated over 100 percent on the negative. it's starts to appear as a double sideband suppressed carrier which on a AM receiver sounds like semi sideband.This is what most of the Japanese radios do for AM.  A better and less complicated circuit would be using a 3SK73 FET dual gate and modulating one the Gates of the device in an IF amp configuration. This would be a two quadrant multiplier modulator that acts like plate modulation and when driven over 100 percent on the negative and cuts the carrier off and does not do the "sounding like sideband thing" I call it synthetic plate modulation and has tremendous positive audio peak potential also.

73 John KI4NR

LPC Wireless
http://www.qsl.net/ki4nr/


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Steve - WB3HUZ
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« Reply #6 on: July 03, 2008, 10:17:01 PM »

Having used the NE602 modulator for years, I've never received a complaint or comment that I sounded like sideband due to excessive positive peaks.

You are correct, in that a four-quadrant mixer can produce very high positive peak modulation, essentially, up to the point of pure DSB.  But in an actual rig like a FT-102, the  RF driver and/or RF final amplifier will run out of head room before the positive peaks exceed 150% to maybe 200% (at most), when running at normal carrier levels (15-25 watts). So, the system is self limiting for the most part.

I've found that 150% positive peak modulation is about the max before many receivers, either due to the detector and/or the AGC, begin to have problems and create distortion. I generally run the audio at about 120-130% positive peak modulation, with the system set up to be able to do 150%. This way, things stay clean, and even if a peak sneaks through, it shouldn't be exceeding the head room of the system. YMMV.


The use of the NE-602 will function as AM Modulator if you unbalance it to create a carrier. The down side of doing this is you now have a four quadrant multiplier modulator and the unbalancing setting is very critical. It will produce great positive audio, but if modulated over 100 percent on the negative. it's starts to appear as a double sideband suppressed carrier which on a AM receiver sounds like semi sideband.This is what most of the Japanese radios do for AM.  A better and less complicated circuit would be using a 3SK73 FET dual gate and modulating one the Gates of the device in an IF amp configuration. This would be a two quadrant multiplier modulator that acts like plate modulation and when driven over 100 percent on the negative and cuts the carrier off and does not do the "sounding like sideband thing" I call it synthetic plate modulation and has tremendous positive audio peak potential also.

73 John KI4NR

LPC Wireless
http://www.qsl.net/ki4nr/



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KD6VXI
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« Reply #7 on: August 24, 2008, 10:41:50 AM »

The use of the NE-602 will function as AM Modulator if you unbalance it to create a carrier. The down side of doing this is you now have a four quadrant multiplier modulator and the unbalancing setting is very critical. It will produce great positive audio, but if modulated over 100 percent on the negative. it's starts to appear as a double sideband suppressed carrier which on a AM receiver sounds like semi sideband.This is what most of the Japanese radios do for AM.  A better and less complicated circuit would be using a 3SK73 FET dual gate and modulating one the Gates of the device in an IF amp configuration. This would be a two quadrant multiplier modulator that acts like plate modulation and when driven over 100 percent on the negative and cuts the carrier off and does not do the "sounding like sideband thing" I call it synthetic plate modulation and has tremendous positive audio peak potential also.

73 John KI4NR

The NE602 has been used for years as a low-level modulator in the CB radio President Jackson Export, as well  as the Uniden HR2510 10 meter radio.  Both have AM.  Both are capable of > 100 percent modulation both positive and negative.

The NE602 is NOT the downfall in the rice burner rigs.  The method of ALC / AMC is.

Put a small negative voltage on the ALC input, and you lose a LOT of the distortion on those radios.  Decrease the ALC to where a standard 100 watt radio is capable of about 150 watts on FM carrier, and keep it at 100 watts PEP AM, and you have a fine sounding radio.

I have used a directly modulated TS850 for quite some time.  More than capable of > 125 percent modulation.

Another problem people attribute to the transmitter is the receiver and the people on the RX end using a(n) noise filters.  Kill your ANL and blankers, and > 125 percent modulation sounds good again.  More than 125 percent positive, and your signal resembles noise to the blanker, and the ANL type circuits diodes can't cope with the waveforms.

Just my two cents worth.  The easiest modulator is a darlington pair of transistors.   Been using it for years, called an AM Shunt Regulator or AM Shunt Modulator.  Capable of Hi Fi operation and works well.


--Shane


LPC Wireless
http://www.qsl.net/ki4nr/



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wj3v
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« Reply #8 on: September 18, 2011, 08:31:50 PM »

Ok guys, I want to build this circuit for my FT 102. I have everything I need except the slug tuned coil. Anyone know where I might find one?
Thanks!
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Pete, WA2CWA
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« Reply #9 on: September 18, 2011, 08:51:39 PM »

Ok guys, I want to build this circuit for my FT 102. I have everything I need except the slug tuned coil. Anyone know where I might find one?
Thanks!

Junk box part: Construction details for the coil are in the upper right of the circuit drawing.
http://www.amwindow.org/tech/htm/602mod.htm
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Steve - K4HX
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« Reply #10 on: September 18, 2011, 09:22:33 PM »

Or order an 8.2 MHz IF can from Yaesu. Numerous ones are used on the optional AM/FM board. You can get the part number from the schematic.
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K1JJ
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« Reply #11 on: September 18, 2011, 10:09:00 PM »

Todd,

As long as you can tune for a peak at 8.2 mhz, the coil form is not critical. You may adjust the value of the 220pf parallel cap that resonates with the coil too.  I used a .29" diameter coil and slug with that documented turns ratio just from junk parts and experimentation. Who knows what the core permability is... Grin  The wire size can be most anything too.

However, make sure you get a good, clean signal out of this transformer to drive the FT-102 IF.  It may take some playing around with these values. You may need to change the turns ratio somewhat too to get a good power/voltage transfer. If the I.F. signal is not strong and clean, the AM audio peaks will look stunted and fuzzy. Once you get it right, the audio peaks out of the rig will be stellar and rise to 300% positive easily. (With greatly reduced carrier as set by the 1 meg pot)  Though, 150% is about the maximum practical limit on the air.

Good luck with it.

Tom, K1JJ
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
wj3v
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« Reply #12 on: September 25, 2011, 09:14:42 PM »

Thanks for the advise. I will be putting this board together soon. I have alot going on right now but will let everyone know how it goes....
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wj3v
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« Reply #13 on: September 27, 2011, 02:53:31 PM »

Looks like the link to the schematic is down. Can someone email me the schematic for this?     wj3v@hotmail.com
Thanks......
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Pete, WA2CWA
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« Reply #14 on: September 27, 2011, 05:59:55 PM »

Try this:
http://web.archive.org/web/20100818210258/http://www.amwindow.org/tech/htm/602mod.htm
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wj3v
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« Reply #15 on: September 27, 2011, 09:16:32 PM »

Thanks! Got it...........
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