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Author Topic: NE602 AM balanced modulator?  (Read 30767 times)
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Glenn K2KL
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« on: March 16, 2005, 10:48:52 AM »

I'd like to breadboard a few low level AM modulator circuits based on W3DUQ and K1JJ's design on the am window for use in my exciter project however, the NE602 is now obsolete  :cry:  there are replacements (SA602, SA612, NE612) but I have no idea if these will perform as well as the original NE602 when used as an AM modulator. There are a few places that sell obsolete components but I'd like to buy around a dozen and would rather not spend the $60 on 12 chips (yes, I'm a cheap ham!)  :lol:

Anyone have any experience with these replacements or know of a US source for any of these devices?

Thanks!
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w3jn
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« Reply #1 on: March 16, 2005, 11:05:00 AM »

Allied has them; I just received a bunch and they were about $2 each.  The SA612 is functionally equivalent.  MAKE SURE you order the DIP package unless you enjoy wrecking your eyes on SMD devices.

Also believe Ocean State Electronics sells them in small quantities.

73 John
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Glenn K2KL
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« Reply #2 on: March 16, 2005, 12:42:01 PM »

Hi John,

You got the SA612 or the NE602 from Allied?.... $2 is great, although I remember having problems placing small orders from them.


Quote from: w3jn
Allied has them; I just received a bunch and they were about $2 each.  The SA612 is functionally equivalent.  MAKE SURE you order the DIP package unless you enjoy wrecking your eyes on SMD devices.

Also believe Ocean State Electronics sells them in small quantities.

73 John
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w3jn
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« Reply #3 on: March 16, 2005, 01:06:38 PM »

THey have 85 in stock at $2.16 each.  I'm sure you can find some caps or other stuff you need!  I've had no problem with small (~$30) orders from them.

http://www.alliedelec.com/Search/SearchResults.asp?SearchQuery=sa612*

73 John
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W2VW
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« Reply #4 on: March 16, 2005, 01:26:55 PM »

Don't forget the AN612 as an alternative. They were used in everything from multimode VHF rigs to slimeband capa-bile C.B.'s along with many H.F. radios of the 80's. Plenty of schematics to get ideas from. R.F. Parts has these for $3.95 so a little shopping should cut that down some. I think the NTE number is 1249.
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KE1GF
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« Reply #5 on: March 16, 2005, 01:48:16 PM »

Glenn I ordered a rail of SA612's from a Phillips' small quantities vendor linked from their website.
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Glenn K2KL
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« Reply #6 on: March 16, 2005, 04:30:27 PM »

Thanks guys!!!  :!:  :!:

Should I be concerned about differences between the original NE602 and the replacements if I'm just using it for a balanced AM modulator?

There's a guy on E-bay selling SA602AN's for $2.35

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=7275&item=6511893556&rd=1&ssPageName=WD1V
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Steve - WB3HUZ
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« Reply #7 on: March 16, 2005, 11:52:20 PM »

The chip in the modulator I am currently using in a FT-102 is a SA602AN. Works FB.

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Glenn K2KL
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« Reply #8 on: March 17, 2005, 12:32:05 PM »

Thanks again folks! I just ordered a dozen SA602AN's from Allied. $26 minimum order which I just made.



Quote from: w3jn
THey have 85 in stock at $2.16 each.  I'm sure you can find some caps or other stuff you need!  I've had no problem with small (~$30) orders from them.

http://www.alliedelec.com/Search/SearchResults.asp?SearchQuery=sa612*

73 John
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K1JJ
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« Reply #9 on: March 17, 2005, 12:57:06 PM »

So, where's the control for  "I'd better drop the MAWL and squash these mud ducks with my 300% audio"  on that thing...  pot #1, 2, or 3 ? [The Mucas Member said that, caw mawn]

Quote from: Steve - WB3HUZ
The chip in the modulator I am currently using in a FT-102 is a SA602AN. Works FB.

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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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Glenn K2KL
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« Reply #10 on: March 17, 2005, 01:44:09 PM »

It's got three pills caw mawn!


Quote from: K1JJ
So, where's the control for  "I'd better drop the MAWL and squash these mud ducks with my 300% audio"  on that thing...  pot #1, 2, or 3 ? [The Mucas Member said that, caw mawn]

Quote from: Steve - WB3HUZ
The chip in the modulator I am currently using in a FT-102 is a SA602AN. Works FB.

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Steve - WB3HUZ
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« Reply #11 on: March 17, 2005, 07:49:29 PM »

That's a front panel control. I can't take the time to open up the rig just to drop the maul on some duckZ!


Quote from: K1JJ
So, where's the control for  "I'd better drop the MAWL and squash these mud ducks with my 300% audio"  on that thing...  pot #1, 2, or 3 ? [The Mucas Member said that, caw mawn]

[/quote]
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Glenn K2KL
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« Reply #12 on: March 23, 2005, 03:18:48 PM »

An important question......

Looking at the W3DUQ/K1JJ NE602 modulator schematic, I see the IF input is from a fixed freq xtal oscillator. If this IF input were say from a xtal oscillator with three different switched xtal freq's, would there be any change to the modulated output?

If I could select from three xtals at the modulator stage without affecting the modulation level or bandwidth at the output it would save an additional stage later on.

Any thoughts?
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K1JJ
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« Reply #13 on: March 23, 2005, 06:35:32 PM »

Quote from: Glenn K2KL
An important question......

Looking at the W3DUQ/K1JJ NE602 modulator schematic, I see the IF input is from a fixed freq xtal oscillator. If this IF input were say from a xtal oscillator with three different switched xtal freq's, would there be any change to the modulated output?

If I could select from three xtals at the modulator stage without affecting the modulation level or bandwidth at the output it would save an additional stage later on.

Any thoughts?


First of all, the bal mod needs no particular IF freq to run at... but, once chosen, it depends upon how sharp your resonant 455 IF output transformer tunes ... also depends on what comes after  the coil. In my FT-102, it gets injected into the IF strip which needs a precise 455.0 IF freq, unless I want to realign it.

Also, I found that the output coil is critical and needs to be peaked well in order to get the highest and most linear audio peaks.

If I understand you correctly ?? - you wish to switch in different xtal IF freqs. I would say not to do it, unless they are VERY close to the freq the board transformer and/or IF strip ahead is peaked for.  It's just a matter of degree, I suppose.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
Steve - WB3HUZ
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« Reply #14 on: March 23, 2005, 09:57:48 PM »

The NE602 AM modulator in question, and the Yaesu FM/AM modulator board the 602 modulator supplants, both operate at the 8.2 MHz IF not the 455 kHz. The tuning on mine needed to be peaked as Tom said, but the bandwidth of the output tank was fairly broad. I measured 3dB bandwidth points a over 100 kHz (IIRC is was closer to 200 kHz). And yes, the 602 would produce full, clean modulation, even with a 100 kHz "audio" input. I could be 200 kHz wide if I wanted to be!

My guess is Glenn wants to use the 602 as a VFO and modulator. If you are looking to use three different crystals, all in the 75 meter band, I would think the output of the 602 would remain sufficiently constant. Looking at it in terms of percentage bandwidth, my 602 modulator showed about 1-2 percent bandwidth at 8.2 MHz. At 3.9 MHz, this percentage translates into a bandwidth of around 50 kHz. I think this would cover the frequency excursions for an AM PW TX on 75 meters.

Of course you never know for sure until you build it.
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K1JJ
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« Reply #15 on: March 23, 2005, 10:56:09 PM »

Yep, the FT-102 is 8.2 mhz, not 455 khz. I was spacing out about the TS-940 project using the MC-1496/ 455kc  I guess.

Anyway, that's an interesting approach to use the NE602 as a direct  75M VFO/modulator. As Steve says, you will probably have enuff bandwidth to do it .  Though watch the tuning of that transformer to insure it keeps the signal looking clean. There were times when I could change the pri/sec ratio a little and saw signs of distortion. Something to keep in mind when you do the freq excursion testing.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
w3jn
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« Reply #16 on: March 24, 2005, 07:01:19 AM »

Glen, that's the thing I've been concerned about because variations in the RF input amplitude will necessarily vary the modulation index.  Just put the adjustment pot on the front panel.  Or run the mudulator at a fixed frequency and use a second NE602 to mix to the proper outpoot freq.  A third option would be a gain-controlled stage between the osc and the NE602 and an AGC detector feeding that stage to keep the level constant.

73 John
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Glenn K2KL
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« Reply #17 on: March 24, 2005, 09:15:47 AM »

Hi Tom;

You just saved me a lot of typing  :lol:  :lol: ... yea, that's the ticket, except the 602 modulator stage will have a xtal oscillator as the LO in, the VFO will be mixed in a later stage... I'll post a block drawing here later today.

I'm torn between two methods, to use the 602 as the low level modulator, or to just series modulate the driver stage of the exciter... I though the balanced modulator approach would produce less distortion, possibly a cleaner signal,....I'll be spending time this weekend hunched over a perfboard fer sure!


Quote from: K1JJ
Yep, the FT-102 is 8.2 mhz, not 455 khz. I was spacing out about the TS-940 project using the MC-1496/ 455kc  I guess.

Anyway, that's an interesting approach to use the NE602 as a direct  75M VFO/modulator. As Steve says, you will probably have enuff bandwidth to do it .  Though watch the tuning of that transformer to insure it keeps the signal looking clean. There were times when I could change the pri/sec ratio a little and saw signs of distortion. Something to keep in mind when you do the freq excursion testing.

T
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Glenn K2KL
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« Reply #18 on: March 24, 2005, 09:21:15 AM »

Hi John,

As my mother-in-law would say... "you hit the hammer right on the nail!"

To avoid any problems, that's what I'll do.. Run the 602 modulator stage at a fixed freq, then exactly as you say, use another 602 to mix the output of the modulator with the switched xtal oscillator... sounds like a plan.


Quote from: w3jn
Glen, that's the thing I've been concerned about because variations in the RF input amplitude will necessarily vary the modulation index.  Just put the adjustment pot on the front panel.  Or run the mudulator at a fixed frequency and use a second NE602 to mix to the proper outpoot freq.  A third option would be a gain-controlled stage between the osc and the NE602 and an AGC detector feeding that stage to keep the level constant.

73 John
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