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PostPosted: Tue Aug 03, 2010 2:13 am 
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Hi all.

I’ve been wanting to play around with the “simple” radios based on the MK484 chip for awhile. Bruce Kizerian (ElmerDude) had kindly sent me some a few years ago and it’s about time to dust them off.

Sorry if all this is “old hat”.
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Reading-up on the MK484, its predecessor the ZN414, and successor the TA7642, I found that folks either love them or hate them. By far the biggest complaint is “they don’t behave well when faced with strong signals”. Those that love them seem to live in the country. :-)

The data sheet for the currently available TA7642’s is no help with the “strong signal problem” -- not much there:

http://www.rapidonline.com/netalogue/specs/82-1027.pdf

The data sheet for the older MK484 doesn’t have much either:

http://kitsrus.com/projects/mk484.pdf

BUT, the data sheet for the original design by Ferranti - the ZN414Z - is a gold mine:

http://www.jaycar.com.au/images_uploaded/ZN414484.PDF

There’s a hint in the operating notes on page 6 as to how to handle strong signals.

It says: “ ---- the gain of the ZN414Z is voltage sensitive so that, in strong signal areas, LESS (my emphasis) supply voltage will be needed to obtain correct AGC action. Incorrect adjustment of the AGC causes a strong station to occupy a much wider bandwidth ---- cause the RF stages to saturate ---- swamping ---- reduced AF output.“

ie. -- not well behaved.

Most of the MK484/ TA7642 circuits I’ve found use the recommended circuit shown on page 9 of the ZN414Z data sheet. This circuit places the same voltage on the power pin (the output pin) and the AGC line - which is the line through the 100k resistor and the tank coil to the input pin. So you have to reduce the chip supply voltage in order to reduce the AGC voltage -- they’re hooked to the same voltage.

And that’s another problem because the MK484/ TA7642 chips work well only over a fairly narrow range of voltage on the output pin. A voltage of ~ 0.9v seems typical. See for example the excellent article at:

http://www.mikroe.com/old/books/rrbook/ ... pter3g.htm

I built several radios following, or slightly modifying, the data sheet circuit - and tried to put ~ 0.9v on the output pin by proper choice of supply resistor. But they all suffered from the “strong signal problem” - and frankly all of them were a pain-in-the-butt or so-so performers.

BUT -- there’s a solution to the “strong signal problem” in an ingenious circuit referenced in:

http://cool386.tripod.com/zn414/zn414.html

It’s called a “Performance Breakthrough” for the MK484 chip. And it is!

Evidently, in ~ 1986, the engineer(s) at a company called Technicraft developed a circuit that made the ZN414Z handle strong signals well. “They” (sure wish I knew who “they” were, to give them proper credit) broke the connection between the supply voltage and the AGC voltage. Each voltage can then be separately and EASILY optimized. Once you do that, the chip is VERY WELL BEHAVED for both weak AND strong signals.

Shown below is my version of “their” circuit -- and it works like a champ, at least for the 2 different MK484 chips, and the 2 different TA7642 chips I tried.

Image

The separated voltages Va and Vb, and the 2 pot adjustments make a HUGE difference.
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The trick to setting up the chip supply voltage, Va at point “A”, is to quickly evaluate how a particular chip (and circuit) behaves. Here’s what I did.

1.) Set the AGC voltage at point “B” to 0v using Pot-B. That’s like having “no signal”.

2.) Adjust the supply voltage at point “A” to a PRECISE value - like 1.05v - using Pot “A”.

3.) Now increase the AGC voltage to about 0.8 - 0.9v or so by adjusting Pot “B” - and start tuning around the band from 530 kHz to 1700 kHz.

NOTE that once you tune-in a signal, you can increase or decrease the volume by adjusting Pot “B”. NEAT!

4.) Take note of the apparent bandwidth and sensitivity as you tune across the band. They change as you go from 530 kHz to 1700 kHz.

5.) Now, reset the voltage at point “B” to 0 again, and adjust the supply voltage at point “A” to a new value using Pot-A. And repeat the above procedure - noting the apparent selectivity and sensitivity. It doesn’t take long.

Here’s an example of the kind of quick characterization I did for each chip:

MK484 #1.

Va = 0.90v; Too sensitive, tends toward instability, Pot-”B” adjustment “touchy”.

Va = 0.95v; Excellent sensitivity & selectivity across whole band. Pot-B not “touchy”

Va = 1.00v; Excellent sensitivity & selectivity across whole band.

Va = 1.05v; Selectivity somewhat degraded at high end of band.

Va = 1.10v; Poor selectivity, lower sensitivity at high end, OK at mid to low end.

“Conclusions:

-- The “good” operating range for Va is pretty small!

-- The target Va for this chip (and circuit) is Va = 1.00v +/ - 0.025v. Use Pot-A to dial-in this optimum supply voltage. Re-adjust Pot-A as the battery ages. No problem.”

All 4 chips I played with behaved pretty much the same - except:

-- the MK484 chips had noticeably better selectivity, but less sensitivity than the TA7642’s;

-- the MK484 chips seemed closer to oscillation than the TA7642’s. I could hear the “sea shell” affect in the audio. Reducing the 100k AGC line resistor lessened the tendency, but I left it at 100k.

-- the TA7642 chips had a slightly wider “good” supply voltage operating range (Va) than the MK484 chips.
-------------------------------------------------------

You adjust Pot-B, if necessary, to change the AGC voltage to suit the station you’re tuned to - weak or strong. When Pot-B is changed, the supply voltage Va on the output pin is also slightly changed from it’s “set” value. Hopefully, it does not get out of the preferred range as determined during the chip characterization exercise described above.

In the graph shown below, I’ve plotted the way the chip supply voltage on the output pin (Va) changes as the AGC voltage at point “B” is changed - by varying Pot-B.

Image

There are a couple of things to note about the graph:

1.) The voltage Va changes as Vb is varied - but seems to stay in the “good” range.

2.) You need an AGC voltage at point “B” near ~ 0.6v to tame very strong stations.

3.) Very weak stations - even right next to strong stations - can be heard by increasing the AGC voltage at point “B” to near ~ 0.9v.

Ordinarily, in the “standard” MK484 circuit, if you had set Va = 0.9v --- then Vb would also be 0.9v, and the radio would respond well to a weak signal, but have trouble with a strong signal.

-------------------------------------------------------

I’ve included a picture of my setup, --- it’s a mess. But it works very well. I suppose it demonstrates that the MK484 & TA7642 are more tolerant than we think.

Image

Without an antenna - just the 4” ferrite rod - and in the basement, the “mess” can easily pick up my moderate - to - strong daytime stations, and a few weak ones too.. And at night, it hears - and separates - lots of DX.

With an antenna/ ground to a Tuggle tuner -- lightly coupled to the MK484/ TA7642 tank, the “mess” becomes a hot DX radio. And it’s a well behaved hot DX radio. :-)
-------------------------------------------------------

Thanks for putting up with another long-winded post. I get excited about these “simple” radios ‘cause it’s all new to me. And great fun.

73, Dan


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PostPosted: Tue Aug 03, 2010 3:52 am 
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Dan,

Thanks for posting this! This is a great breakthrough on dealing
with that chip!

Also, did not now there was a new replacement. Did I miss
where that was available?

Not long-winded, just lots of good info.

Long-winded could be two sentences, that mean nothing :shock:

Thanks again!

BTW: Couldn't make the link work for the TA7642 spec sheet.
Did find it here:

http://www.datasheet4u.com/html/T/A/7/T ... C.pdf.html

Mike

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PostPosted: Wed Aug 04, 2010 5:12 pm 
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Location: Boston, MA
Hey Dan,

A few questions:What is the value of the cap at +vdc,it just says "100" but has no unit indication.I have an idea of value but want to be sure.

Re varying voltage: It seems that there is an ULTRA narrow range of "Va" voltage to optimize performance. Is there enough arc in the motion of the pot to be able to find this sweet spot "by ear",even as the battery voltage drops with age -OR- does the radio have to be measured w/ a digital volt meter every time to set this critical voltage.

I suppose one could include DVM banana jacks on the radio front panel.



K


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PostPosted: Wed Aug 04, 2010 5:50 pm 
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Location: The Netherlands
You could use a seperate Q-multiplier for enhancing both the sensivity and the selectivity with a receiver based upon the MK484.


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PostPosted: Wed Aug 04, 2010 6:42 pm 
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Location: Allentown,PA
Hi Guys.

Mike - yes, pretty clever these Australians. :-)

Glad you found another link for the TA7642. The ZN141 data sheet is BY FAR the best. I found the TA7642's at:

http://www.electronics123.com/

krystallo, hope your set turns out well.
-- all caps are in uF ( I forgot to specify that); I used 100 uF there only because I had one. :-)
-- the "good" Va range IS very narrow. Others have called attention to that too. I used a DVM to measure Va - and judged "goodness of response" across the band by ear. I like to measure stuff when I can - it just removes another guesstimate that confuses me. Pot-A shouldn't have to be readjusted often (if nothing is changed in the circuit once set) but it's NOT a set-and-forget deal either.

DrM - good point. I think you can fairly easily introduce a bit of regeneration into the circuit to increase selectivity & sensitivity. I had some of that by accident in one set-up. Would be fun to play with that in the future.

73, Dan


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PostPosted: Wed Aug 04, 2010 8:59 pm 
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Location: Boston, MA
Also forgot to ask, what would be the desired Z at the audio out?

K


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PostPosted: Wed Aug 04, 2010 9:21 pm 
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Hi K.

I don't know.

I'm using a Radio Shack Amplified Speaker for convenience while fiddling around. I think it has ~ 5k input.

Also use a piezo ringer element with 1 meg across it - works good.

73, Dan


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 Post subject: LMF501
PostPosted: Wed Aug 04, 2010 9:42 pm 
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Posts: 187
There's also the Mitsumi LMF501 chip:

http://www.datasheetarchive.com/pdf-dat ... 00872.html

The real trick is finding a source of any of these obsolete and
elusive devices.

Dave


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PostPosted: Wed Aug 04, 2010 10:42 pm 
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Location: Western Washington State
Dan,

OK, I see that your original link for that TA7642 IC
works now. Glad you have found a source.

I made your posting into a PDF if that's all right?

Anyone can e-mail me for that, if that's OK with you.

I thought the posting was so great that I wanted to
put it into my MK484/ZN414 files for future reference.

I do have two, overwhelming stations here, and that has
detered me from doing a lot of experimentation with
those chips, of late. This may be a great breakthrough
to overcome that "selectivity" problem with those chips.

I still have a few ZN414's, but have never compared them
with the MK484, as to a direct comparison. Has anyone done that?

Thanks for this great info!

BTW: I have lots of MK484 chips, so e-mail me if anyone wants any.

Mike

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PostPosted: Thu Aug 05, 2010 2:24 am 
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Dave - thanks for the LMF501 info. They show a slightly different audio amp than the other data sheets. That’s helpful for a beginner like me.

Mike - fine by me. See below for the author who uncovered this “breakthrough”.

Just wanted to mention that it was John Hunter in Australia who wrote the very informative article at:

http://cool386.tripod.com/zn414/zn414.html

that brought the MK484 “Performance Breakthrough” circuit to my attention. (I got permission to mention his name on the RadioBoard so I could give credit where credit is due.)

John has a LOT of interesting stuff on his web site:

http://cool386.tripod.com/index.html

73, Dan


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PostPosted: Thu Aug 05, 2010 8:14 am 
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OK Dan,

Fine biz.


I just wanted to see if there was any degrade by using the wrong Z reproduction device. But this doe not seem to matter,at least per your experiences.

K


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PostPosted: Fri Aug 06, 2010 2:11 pm 
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Hi all.

Here’s an update.

1.) Reduced the Vcc supply to an AA rechargeable battery (1.33v). I’ll let this run continuously for a few hundred hours to see how the set handles a drop in voltage and increased battery resistance.

--- Eventually, if all goes well, I’d like to put a solar cell on the radio to trickle charge the battery.

2.) Added just a touch of + feedback to narrow the selectivity a bit. It also reduces the audio bandwidth a little which helps my bad ears. I don’t know if this is really worthwhile or not -- have to play with it some more. It’s fun to fiddle with.

3.) Added a headphone amplifier as described in the LM501T datasheet. (Thanks WA4QAL, Dave.) I had tried a few others, but this worked about the best - at least for this setup. Plenty of volume even for night time DX.

--- A “Walkman” type headset is used with the amp. Very comfortable. All I have is a flea-market unit that measures ~ 36 ohm per element. I’m only using one element -- eventually I’ll wire both elements in series to give ~ 70 ohms and maybe pick up even more volume.

4.) Put the whole thing on a “Lazy Susan”. The radio can be easily rotated since there is no external antenna or ground hook-up.

--- This REALLY helps null-out a particularly loud daytime station 3 miles away. At night - when they reduce power - it’s not needed. It’s kind of fun to rotate the thing and play “direction finder” games.

Image

Image

I’ll post another update when the battery runs down.

Have to start learning about solar cells. :-)

Great fun.

73, Dan


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PostPosted: Tue Aug 17, 2010 1:13 am 
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Location: Brisbane, Australia
Hi guys,
been on 'holidays' from my radio parts box for a while now - bigger fish to fry elsewhere for a while. The original article on the ZN414 receiver chip appeared in an edition of Electronics Australia (now defunct) magazine. I'm not sure which month it is, but you can still get hard copies from Silicon Chip magazine, who now owns the copyright on all SC, EA and even Radio, TV and Hobbies mags, right back to around 1934.

The original article was written based on the circuit and pc board layout designed by Technicraft, and they even supplied a complete kit of parts, including an old style vernier dial for fine tuning.

They charge the locals around $9.00 AUD for an article, posted within the land of Oz, and maybe a little more for overseas. I'm not sure whether they can online it for you via the Net, if you use a credit card, but there's no harm in asking.

Austin Hellier
Downunder


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PostPosted: Tue Aug 17, 2010 6:49 am 
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Image

Hi Dan ,

What if you put a diode such as a 1n4148 in series with the bottom legg of pot B so there would be a constant voltge drop of .6 volts across the diode ,so the bottom of the pot would be .6 volts . This would alow you to use a switch and two resistors to have repeatable voltages buy shorting out one resistor so you would have .6 volt when one resistor was shorted and .9 volts when the switch was open or you could use a smaler value pot that is .9 volts centered and .6 all the way to one end .

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Mark


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PostPosted: Tue Aug 17, 2010 3:39 pm 
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Austin - thanks for the additional info!

Mark - I’m finding that you really want to be able to vary the voltage at point B. It acts as a VERY nice volume control.

73, Dan


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