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 Post subject: Simple Solid State TRF with Selectivity
PostPosted: Jan Sun 06, 2013 5:20 am 
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I know, I know, Transistors and Solid State in general are treated like second-class citizens around here, but, oh well, lol.

Some time back (about 24 years ago), I ordered some ZN-414 IC's, from Circuit Specialists, in Arizona. A year ago or so, I obtained a PC Board for a TRF circuit. I went ahead recently and scrounged the rest of the parts from my ample supply of junked Transistor radios, and proceeded to build the unit.

The performance floored me. This circuit with the antenna Ferrite bar and coil I used operates as well as any 5 Transistor Superhet, and in fact seems to not be as receptive to spurious noise as most Superhets are.

It tracks stations from 540 to 1650 Kilocycles, and discriminates between closely spaced transmissions as well. Swamping the dial by blowtorches is almost non-existent.

One very important aspect I discovered is the particular coil used, especially the oscillator winding (which is employed to provide feedback). I experimented with quite a few before finding one that caused excessive oscillation until I shortened it's lead going to the PC board. Using as long a Ferrite bar with it, as I could fit in the case, was also helpful in increasing the gain.

I used the front half of a Transistor radio cabinet backwards, and cut a piece of Acrylic plastic to fit, drilling the circular pattern of holes for the speaker. I know I'm not as proficient as some of you are AFA cosmetics go, but I'm happy with the result.

I used a piece of counter laminate for the sub chassis. I had originally placed the battery supply vertically between the speaker and the PC board, but found that it diminished signal reception, so re-oriented it along the bottom.

The PC board in the second scan is larger than actual size. I can post an actual size of it if anyone wants to make a copy.

In the third scan, the TRF IC is the black Transistor looking unit in between the tuner on it's left, and the E-Cap and resistor, to the right. Although the Ferranti made ZN-414 is no longer made, identical units are available (MK-484, and one or two other designations as well).


Attachments:
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finished 1.jpg
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Inside case.jpg
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component side.jpg
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PC board.jpg
PC board.jpg [ 63.67 KiB | Viewed 3522 times ]
Schematic.jpg
Schematic.jpg [ 127.23 KiB | Viewed 3522 times ]

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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Jan Sun 06, 2013 7:19 am 
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I've seen the ZN414 circuit on the net, and I'm glad you had good results with it.
I always wondered what it would do.
It's cute, actually.

Now the $64,000 question, fitzies...

How many times during the making of this creation did you shun off the wife and husbandly duties?
HAHAHAHA!

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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Jan Sun 06, 2013 10:24 am 
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RepairTech wrote:

Now the $64,000 question, fitzies...

How many times during the making of this creation did you shun off the wife and husbandly duties?
HAHAHAHA!

Nothing that an offer for dinner out didn't cure! :wink: (I know my woman by now! Only took most of 40+ years.)

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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Jan Sun 06, 2013 6:18 pm 
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Normally the tuned circuit would be between points C and D. That is a novel circuit, since it minimizes loading on the tuned RF stage, giving higher Q and better selectivity. I like that approach!

Pete

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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Jan Mon 07, 2013 8:45 am 
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It also gets around the problem of having to "float" the variable capacitor frame, as in typical ZN414 configurations:

Image

Rich


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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Jan Tue 15, 2013 4:24 am 
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Another way not to float the cap frame is to ground the cap then insert the 10nF cap between the cap frame and coil as seen in my MK-484 circuit.

Image


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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Jul Sat 29, 2017 3:09 pm 
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Hi Everyone,

I found a very interesting article about the history of the many different versions of the simple AM one chip radios.

From the mid 1970's onwards, if you wanted to build a simple AM radio, chances are the circuit you would use was based around the ZN414.

n the 1980's, two more versions of the IC appeared in an 8pin DIL package. These were the ZN415 and ZN416 and were simply a ZN414 with an audio stage capable of driving low impedance headphones.

Eventually, Ferranti went the way of many semiconductor companies, and the ZN414/ZN415/ZN416 were no longer being produced. However, all was not lost as the Asian manufacturers had cloned it under a number of different types. First was the YS414 and then came the MK484, LMF501T, TA7642, and various other xx7642 types. The MK484 and TA7642 are the most common types these days.
It would be interesting to know the development of the ZN414. It is possible the concept came from one of two places. In the U.S, National Semiconductor had their LM372 in the late 60's.
Functionally, this is a very similar IC. However, it has a low input impedance, requiring a tapping on the aerial coil when used for a TRF receiver, and runs off a higher supply voltage. The LM372 has a gain of about 60dB (slightly less than the ZN414), is in a different package, and has more of its internal circuitry available to the outside world. It was intended to be used as an IF amplifier/detector, but Electronics Australia published a TRF circuit with it operating in the broadcast band, fed from a ferrite rod aerial, as one would with a ZN414. A later project saw it used in a 27Mc/s superhet remote control receiver.

The complete article with many schematics is located at: http://members.iinet.net.au/~cool386/zn414/zn414.html


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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Oct Mon 30, 2017 7:17 pm 
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I recently found another interesting receiver using the MK484.

Let me know what your thoughts are about this particular design.


Attachments:
Design1.jpg
Design1.jpg [ 79.32 KiB | Viewed 686 times ]


Last edited by Frank512 on Oct Tue 31, 2017 3:07 am, edited 1 time in total.
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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Oct Mon 30, 2017 7:53 pm 
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The print has symbols showing that it's most likely a design out of Europe or Britain.
Seems somewhat complex, and I'm wondering why the nine volt supply, for a chip designed to run on about .9 to 1.1 volts (plenty of Transistors can operate on 1.5 volts).
Interesting that they use an IF coil as the input tickler to the chip. Let's see, 6 coil windings, same number of Diodes, and I can't count high enough for the resistor and capacitor numbers, lol.

Is there any review on how well it's supposed to operate?

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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Oct Mon 30, 2017 8:21 pm 
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The "Design1.jpg [ 79.32 KiB" circuit is a superhet. So it was pretty natural to use an IF transformer as the IF transformer. If you inserted a proper IF filter there, this circuit should work about as well as the common 5 or 6 transistor radios did.

The MK484 is used for the IF & detector stages. It's run on about 2VDC.

Ted


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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Oct Mon 30, 2017 9:01 pm 
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If Q1 is a PNP as shown it cannot work.

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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Oct Mon 30, 2017 9:52 pm 
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Peter is correct, it will not work..

Maybe its a misprint.


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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Oct Mon 30, 2017 11:37 pm 
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You are right. PNP won't work. Symbol and specified BC558 are PNP.

There is at least one more schematic error: Varicap D1 has no DC bias for its anode. The whole varicap tuning scheme seems suspect. I don't see how they would get it to track, and the oscillator tuning needs the series pair of diodes more than the antenna tuning does.

I suspect that this schematic is a mash-up of parts of other schematics. I wonder how many poor fools tried to build it. The basic idea may not be bad, but details do count!

These sorts of errors are very common in circuits found on the internet, as they were in the old "Design Ideas" columns in trade magazines.

Ted


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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Oct Tue 31, 2017 5:51 am 
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Usually Lurking wrote:
The MK484 is used for the IF & detector stages. It's run on about 2VDC.

I don't think so. Much above 1.5 volts will destroy it. It was designed to perform best at right around one volt, but can tolerate up to about 1.6 volts.

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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Oct Tue 31, 2017 8:53 pm 
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fifties wrote:
Usually Lurking wrote:
The MK484 is used for the IF & detector stages. It's run on about 2VDC.

I don't think so. Much above 1.5 volts will destroy it. It was designed to perform best at right around one volt, but can tolerate up to about 1.6 volts.

Don't the 3 diodes perform as a voltage regulator of about 2.1 V and doesn't the 1K R10 further drop the voltage to the required value?


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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Oct Tue 31, 2017 9:19 pm 
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LM386 wrote:
fifties wrote:
Usually Lurking wrote:
The MK484 is used for the IF & detector stages. It's run on about 2VDC.

I don't think so. Much above 1.5 volts will destroy it. It was designed to perform best at right around one volt, but can tolerate up to about 1.6 volts.

Don't the 3 diodes perform as a voltage regulator of about 2.1 V and doesn't the 1K R10 further drop the voltage to the required value?

The power supply in that circuit is 9 volts, and you stated that the MK-484 was run on about 2VDC, which is incorrect.

Yes, I would imagine that the Diodes and resistor drop the voltage, but considering the errors in that circuit previously pointed out, we can't be sure that they will adequately do the job; the entire circuit design is suspect.

The best operating voltage for an MK-484 is between about .9 and 1.1 volts. An NiCd 1.2 volt battery would be ideal.

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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Oct Tue 31, 2017 11:28 pm 
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fifties wrote:
you stated that the MK-484 was run on about 2VDC, which is incorrect.

No I didn't.

Although a Google image search reveals most MK484 circuits use 1.5V, many use a higher voltage and compensate by using a larger series load resistor.

It is also difficult for me to understand how 2V connected through a 1K resistor for a device which draws 0.3 ma. could damage any semiconductor device. It is likely that the recommended maximum voltage 1.5V is required so that the internal AGC circuit remains effective. For those of us that don't have your insight I was hoping that you could explain the failure mode that will "destroy" it if the circuit is run from 2V.

Although the data sheet recommends a maximum voltage Vcc of 1.5V this is the voltage on the output pin, the same data sheet indicates that the usable circuit voltage using a 1K load resistor can vary between 1.1v to 1.8V the higher value of which can usually be obtained buy using 3 diodes in series as a voltage regulator.


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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Nov Wed 01, 2017 1:05 am 
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LM386 wrote:
fifties wrote:
you stated that the MK-484 was run on about 2VDC, which is incorrect.

No I didn't.

You are correct; Usually Lurking posted that. Apologies.

LM386 wrote:
Although a Google image search reveals most MK484 circuits use 1.5V, many use a higher voltage and compensate by using a larger series load resistor.

It is also difficult for me to understand how 2V connected through a 1K resistor for a device which draws 0.3 ma. could damage any semiconductor device. It is likely that the recommended maximum voltage 1.5V is required so that the internal AGC circuit remains effective. For those of us that don't have your insight I was hoping that you could explain the failure mode that will "destroy" it if the circuit is run from 2V.

Although the data sheet recommends a maximum voltage Vcc of 1.5V this is the voltage on the output pin, the same data sheet indicates that the usable circuit voltage using a 1K load resistor can vary between 1.1v to 1.8V the higher value of which can usually be obtained buy using 3 diodes in series as a voltage regulator.

The only reason for a circuit employing an MK-484 to use a voltage higher than a 1.5 Volt battery, is if there are other active elements (Transistors) that require the higher voltage. If you'll notice the circuit in my OP, there are two resistors between the 3 volt supply (needed for the Transistors), and pin 3 of the IC, in order to reduce the voltage.

I have not personally tried frying one by feeding it more than 1.5 volts, but my understanding from researching it is that anything at or above 1.8 volts will do it in.

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 Post subject: Re: Simple Solid State TRF with Selectivity
PostPosted: Nov Wed 01, 2017 5:12 am 
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fifties wrote:
The best operating voltage for an MK-484 is between about .9 and 1.1 volts. An NiCd 1.2 volt battery would be ideal.


I tried a NiCd 1.2V, wouldn't work. Ended up with a "D" cell and resistor to get the 1.2V.


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