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Old 7th Nov 2018, 7:12 pm   #21
samoca58
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Default Re: ZN414 and TA7642 radios

This is one of my TA7642 radio superhet experiments.
A dualgate mosfet as a self oscillating mixer feeding an IF transformer feeding a ceramic filter followed by another IFT feeding the TA7642.
Works very well . Sensitive and selective.
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Old 7th Nov 2018, 7:27 pm   #22
Biggles
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Default Re: ZN414 and TA7642 radios

I like that. It's neat and compact.
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Old 7th Nov 2018, 8:02 pm   #23
Neil Purling
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Default Re: ZN414 and TA7642 radios

To those who have contributed images of their own creations: Jolly 7, Ian G4JQT and Samoca 58.
I am sure that we'd like to know more about them and see more images.

Here's that little kit I got. it seems to be of Thai origin. The instructions were written in Thai and English. I haven't tried it yet. Operating voltage is 3V & the o/p is into 32 ohm earphones or headphones.

I have another circuit I made on stripboard that I really ought to mount onto something.
It was a modification of an article out of 'Electronics Australia' that exerted more control over the ZN414. I will provide the link, when I find it again. My modification was to remove the audio stages. I am not sure whether to get one of those ready-made TBA820M circuits or feed it into a Mazda SP61, to see what output I get.
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Old 8th Nov 2018, 1:11 am   #24
Jolly 7
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Default Re: ZN414 and TA7642 radios

Here is the circuit diagram of the radio I made. For the audio stage I used an LM386N based amplifier from eBay. The radio section works fine from around 0.9 V to 1.5 V but as expected the amplifier needs around 9 V.

From my experience I found that the radio had the best gain when assembled on copper stripboard rather than perforated board. Also keeping the MW coil to one end of the ferrite rod rather than the centre gave me the maximum number of stations without any duplicates. Something to bear in mind is that often radio programs are simulcast on different frequencies, so they are not necessarily unwanted image frequencies of the same station.
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Old 8th Nov 2018, 2:50 am   #25
Jolly 7
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Default Re: ZN414 and TA7642 radios

Quote:
Originally Posted by samoca58 View Post
This is one of my TA7642 radio superhet experiments.
A dualgate mosfet as a self oscillating mixer feeding an IF transformer feeding a ceramic filter followed by another IFT feeding the TA7642.
Works very well . Sensitive and selective.
Very compact and interesting design. Might it be possible to have the circuit please ?
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Old 8th Nov 2018, 10:57 am   #26
Neil Purling
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Default Re: ZN414 and TA7642 radios

Here is a page of ZN414 circuits. That Thai Future Kit is there too..

http://members.iinet.net.au/~cool386/zn414/zn414.html
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Old 8th Nov 2018, 12:11 pm   #27
nutteronthebus
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Default Re: ZN414 and TA7642 radios

My Dad worked at Ferranti's and got me one of the pre production ones in the early 1970's ( I was still at school) I built a matchbox radio with it

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Old 8th Nov 2018, 12:50 pm   #28
samoca58
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Default Re: ZN414 and TA7642 radios

I have included some more pictures and a circuit as requested.
The first picture shows the mini TA7642 superhet board transplanted into a Perdio case. The new circuit uses the Perdio tuning cap, Ferrite aerial, wave change switch and volume on/off control. The other mini circuit is an LM386 audio amp. The Original OC45 IF circuitry on the left is isolated.
The next picture is the circuit.
The ceramic filter and IF transformers I purchased on eBay. A lot of 10 SFl455c devices can be obtained relatively cheaply.
The 3rd picture is my very first experiment using aerial, tuning cap and osc coil from an old Hong Kong radio.
Lastly is a picture using a similar dual gate mosfet version using Fab old Denco coils with varicap tuning !
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Old 8th Nov 2018, 3:36 pm   #29
Jolly 7
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Default Re: ZN414 and TA7642 radios

Many thanks for sharing these.

In your circuit you have used two ceramic resonators with a resonant frequency of 450 kHz. Do the oscillator coil and two IF transformers also need to have an intermediate frequency of 450 kHz ?

I can make out the red oscillator coil but not the colours of the two IFTs.
Which IFT would these correspond to on a standard superhet radio ? Yellow, white or black ? I have a few different IFTs and resonators in my parts box.
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Old 8th Nov 2018, 3:51 pm   #30
samoca58
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Default Re: ZN414 and TA7642 radios

The ceramic filter that I purchased comes with a matching If transformer. I purchased 10 of these and the IFt and the ceramic filter can be removed from the original case easily. For the 2nd IFt I just used another. In the Perdio the osc coil was originally feeding a 470khz If chain. It didn't present any problems feeding the 450khz filters. Just a slight tweak to align. The two IFt s are identical.
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Old 8th Nov 2018, 5:13 pm   #31
Jolly 7
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Default Re: ZN414 and TA7642 radios

That's brilliant. I hope to build one similar to yours in the near future. All the best.
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Old 8th Nov 2018, 5:26 pm   #32
Neil Purling
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Default Re: ZN414 and TA7642 radios

Was the Perdio radio defective or had poor performance? It looks like the additions could have improved performance by adding more IF gain.

I was going to try that Thai Future Kit. I was sure that I had some tiny 32 ohm speakers, but can you ever find stuff when you need it? I didn't fancy trying to solder those tiny wires of that daft ear bud they supplied.
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Old 12th Nov 2018, 6:52 pm   #33
Neil Purling
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Default Re: ZN414 and TA7642 radios

The Future kit is a pair of BC458's for audio amplification wheras the 'Electronics Australia' circuit had only one. I am wondering what will serve as the power amp TDA2822M or a LM386?
It depends on if you can change the gain by any means, as you can with the LM386. I certainly have a bunch of TDA2822m & there are some LM386 somewhere.
I need to get my 'scope & sig gen out to do some testing.
Perhaps I ought to make amends by amplifying one of these boards with hot bottles, rather than cold Silicon?
I could power the radio section from a pair of LR44 shirt button cells (in parallel) & they'd last for ages.
But'i'd need to do something about the impedance mis-match.
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Old 14th Nov 2018, 10:59 am   #34
Neil Purling
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Default Re: ZN414 and TA7642 radios

Has anyone tried measuring the output from the chip in it's simplest form using a signal generator?
I was just wondering. So I can use the most appropriate audio amplification.
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Old 14th Nov 2018, 3:41 pm   #35
samoca58
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Default Re: ZN414 and TA7642 radios

Although I have not scoped it the data sheet for a TA7642 indicates the audio output at only about 20mV. With this connected to a lm386 I had to configure the lm386 for maximum gain.
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Old 14th Nov 2018, 3:58 pm   #36
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Default Re: ZN414 and TA7642 radios

I've never looked at the output of a ZN414 on a scope, but I recently took a peek at the TA7642 and found it to be rather non-linear, compressing the bottom of the audio waveform (supplied from a sig gen with sine-wave mod).
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Old 14th Nov 2018, 4:37 pm   #37
Neil Purling
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Default Re: ZN414 and TA7642 radios

Andrew2: What frequency did you inject the signal at & what p-p voltage?
I was hoping that somebody had looked at what was the sort of level of an incoming broadcast and used a signal generator at the same sort of level.
Wondering how many millivolts p-p the sine wave o/p was.
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Old 14th Nov 2018, 4:56 pm   #38
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Default Re: ZN414 and TA7642 radios

Hi Neil - I can't remember now, but it was pretty small (about -60dBm to -30dBm, this being about the level of our local signals) at 1 MHz, 90% modulated at 1 KHz.
The envelope waveform directly from the generator was fine.
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Old 14th Nov 2018, 7:00 pm   #39
G6Tanuki
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Default Re: ZN414 and TA7642 radios

There was a design for a ZN414 radio [using a Plessey SL403D audio amp] in a late-1970s Practical Wireless, they called it the "Pop Star". I built a sort-of clone of it but used a LM380 as the audio-chip.

It never really satisfied - biggest problem I found was that even with the moving-vanes/chassis of the variable capacitor well earthed to supply-negative through a good 0.5uF capacitor paralleled with a 4.7nF, it suffered with bad hand-capacity-induced hum which made tuning a burden.

In the end I remember I tried - and failed - to convert it to varicap tuning using a BB113 and a 10-turn 'helipot'.

I've still got the ZN414 somewhere - and am sort-of inspired to dig it out for some experimentation!

BTW there was also in a similar-era PW one of their "Take-20" circuits that used four Lockfit transistors (BC149?) and a handful of passive components in a ZN414-equivalent circuit.
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Old 14th Nov 2018, 9:51 pm   #40
Ed_Dinning
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Default Re: ZN414 and TA7642 radios

Hi Gents, the cheap and cheerful TL431 voltage ref can be configured as an audio amp for this type of set, plenty of app notes on the web.

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