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Old 5th Nov 2018, 8:15 pm   #1
Neil Purling
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Default ZN414 and TA7642 radios

Is it usual that the selectivity is apparently wider at the HF end of the band if you had one working on the AM broadcast band?
It just seems like that.
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Old 5th Nov 2018, 9:34 pm   #2
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Default Re: ZN414 and TA7642 radios

That sounds right for fixed L tuning.

Lawrence.
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Old 5th Nov 2018, 9:41 pm   #3
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Default Re: ZN414 and TA7642 radios

My experience is that selectivity isn't great on these radios. Not surprising with one tuned circuit. Strong local station breakthrough is another problem. I live less than five miles from a 50Kw Radio Five Live transmitter. You can imagine what happens. Try experimenting with a double tuned RF input circuit?
Alan.
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Old 6th Nov 2018, 12:23 pm   #4
Neil Purling
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Default Re: ZN414 and TA7642 radios

I was wondering about the choice of components around the chip & whether they would make any difference.
How would you make up the double-tuned RF input circuit?
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Old 6th Nov 2018, 12:29 pm   #5
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Default Re: ZN414 and TA7642 radios

I doubt it so far as making the selectivity more constant across the band, selectivity is always better at the low end of the band if it's variable capacitor tuning.

Lawrence.
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Old 6th Nov 2018, 2:31 pm   #6
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Default Re: ZN414 and TA7642 radios

I enjoy experimenting with the TA7642 . The third version I built works mostly fine where I live in the Southampton area. I can tune in from 531 kHz to 1557 kHz, but no further down than that. In London the reception was good in West Drayton, but poor in Southall due to a local transmitter signal being 'smeared' across a large portion of the dial.

I used a 4 inch ferrite rod and MW coil from an old radio to get the best results and a good quality polyvaricon tuning cap.
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Old 6th Nov 2018, 2:52 pm   #7
Neil Purling
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Default Re: ZN414 and TA7642 radios

There's a DIY kit that utilises the oscillator coil from a superhet radio as the inductance, which is interesting. I bought one.
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Old 6th Nov 2018, 3:12 pm   #8
Jolly 7
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Default Re: ZN414 and TA7642 radios

Can you share the seller's details please. I found a page in Japanese where the designer has used two oscillator coils.
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Old 6th Nov 2018, 3:13 pm   #9
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Default Re: ZN414 and TA7642 radios

I purchased from China 50 TA7642 for 1.66! and have experimented with them in simple superhets using the TA7642 as the If amp and detector. I have been impressed with the results for such simple circuits.
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Old 6th Nov 2018, 5:04 pm   #10
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Default Re: ZN414 and TA7642 radios

Many years ago, back in the days when a tablet computer was something that took up an entire floor of a drug company HQ, I built an HF radio by using a 455kHz IF transformer as the tuned circuit of a ZN414. The link winding was driven by a FET frequency changer. It worked, but selectivity wasn't good, as you'd expect.
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Old 6th Nov 2018, 5:44 pm   #11
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Default Re: ZN414 and TA7642 radios

Here is my completed TA7642-based radio. It picks up lots of stations in the evenings including Radio Caroline.
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Old 6th Nov 2018, 7:24 pm   #12
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Default Re: ZN414 and TA7642 radios

The L/C ratio, varying as you tune across the MW band, will result in varying selectivity, yes.

[Perhaps look at permeability-tuning?]

I never really had much luck with ZN414-type radios, though I did use one - fixed-tuned to 465KHz - along with a single-transistor BFO to "SSB-ize" a little Daiwa "Search-9" VHF receiver back in the 1970s, following an article in SWM. The result wasn't too good as the Search-9's free-running VFO, though good enough for FM, didn't really match up to the needs of SSB reception.
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Old 6th Nov 2018, 7:27 pm   #13
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Default Re: ZN414 and TA7642 radios

Quote:
Originally Posted by Karen O View Post
I built an HF radio by using a 455kHz IF transformer as the tuned circuit of a ZN414. The link winding was driven by a FET frequency changer.
The idea of using a ZN414 as an I.F. stage for a superhet receiver was explored in F. G. Rayer's book 'How To Make Walkie Talkies' (Babani BP43, 1977).
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Old 6th Nov 2018, 11:21 pm   #14
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Default Re: ZN414 and TA7642 radios

I once tried a ZN414 to convert an FM only radio scanner into an AM/FM model, way back in the eighties. It took a sample of 455KHz IF signal from the IF chip, before the limiter, and demodulated it. It didn't have the benefit of AGC and used to overload and "flatten out" on large signals, but it worked after a fashion. Neil; The double tuned RF input circuit I had in mind was to top couple two ganged tuned circuits with a low value capacitor. I thought this may sharpen up the selectivity a bit but probably not by much.
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Old 7th Nov 2018, 10:06 am   #15
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Default Re: ZN414 and TA7642 radios

There was an article covering the ZN414 in August 1978 Practical Wireless.
(A.M. Receivers - Devices and Circuits - Part 1)
https://www.americanradiohistory.com...PW-1978-08.pdf
This includes a double tuned circuit.

Andy
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Old 7th Nov 2018, 10:17 am   #16
Neil Purling
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Default Re: ZN414 and TA7642 radios

A TA7642 as an IF amplifier sounds interesting.
Is that from the original Ferranti ZN414 tech sheet?

radioman: There's a missed page of the article mentioned.

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Old 7th Nov 2018, 10:38 am   #17
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Default Re: ZN414 and TA7642 radios

Quote:
Originally Posted by SiriusHardware View Post
The idea of using a ZN414 as an I.F. stage for a superhet receiver was explored in F. G. Rayer's book 'How To Make Walkie Talkies' (Babani BP43, 1977).
I used this idea in a fixed-tuned SW pocket radio in the 80's. I don't know how many remember 'Radio Fax' in the 49m band, but I wanted something I could listen to it on at work and I didn't want to take my posh radio in.
So it was a tuned whip antenna feeding a 40673 RF amp, followed by the then new NE602 as mixer. Can't recall the oscillator (I may have used the NE602's on-board osc) but it had a fine tune control. The resultant 455 KHz IF was handled by a Murata ceramic filter and a ZN414 which drove an earpiece. It worked remarkably well!
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Old 7th Nov 2018, 10:45 am   #18
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Default Re: ZN414 and TA7642 radios

I made a radio for our bathroom that only receives 198 kHz Radio Four. It's a ZN414 and LM386, completely standard circuit, although with an adjustable 1.5V to set the gain of the ZN414 (actually it's an MK484 equivalent) for LW. No idea how much longer 198 kHz will be in use (at east a few more years I believe), but it gets a great deal of use.

General points using ZN414 or equivalents: make sure the shaft/moving side of the variable capacitor is connected to the earthy side of the tuned circuit and not to the IC input. This helps reduce hand-capacity effects.

The capacitor on the output of the IC has various uses in conjunction with the AGC resistor (See ZN414/MK484 data sheet) and can also effect treble cut. I've found 100nF is about right.

It's a good idea to have some sort of voltage adjustment (and many circuits show this) for the ZN414 to adjust its gain for your location because they can overload if you're near a strong station. Likewise you can turn up the gain if there's not much around you. More than about 1.65 volts on MW leads to instability - depending on component layout.

Ian
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Old 7th Nov 2018, 11:20 am   #19
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Default Re: ZN414 and TA7642 radios

Quote:
Originally Posted by Neil Purling View Post
...
radioman: There's a missed page of the article mentioned.
Oops - sorry I should have checked...
I've scanned the article from my own copy of P.W. and it's attached below.

Andy
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Old 7th Nov 2018, 6:21 pm   #20
Neil Purling
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Default Re: ZN414 and TA7642 radios

There are kits that employ the TA7642 and two transistors to give speaker sound.
There's another that uses a TBA820M for audio, ZX2026. I am interested in that one as well, as it would have more audio 'oomph'. The TBA820M is that which is used in your amplified PC speakers.
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