TA7642 radio

[Neil Purling]

What are your recommendations for circuits that give the best chance of success.
Some way of adjustment for a local or distant signal would be useful.

I have the datasheet for the TA7642. The connections for the first and third wire are reversed, compared to the original Ferranti ZN414 data. I know to be aware of that if the diagram I am working from is for the ZN414.

[Neil Purling]

Preferably something that you have built yourself, so you can vouch for it. I see selectivity as being a potential issue, which is why I suggested examples you can vouch for.

[radioman]

Hi Neil,

I came up with the attached radio project for some electronics students. The performance is much better than you'd expect - but, this will be largely down to the antenna coil, since this is the only tuned circuit.
If you use a ready-wound (Litz) coil the Q could be so high that the circuit becomes unstable so damping with a parallel ~680k-1M resistor will cure that.
Alternatively, about 70 to 80 turns of 30swg enameled-copper wire will be fine.
Selectivity was not a problem and there is a degree of AGC due to the 680 ohm resistor.

Andy

[radioman]

I've just noticed that the GIF within the ZIP file for the PCB layout has lost it's holes !
- so please find attached a PNG file with holes for those of you who fancy a go at etching your own board.

Andy

[Neil Purling]

Can you draw the tracks on a copper-clad board in ink and then dip the board to etch it?
Does it need to be a special kind of ink to defy the etching solution?

[ParcGwyn]

This site has some really useful information on ZN414 radios.

http://members.iinet.net.au/~cool386/zn414/zn414.html

Dave

[Neil Purling]

I had a almost finished radio on a bit of strip-board and I couldn't remember where the circuit was from.
It was the Electronics Australia 1986 circuit omitting everything past the 22uf electrolytic capacitor.
The reason was because I had already made several LM386 amps on strip-board.
To finish it: I need to solder in a wire link, wind a coil on some ferrite rod and get a 100K lin pot for the RF gain. if I use a switched pot I can turn the amplifier on and off too.

[radioman]

Originally Posted by Neil Purling

Can you draw the tracks on a copper-clad board in ink and then dip the board to etch it?
Does it need to be a special kind of ink to defy the etching solution?

As long as the ink is solvent and not water based, you should be ok. I've used Steadtler OHP/CD pens in the past with success.
However, I've had a rummage around and managed to find an etched but un-drilled board for this project that you can have FOC if you'd like it - just PM me your address and I'll pop it in the post.
(It's been cut into two parts to fit the original case, the small board being the LM386 audio amplifier.)

Andy

[Neil Purling]

Thank you for the offer of the circuit board. I will take it and use it if this stripboard effort does not work. I was very careful to scrutinise the solder blobs that no whiskers might bridge adjacent strips. I might be tempted to put the gubbins into a box, or re-cycle a scrap radio case.

[Restoration73]

re post #3, if you use a coil for long wave you might need a lower value resistor.

[Neil Purling]

The ferrite rod and coils I have got are salvage from a 'trannie. I assume that they are wound of Litz wire, they usually are.

[Neil Purling]

Amongst the six-transistor radio kits on-line I found one which remarkably is a TRF: It consists of a TA7642 IC plus 9013, 9015 & 9013 transistors.
Even though the postage was three times the cost of the item I couldn't resist temptation. A pity that the kit is not more widely available from UK sources to reduce the postage. It is known as a D08.
There is that Paensonic whose AM section also uses a TA7642. Prices of those vary from just over three pounds, which is nothing for a bit of fun on a dark winter evening.

[Neil Purling]

I have done the Electronics Australia, February 1986 circuit on stripboard.
The coil is wound on some really coarse ferrite bar & is 62 turns as specified. The variable capacitor I tried first had two gangs of 120 & 180 pf, but I only used the 180pf gang.
The 100K pot varies the gain & should be a far lower value. The commentary suggests you could use a value as low as 10K which I believe would be about right.There will be some pictures of my experiments slightly later when I re-size them. I fed the signal from the circuit into my Mullard 3-3 for now, but i'd like to knock up something else, if a LM386 on full gain of 200x is not enough.
I believe an output of 15mV was quoted, but I have not gort around to making a test with a signal generator and a 'scope.

[Neil Purling]

The Electronics Australia circuit is quite good. It has exposed the shortcomings of my home built LM386 amplifier section. Built on stripboard it is just too spread out for total stability with a gain of 200.
It is quite remarkable just how well the LM386 IC can do into a decent speaker fed by a radio circuit like the one I have already made. I have no idea how potent the IC's I have actually are.