TA7642 AM radio chip

[ka1iic]

Has anyone played around with the TA7642 AM radio chip?  I've been digging around in my junk box and found a few of these chips and figure it might make a nice winter project. Not just for AM radio but for a cute/small AM unit for what few SW broadcasters left...

What say folks...

[Zazzle]

Hi there!

The TA7642 is the follow-up to the famous ZN 414 (MK). It has ups and down. They go along with the simplicity of it. You can build an AM-Radio just with a few components. But on the other hand you won't get great results from it. It can't handle large signals and with a powerful station around you will have a hard time receiving any other stations. And forget about selectivity. During night in a good location without much noises it sounds like you're in a big room with a dozen people talking.

But you can thinker with it and explore how IF-Stages work. It can be used as a -Last-IF-Amp-and-Demodulator-in-one-Chip component.

Given it's simple features it won't make much sense to use it in a receiver with a high class and complex IF setup. More than one leading IF-Stage doesn't make sense.

What I recommend is to build a classy radio frontend. Tuned circuit that mixes with a second frequency (let's say 455kHz higher than the desired frequency) followed by a simple IF-Filter. The filter can be a classy inductor based or a ceramic filter.

If you still experience trouble with strong stations still going through you can add a leading notch filter or something like that.

I hope that got you some ideas for your winter project to come.

Kind greetings,
Zazzle.

[ka1iic]

Hi Zazzle!  Thanks for your reply :-)

Your input is great-fully taken and fills in a few holes on the situation.

I didn't plan to do any serious work with the chip it's just one of those lets see what the thing can do <heh>  As in when winter gets boring and everything else just isn't in the play list.

I do have an IF stripe I built and am having great fun with... that is for a more serious receiver.  It's a dual stage cascade thingie and has a lot of gain at the moment I have the strip tuned to approximately  9500khz and it is wide band.  When I hook an antenna to the mixer input I receive everything from 9000 to 10000khz.  but I digress... 

I'm going to find out what this chip can do and that does not necessarily mean what the thing was designed for.  I can't imagine a commercial receiver being built and sold with such a chip but some folks will buy most anything.  Perhaps the intent was for rural areas with a single local transmitting station?  I've live in such areas and this would be a cheap alternative to a more costly receiver. Who knows? <heh>

The wide band part of this item is giving me a few ideas but more on that when I get going with the proto, from the original intent to what I hope might work :-)

Thanks again
73
Vince

[Stretchyman]

Tis nothing more than an amplified Xtal set aka TRF, not much use on SW (intent was for MW?) as it's got NO SELECTIVITY. Much better to use something with an I.F. albeit a low one and the TDA1072 is still available. However somewhat more complex but as simple as it gets for a 'Decent' Rx.

Si4734 is the ONLY chip worth bothering with these days and someone needs to write some software for it!

[Zazzle]

TDA1072

TDA 440

[ka1iic]

Tnx again Zazzle I'll keep that other chip in mind if I run into one cheap... as I am broke.

The TA7642 internals remind me of the old tube RF radios but with u-tuned stages.  It might have better selectivity with a couple of wave traps to null out more powerful stations but we shall see.

As for SW... well...  A tuned RF stage placed on the front end might make an interesting Solar Noise detector but I might be going into the Twilight Zone on that one... On the upper parts of the SW bands, they tend to be vacant these days.

I've got about 20 of these little beasts but I don't remember where they came from.  Someone must have given them to me to play with... I suspect that is the fact ;-)

Anyway... I'm off.... (way off) broke don't go anywhere but I have fun anyway <heh>

BTW... Do you know of any commercial radios that used this chip???