TA7642 (ZN414) IF Amp

Now that is an interesting coincidence. But it is quite a simple and common technique.

Yes, there is a new-member link-posting delay. This to prevent the occurence of the SPAMmer who registeres abd then posts loads of links for Rollox watches and the like. We had quite a lot of those on the previous forum.

Once you are a member and waited a week then you can post as soon and often as you like. It is only a problem for new members for a few days, and it makes liafe a LOT easier for me and the two moderators. I do hope you understand.

BR Harry

Since I promised schematic, here it is:
https://servimg.com/view/17950553/2
I forgot to address the link you gave yesterday, Harry. Sorry about that, and yes, the similarities is just a "happenstance".
If there is something I could improve on the circuit, but still keep it simple, do not be afraid to let me know, and that goes for anyone.
It seems like there is a 7 day link posting denial for new users here. Can't even upload an image.

Interesting. The link I gave you http://www.sm0vpo.com/rx/zn414.htm is also a receiver for 145MHz, 27MHz and was used in my AM R/C TX/RX project.

Ok on the coils. Good to know that you have gotten to the bottom of the troubles. Nice to know you had a nice weekend.

BR Harry - SM0VPO

If you search on the net for the GEC Plessey datasheet/application note for ZN414, you will at he end find an example for a 27MHz RC receiver. Instead of the Mullard LP1175/2 filter I have something similar with just an IF coil and a ceramic filter, and in the same schematic you will also see they use a 5 Volts supply with a 10k resistor to supply the chip. I am using a red LED as a "zener" and a 4.7k, so I believe I'm actually starving the TA7642 a bit.
As of now, I can't see that I have problems actually. Everything seems to work just fine and the problems I thought I had in the beginning disappeared completely when I swapped my homebrewed mismatched(to the ceramic filter) IF coil with a proper one.
I have made so many undocumented changes to the circuit now I can hardly remember where I was, and what I did. There is nothing special about the circuit I believe. It has all been done before.
I will try to post a schematic as soon as I can.

I would love to see some schematics. As I understand it, the TA7642 is identical to the ZN414, and also the YS414, MK484, UTC7642, LMF501T and LA1050.

When the ZN414 died I didn't realise it had a family. But about a year ago I got a load of MK484 from China.

Now this particulr thread I did not follow since I have been busy the past couple of days, but as I understand it there are problems with filter BW and device gain. Is this correct?

The ZN414 had a 100K resistor from input to output but the "collector" resistor was also rather critical for the gain. The problem with this device is that it runs from a very low supply voltage so a DC supply voltage change of 0.15 V represents a 10% variation.

The projects I made taught me several things, for example:

1 - The supply voltage should be about 1.35 to 1.4 V
2 - The input load must be fairly high impedance

No 2 is quite important because it allows some negative feedback as well as DC biasing, but if you use a low input impedance then the gain rises to unwanted levels.

I am looking forward to the schematic. Is it anything like my [url=http://www.sm0vpo.com/rx/zn414.htm]http://www.sm0vpo.com/rx/zn414.htm[/url] project?

Very best regards from Harry - SM0VPO

Photos and schematic are coming up later, I hope.

Excellent, glad it worked for you.

Do you have any photos of your project to share? I for one would love to see it

Sorry, I haven't copied the whole thread, so it seems like I'm talking to myself here.
-----------------------------------------------------------------------------------


Just a simple review of the ceramic filters that I got in the mail yesterday (Friday 25.11.12).


42 of them where bad. Some just plain duds. Some with a dip on the frequency. Some far off the specified frequency (like 400kHz).

39 somewhat usable, with reasonable Q and not too far off the frequency (+/- 5kHz).

22 Just excellent.

Was it worth it? Well, I'll guess so, since I probably will have to pay close to a dollar for one piece, or maybe even more, from a "good" source.
I kinda expected there was some duds in the pack so I'm not really disappointed.

Tomorrow I think I'm going to play with a suitable oscillator for it. Maybe let it cover the 49 and 41 meter bands by a 6 to 8 MHz hooter. Oscillator will then work OVER the RF on 49 meter, and UNDER on the 41 meter band. So far the simple radio is working excellent with the IF filter setup and the TA7652 amplifier. It beat my Chinese "Denver" pocket shortwave radio with a digital frequency readout, in both selectivity, sensitivity and noise. Not bad IMHO.