Self built MW set with circuit diagram

[squinancy]

More knowledgeable people than me, please help.
I bought a DIY kit online for a child of a friend who expressed interest upon seeing my modest collection of vintage radios. Contains the circuit board, instructions and components for building a simple MW receiver set. I helped with the assembly, strictly according to the instructions. See the attached circuit diagram.
Well, it doesn't work. Not a beep comes out of it. A very slight hiss from the speaker, but can only tell when you switch on and off. At the same time, the ammeter (built in) shows 5 uA current when the pot is at its maximum and 3 uA at its minimum position when just switched on. So what could be wrong? Can someone comment please. Many thanks.

[Restoration73]

It would appear to be a modern version of the ZN414 receivers of 4 decades ago.
T1 is a class A amplifier, but I have not seen the variable resistor used in this way.
It might work if C3 is put in the top connection of the variable resistor instead of as
drawn. The main problems with the circuit:
(a) ampifier has insufficient voltage gain to drive a speaker. (needs darlington or op-amp)
(b) speaker impedance too low to get sufficient current with 1.5 v supply
(c) no dc supply decoupling for the IC1

I have made a simple diode receiver with the output feeding a darlington transistor that
can drive a 64 ohm speaker to a reasonable volume.

I hope the kit supplier provides a get you working service.

[G8HQP Dave]

Originally Posted by Restoration73

It might work if C3 is put in the top connection of the variable resistor instead of as drawn.

No, that would leave T1 with no forward bias.

This sort of set needs a higher impedance speaker. Philips used to make 80 ohm speakers; I don't know if there is modern version.

As always, when something doesn't work the first thing to do is measure DC voltages. That might show whether there is a construction error or a design error.

[squinancy]

Will measure voltages and report here. In the meantime rummage around or buy a high impedance speaker. Thanks.

[Restoration73]

Sorry I missed that - T1 would need a resistor ca 100k from base to collector. (for a
darlington transistor 4.7 Meg is used) you can use an old telephone earpiece/insert ;
I have some speakers if you are stuck.

[kellys_eye]

Golden rule of kit construction - test your own work! The transistor may have a different pin-out from the device supplied (the TO92 case is notorious for swapping the b-c leads, sometimes base is centre pin, sometimes at one side).

[Radio Wrangler]

With regard to that poll at the top, there's one option missing "Wrong design"

The circuit of this little radio shows evidence that it was designed to minimise components at all costs and to use a single cell. One of the costs paid is that of probability of success, given the crude bias arrangement for the transistor.

It's not a broadcast receiver, but for something that goes on a small board and is easy to get going, try George Dobbs' "Sudden Rx" getting signals from all over Europe and sometimes the US might be more interesting to a youngster than the routine stuff.

My definition of a simple circuit is one that's easy to get going. Components are cheap, knowledge and confidence building are precious.

David

[paulsherwin]

I agree with all the other comments. That isn't a good design and is unlikely to give good results even if you build it correctly. The most likely explanation for it not working at all is incorrect wiring of the transistor.

A much better approach for a young beginner is to build a crystal set, which is almost impossible to build incorrectly and demonstrates the essential principles involved clearly. It's then possible to add a one transistor audio amplifier to drive a speaker at low volume. If there is still interest at this stage then more sophisticated designs can be tried.

http://vintageradio.me.uk/radconnav/transtrf/

[ukcol]

Originally Posted by Radio Wrangler

The circuit of this little radio shows evidence that it was designed to minimise components at all costs....

It doesn't look like he has even succeeded in achieving that goal. See application circuit below.

When checking your circuit, make sure you have put the TA7642 in the right way round. See TO-90 picture below.

[Herald1360]

I suspect you'd get much better results using the simple application note circuit given in the TA7642 datasheet!

http://www.shenzhensum.com/products/...eet/TA7642.pdf

To use the 8ohm speaker, I'd grab an output transformer from a dead old tranny.

[Radio Wrangler]

And even the apps note circuit needs a 32 Ohm speaker.

I'm not sure there is any great merit in using a single cell.

Crystal set into a computer speaker would be interesting, then move on to a proven TRF or maybe the Sudden (George's parish on the outskirts of Rochdale was called Sudden)

The problem with flaky designs for beginners is that they strongly discourage the beginner from going any further... If a simple set is this difficult, perhaps they don't want to ever try anything more complicated looking?

David

[wireful3]

I must admit that some kits can put a beginner off. I favour Paul's approach. Starting with a simple crystal set can fire enthusiasm even if selectivity is non existent. Hearing sound for the first time from a self built set can be exciting. Initial failure because the kit is not really foolproof is similarly very discouraging.

[llama]

I deplore the drawing practice adopted - the difference between a "join" crossover and a simple crossover is as slight as a smudge of ink!

I know "bridge" crossovers are infra-dig so for the joined case the approaches should be staggered. I guess what I'm saying in this context is to check and double-check which ones are actual joins.
Graham

I remember a 'Take 20' design in PW (Practical Wireless) that would give 'speaker reception with just one transistor. The bit I remember is 'until cruelty to transistors is outlawed'. A classic reflex design using a modern 2N3819.

[Restoration73]

The ultimate
http://oldradiobuilder.com/One%20Tra...r%20Radio.html

[squinancy]

May I thank all for the valuable comment. Absolutely true: if the failure had occurred without me trying first, it would have put off the young man. Possibly with long-term consequences. In the event, I keep delaying delivery of the present until I find something simple that is actually working. Either tinkering with this one, or getting an entirely new kit. Alas, only have time during week-ends.
Great help. Thanks.
Miklos

For the education of the young man try a crystal set first, lots of fun rigging an aerial and earth (you wont need much aerial where you are). We all stand on the shoulders of giants, best not leave a gap where we can fall through.

[paulsherwin]

Any project for a beginner involving an IC is dubious. Solid state electronics is difficult enough for non physicists to understand to begin with, without putting lots of transistors and associated components in a tiny package and viewing it as a magic black box.

[Telleadict]

Bummer when that sort of thing happens -- it would be nice if people selling things checked they work when built as published though there are threads somewhere on here suggesting "'twas always thus"

More seriously, I see that Rapid electronics have some simple IC based radio kits here and as they seem to supply to schools presumably they actually work. In any case it looks as though the build notes are available to download without ordering the kit so you could probably check the circuit looks reasonable before starting.

Another thing might be the old Ladybird books project radio that I recall being disscussed in the forum not so long ago. The origional uses what are now obsolete components but I'm sure some folks had reworked it with parts that are easyer to get.

Sorry for poking my nose in late but just had these ideas.

Will

[G6Tanuki]

The cynic in me wonders about the true wisdom of building simple medium-wave radios for kids these days: you have to ask yourself if they will be at all impressed by something that only gives crackly earpiece-grade reception of a phone-in on BBC Radio Borsetshire where the elderly complain about the price of a fish-supper or the postmen dropping rubber-bands in the street.


There are, it has to be said, many more "entertaining" electronic constructions that can be done - and with a higher probability of success - using the various kits available from places like Maplin.