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Old 2nd Sep 2013, 7:58 am   #21
KeithsTV
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

A friend, who wrote several articles for PW, told me a story about F.G.Rayer and his reponse to people who wrote in about his projects which they couldn't get to work. The standard reply was "Oh they've just used the wrong components".

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Old 2nd Sep 2013, 8:43 am   #22
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

F G Rayer kept re-using a lot of the same components, as a friend of mine put it...Keep reading his articles and recognise old friends each time they made a reappearance.

To my mind, a good design is one which will work to the required standard provided all the parts are within spec and there are no constructional errors.

When components have to be selected, the end result is disaster. For a homebuilt project, the poor constructor had ordered up the exact number of parts listed and didn't have spares to try. If he trekked back off to town to buy another, then it likely came out of the same box and the same batch and was no more likely to be a go-er. And so on.

For stuff built in a factory, as I designed, if an in-spec part wasn't good enough, you soon had all the people on a production line changing them, but all the parts in stock would be from the latest delivery, so the yield would be rather low. Customers would soon be asking where their instruments were, bosses and accountants would be forming a lynch mob...

Either way, poor design causes misery.

As a kid, I built quite a few things from magazines and learned that even with the right bits, a lot of designs were not sure-fire.

The absolute nadir was buying a kit for a Lasky's Skyrover portable. I resent every last penny of the £6-19-6 I paid.

There was a lot of junk around, and the big problem is that you needed the level of knowledge to have designed a proper one for yourself inorder to see what was wrong with the magazine design.

Of course, there were good designs published. G2DAF receivers come to mind.

David
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Old 4th Sep 2013, 9:54 pm   #23
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

I spent a fortune (pocket money) in parts for projects that never worked. I was always convinced it was my fault or we had got duff components. A school friend had the same experience. We did begin to wonder if some of the projects had ever existed...
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Old 5th Sep 2013, 1:27 am   #24
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

My reaction to this thread is the same as that of many others: Thank you! This book, found originally in the library at school, took me from no prior knowledge of electronics to completely hooked in the space of one short read, and led ultimately to a lifetime of continuous involvement with electronics.

I was never able to build the actual circuits in this book because I had no access to the Denco coils used in the projects, but it gave me enough of an understanding to want to go and find other books, and many of those did have circuits in which I did build. Like others, I remembered the original appearance of the book but not the exact title or the name of the author.

Of course, all this nostalgic interest is going to cause a mini run on and subsequent price hike of those few copies which will be available right now, so I'll have a look again in a few weeks' time.

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Old 14th Nov 2013, 2:09 pm   #25
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

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Originally Posted by saxmaniac View Post
Talking about dodgy designs, I tried to build several projects from Practcal Wireless by F.G Rayer, and I came to the conclusion that absolutely nothing designed by this writer ever worked! I dont know if anyone else had hours of frustration as a result of dodgy projects
Oddly enough I never had any problems with F.G Rayer designs. http://vintageradio.me.uk/radconnav/...rc/circuit.jpg This circuit gave me many hours pleasure and I later modified it with an RF amp and a product detector.
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Old 14th Nov 2013, 3:22 pm   #26
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

I once had trouble with an F.G. Rayer design (Tx coils for 2m were clearly too high in inductance, perhaps came from a 4m design?). A schoolfriend said "Well of course. You don't think he has time to actually build and debug all the circuits he publishes, do you?". That was the point when I began to doubt what I read in electronics magazines, especially from regular contributors. Editors can't/won't/don't check articles for technical accuracy.
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Old 14th Nov 2013, 5:24 pm   #27
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

Hi Gents, seconded on the lack of checking and proof reading in many magazines, especially the "Practical" group of some years ago; you had to wait for at least 3 more editions to see all the corrections.
WW was not immune either, as I had written for them and used to ring up when I saw the published article.
My experiences with Elektor are totally different; everything is well checked and there is a lot of feedback to authors.
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Old 14th Nov 2013, 6:05 pm   #28
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

I think a lot of reliance is now dependent on spell checkers, ignoring the fact that the article might have "real" words in it but factually incorrect.
Some local newspapers are dreadful in that respect.

When I wrote for "Television" when it was edited by the late John Reddihough he read my submitted text in great detail and phoned me if he thought there was anything incorrect.
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Old 14th Nov 2013, 6:21 pm   #29
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

Elektor had a good reputation.

The ARRL also check what gets published in QST. THey have a good lab, they also do their own reviews on anonymously purchased equipment as well.

Sometimes you learn a lot more from the ones which don't work first time...

David
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Old 14th Nov 2013, 7:18 pm   #30
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

Quote:
Originally Posted by Mike Phelan View Post
.When I wrote for "Television" when it was edited by the late John Reddihough he read my submitted text in great detail and phoned me if he thought there was anything incorrect.
Most of the articles I wrote for Television were published without any contact from John Reddihough but on one occasion he phoned me to check the article on adding a printer to the Amstrad PCW was correct after it had been prepared for publication.

Keith
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Old 14th Nov 2013, 8:23 pm   #31
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

Quote:
Originally Posted by G8HQP Dave View Post
That was the point when I began to doubt what I read in electronics magazines, especially from regular contributors. Editors can't/won't/don't check articles for technical accuracy.
Unfortunately, same criticism can be easily levelled at a lot of ccts. / projects (and their accompanying texts) that appear in all editions of the RSGB handbooks.

Al.
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Old 14th Nov 2013, 10:36 pm   #32
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

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Originally Posted by Mark GØOIW View Post
Look at the circuit and see what you think. There was both a board layout and photos of the set, and the book ran to several editions, but it can't possibly have worked whan constructed according to the instructions. How was such a major error allowed to occur, and be repeated without amendment?
Interesting (old) thread.

Although the OP will have long given up on this circuit here is my initial analysis.

First of all I'm afraid I have more questions than answers because I know very little about germanium transistors or denco coils but nobody seems to have 'corrected' the circuit 'correctly'. The main area of concern seems to be D2 because it clamps across the transistor. So here is my first question:

What is the Vf of an OA81 when biased at just over 1mA? If it is several hundred millivolts then I think I can begin to argue a case for this rather odd circuit as being drawn correctly.

This is because you don't need much in the way of collector-emitter voltage to get a transistor to amplify small signals or to oscillate. I don't know if this applies to germanium transistors as I have never designed with them.

So, if we give the author of the book a chance, I can argue a (fairly weak) case for D2 and also D1 being correct as drawn. I think both are involved in controlling the way the detector behaves at the onset of oscillation.

What is very odd about the circuit is the poorly defined feedback path. So here is another question:

Does the book state anything about how to align the blue coil in relation/orientation to the yellow coil? Does it say anything about placing wires together to get some feedback? In the images the coils seem a long way apart so I'm a bit lost on how well the feedback is defined.

Which brings me full circle back to D1 and D2 because I think these may be there to help the circuit act as a super regen detector. There's an RC circuit with a long time constant to define the squegging and I think D1 and D2 is involved with defining the squegging too.

I'm going to give this very odd looking circuit a 'chance' that it is drawn correctly.

However, what is apparently unforgivable (for a book aimed at beginners) is the way the feedback is arranged because I suspect that a lot of builders will get this wrong if they arrange the layout differently.

So my final question is: Can someone post up the relevant text from the book please? Does it state anything about the layout?
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Old 14th Nov 2013, 11:53 pm   #33
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

Having looked a bit closer at the circuit I really do think it was meant to be a super regen detector and 'if' it works the way I think it is meant to work then it could be quite a neat little circuit.

Therefore, I'm far from convinced that the circuit is 'wrong' and I do think that TR1 can oscillate even with just a fraction of a volt across the collector-emitter (i.e. it can function despite being clamped at this voltage due to D2). The diodes D2 and D1 look like they work together to define the squegging threshold and this will help act as an ALC system for the regenerative detector.

The bit I'm not so happy with is the feedback because this will be layout and wiring dependent. Also, I don't know enough about germanium transistors to guess the amount of C-B capacitance there might be.
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Old 15th Nov 2013, 12:48 am   #34
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

I tried googling for this book and couldn't find it but someone on the thread mentioned a reflex receiver.

I've never had any experience of this type of receiver (had to google it) but maybe this receiver is a reflex receiver and not a super regen as I first thought. That explains why I couldn't see any well defined feedback path for oscillation.
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Old 15th Nov 2013, 1:24 am   #35
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

I mentioned reflex, but with a capacitor between the collector and the junction of the diodes. I didn't mention that VC2 should stay connected to the collector/coil junction.

Whether the circuit would be stable with its tuned collector tuned base arrangement (shades of TPTG here) I don't know. A typical reflex would have an aperiodic RFC as the RF load on the transistor but a parallel tuned circuit should work instead.
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Old 15th Nov 2013, 2:33 am   #36
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

Sadly, I'm probably not going to be much help here because I have zero experience of reflex receivers.

However, now that I know what a reflex receiver is I still think the circuit 'may' be OK.

Because the diodes are presumably acting as an AM detector for the 'reflex' mode then I still think there is a chance the circuit is drawn correctly in this respect at least.

This is because I think the detector will need to be forward biased for both diodes because it has to detect tiny signals and so the detector needs to be operating close to the linear region. If this bias is not there then the detector will become very inefficient and non linear. Not good for audio quality?

I think if you added the blocking cap the diode detector would not get enough forward bias to keep the two diodes in the linear region. But then the circuit as drawn isn't biased ideally either?

I can't say how well TR1 will work with such a tiny supply voltage at the collector because I don't know much about Germanium transistors but it might still be able to function for small signals.

It does look a bit odd that there is no RF decoupling cap on the cold end of the yellow coil but maybe it isn't needed for satisfactory operation?

It's late and I can't think any more... off to bed
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Old 15th Nov 2013, 9:30 am   #37
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

I'm sure that the circuit shown is intended to be a reflex: the 2 diodes are intended to be a voltage-doubler detector feeding the audio back to the base of the first transistor.

However the 2 diodes should be isolated from the transistor collector by a small (100pF) capacitor, and VC2 should connect direct to the junction of the transistor's collector and the "Pin 1" of L2 so VC2 and L2 form a tuned-collector load.

Built like that, it might work. But even then, the biasing of the first transistor would still be wrong - the rectified DC component of the signal from the two diodes is not isolated from the base circuit of TR1: varying degrees of this DC component (in response to different signal strengths) will shift the bias-point.

Possibly this is an attempt at a form of AGC? I'd say it was guaranteed to shift the operating-point of the transistor towards greater distortion.
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Old 15th Nov 2013, 1:51 pm   #38
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

I reckon the time for theoretical discussion is past. Anyone fancy building it to see what happens? It wouldn't surprise me if it worked with a suitably leaky old OC44 installed. It's quite likely that if Mr Gibson actually built it, it worked, but that would be no guarantee that if he built another that would too- I suspect it's a rather device dependent issue.
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Old 15th Nov 2013, 3:06 pm   #39
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

Here is an example of the Superhet which was the thing we all wanted to build.

I have had no time to try this since I bought it ready built a couple of weeks ago although I have made some mechanical repairs.

It's been built pretty much built verbatim to the original design just a couple screw terminals added for Aerial, Earth and Battery.

Interestingly the trimmers mentioned in the text but not shown on the circuit diagram to improve clarity are not fitted.

The other intriguing thing is this book was originally published in 1970 by which time OC170's were well past their prime, even my 1964 Philips Electronic Engineer kit was using AF116's

So far I know no one who has had success with the 2 transistor circuit....... Maybe one of us will!

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Old 15th Nov 2013, 8:35 pm   #40
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Default Re: "Illustrated Teach Yourself Radio" by David Gibson

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It's quite likely that if Mr Gibson actually built it, it worked, but that would be no guarantee that if he built another that would too- I suspect it's a rather device dependent issue.
I suspect you are right.

The main point I've tried to make here is that you 'can' operate a transistor as an RF amplifier at very low Vce so I don't see the clamping effect of D2 to be a showstopper. So the OP 'might' be missing the reason this circuit proves to be so unreliable. But I don't know enough about germanium transistors to say this for certain.

But, what may well be a killer is the need for TR1 to have enough current gain at this low Vce to be able to rob a decent chunk of the initial startup (1mA) bias current from D2. This robbing process lowers Vce even further but it allows the amplifer to swing better for small signals.

I don't know much about germanium transistors but you can still get lots of (very small signal) voltage gain from something like a BC109 even if its Vce is down at 400mV dc.
What kind of Hfe can you get from a 1960s germanium transistor with a very low Vce? I suspect the circuit needs a decent current gain in TR1.

200 would be nice, 150 probably adequate and 100 would be a problem (I think) This is because the resistors in the circuit suggest that the bias current into the base of TR1 will be about 4V/1Meg = about 4uA.

So to rob a big slice of the 1mA that starts up in D2 the transistor TR1 needs a high Hfe. Otherwise it will begin to lose the fight for bias current against D2. I can see why adding the blocking cap makes things better for TR1 but I think the circuit 'as drawn' might actually have a chance of working if TR1 has a decent current gain.
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