Sorry, but this is just a long, rambling, disjointed page…
Of VFOs and Superglue.
5th January 2015. This is the second prototype for the VFO, based on a similar but rather more elaborate circuit gleaned from the ’net. Nothing would work without Superglue, of course. The prototype is ‘lashed up’ on a bit of (single-sided) copper coated fiberglass board. This is quite cheap. The construction method was developed many years ago (I think in the U.S.A.) whereby small squares or rectangles are cut from scrap bits of PCB. These little ‘islands’ are Superglued onto the main board, in convenient places. Components are soldered to the copper ‘islands’ – which are insulated from the copper ground of the board beneath, by the fiberglass. Other components – transistors, resistors, capacitors &c – are soldered to other islands, as required. Many of these resistors &c. have to be connected to ground potential (earth, or ‘deck’ it was often called) and the fact that there is a large expanse of copper ‘deck’ around the islands makes this extremely easy. You can tack things down to earth wherever is convenient. If you are old enough to remember how tedious and difficult it was trying to solder several resistors and capacitors to solder tags stuck deep in the corner of a chassis, being able to solder ’em down anywhere, is truly marvellous. The device above is working quite well now, so we can attempt a ‘proper’ build. This will involve exactly the same technique, but all the long leads will be cut down, the coil will be Superglued down to stop it wobbling about, all will be tidied up and so on. This should all get done tomorrow.
Now onto the really important bit. Superglue – where would we be without it? Of course it has its hazards, so we are very careful when we use it & we don’t let kids mess about with it. BUT –there comes that time, often all too soon, when our bright new tube of Superglue gets it top stuck on. Yes, even the ones which the packet tells us have cunningly designed patent ‘non-stick caps’. It used to be said that the Colman’s Mustard Co. made most of its money from the mustard we left on the side of our plate, rather than from the mustard we actually ate. Likewise, the Superglue manufacturers must make most of their money from the glue left in our tubes – the glue we can’t get at, because the top won’t come off, and we know better than to force it, in case the tube suddenly bursts & the Superglue fastens us together &c. In extremis, we might carefully snip off one corner of the tube, and get a little more out of it. Perhaps, even, the time after that when we need it, we can snip off the other corner, but that’s definitely the end of the line. The superglue left in our still well-filled tube, might as well be on the far side of the moon for all the use it is.
Now I’m sure you know this already, but ACETONE is the stuff. As Toad of Toad Hall might declaim it: ‘Acetone is THE ONLY THING!’ It is readily available, not terribly expensive, and most easily obtained by buying nail varnish remover. Quite often, pharmacies actually stock pure acetone, but if they haven’t got any, just get nail varnish remover, which is almost entirely acetone. Get a little acetone on a paper kitchen towel & wipe clean the pointy spout of the Superglue tube, before you put back on the patent non-stick cap. I assure you that then, it will not stick.
Mind you, this procedure requires a lot of will-power. Because if you are repeatedly using the Superglue, you will be tempted to leave it open & lying about AND NOT CLEAN IT AFTER EVERY SHORT PERIOD OF USE. It’s fine to stick down several pieces of circuit board, or stick back in place two or three shards of the Ming Pot you are restoring, as this will only take a few seconds. But then, clean the spout, replace the cap, and in addition, as a fail-safe, always keep the superglue tube vertically in any case. An anchovy jar, as employed above, or anything similar will do admirably.
By way of proof, the tube above has been opened, used, wiped & closed, probably about 30 or 40 times. There is no resistance whatever when I unscrew the cap – I’ve just tried it. Ta-raah!
Hazard warning: Acetone is highly volatile and flammable, so be careful. Apart from that, it is apparently not regarded as especially poisonous, or carcinogenic. That makes a change! Remember ‘Thawpit’? You don’t? Good! Many years ago it was commonly used domestically as a stain remover. ‘Thawpit’ was Carbon Tetrachloride, and is a very nasty substance with far too many hazards to list here, so it’s a good job it’s gone out of use.
Page re-formatted 19th December 2015. It seems to be complete, but rather purposeless? H’mm.
A home-brew VFO for 3.5 MHz.
The VFO is completed – ta-raah!
23rd January 2015. The finished VFO board. Yes, it is ugly, isn’t it? And yet, thanks to the short, vibration-free leads, the general inherent rigidity of the small board, and mainly because we decided to make it operate on 1.75 MHz rather than 3.5 MHz, it’s works quite well. That is, works well for something I have made! 8^) At ‘a’ there is a 2N5458 FET, which is the oscillator itself, flanked by the two large silver mica capacitors. It is a Colpitts oscillator. My favourite is the Clapp (or Clapp-Gouriet, or Series-tuned Colpitts – whatever you want to call it; after 60-odd years they’re still discussing the correct name for it. See Wikipedia!), but for 1.75 Mhz we just followed the circuit. The coil, or should I say inductor? No; it’s a coil as far as I’m concerned, and is wound round two stacked Amidon T-50-2 toroids in order to get the higher inductance needed for 1.75 MHz. At ‘B’ is another 2N5458 FET, a buffer to stop frequency pulling when power is drawn from the oscillator. ‘C’ is a plain old 2N3904 to act as an amplifier, for the oscillator circuit is working on only 5 Volts, so doesn’t give much output. The black toroid next to it is an FT-50-43 as a choke. At ‘d’ are seen two varicap diodes, which will give us our tuning range, when a suitable voltage between 0 and 12 V is applied to them. The board as a whole is supplied with 12 Volts.
This disgracefully out-of-focus shot is the little frequency doubler board. This was necessary because we want our transmitter to work on 3.5 MHz. Having the VFO on 1.75 MHz helps us to make a stable circuit, and it is easy to double it to 3.5 MHz. The transistor is another common-or-garden 2N3904, and in its collector, is a tuned circuit made up of the yellow T-50-6 toroid, and a 100 pF polystyrene capacitor – underneath it – plus a small 100 pF variable capacitor to adjust the resonant frequency of this ‘tank’ circuit to 3.5 MHz. Why they call it a ‘tank circuit’ I have no idea. But I know how it works. The oscillator at 1.75 MHz will produce a strong second harmonic – twice the fundamental, or 3.5 MHz – and a circuit resonant at 3.5 MHz offers a high impedance (or opposition) to that frequency. So the 3.5 MHz component cannot ‘escape’ to ground through the tank circuit; it can only pass through the upside-down 0.1 µF capacitor to the right of the 100 pF trimmer, and be led away through the pink wire. In other words, it’s got us exactly where we want it. <8^)
Well then. It was simply a matter of drilling a few holes in the magnificent (and not really terribly expensive) die-cast aluminium box we ordered a couple of days ago, and mounting all inside. It is always a temptation to stick the thing ‘as-is’, in a plastic box, or even leave it en plein air. To be sure, I have often done so myself. But there is little else more satisfying, than to have your frequency as stable as possible. And I remember once, I upset a cup of coffee – or it might even have been a glass of beer – into the open-plan VFO, and all Hell broke loose. There are only four externals here: 1. A phono socket to take out the Radio Frequency product of our unit to the power amplifier; 2. A knob (you can’t see it, because it’s black) which turns the 47 KΩ pot that varies the voltage to the varicap diodes, giving us our tuning range; 3. An offset switch. When down, we get our operating frequency. When up, it puts a 10 KΩ resistor in series with the pot, shifting the frequency about 10 KHz away, so that it won’t swamp the incoming signals in receive mode; 4. A phono socket for the 12 V power for the VFO. It’s true that QRPers (which is what I basically am) are much wont to use phono sockets for power input. They are cheap, and perfectly OK for the low current we tend to use. Downside of course, is that if you miss the socket with the plug, you are highly likely to short out your power supply. Also, if you leave a phono lead with a live centre pin lying around on your bench, a considerable number of undesired consequences may occur. I only write this to make plain that I am aware of these things, and even though I do it, I do not really condone it! 8^)
And here, at last, is the finished product. A stark, plain box with a knob and a switch on it. It works from 3.5 MHz to ~3.62 MHz. Our own operating window is most likely to be the narrow one of 3.558 MHz (the FISTS calling frequency) and 3.560 (the QRP calling frequency). We were tempted to slug the pot so that it would give just a tiny range of say 3.550 to 3.570; but left it as is. You never know if you might hear an old mate calling CQ on 3.530 – then you wouldn’t be able to call him & say ‘Aa-doo’, as we have it in this locality! As to the stability of the device, which has four little self-adhesive rubber feet on the bottom, just right-click on the link below & open in a new tab, and you will hear the VFO idling on about 3.550 MHz, before being bashed pretty hard with my fist. Even I was surprised – and gratified – by its steely resolve.
Don’t miss the next thrilling episode. Will the PA ever work? Can we get rid of the chirp? Why is there a chirp anyway? Will we ever make a contact?
Page re-formatted 19th December 2015. Pity. It looks like another incompleted page…
The W7ZOI active low-pass filter for CW.
January 7th 2016. The perfunctory lash-up you see above, is of a circuit designed way back by the renowned Wes Hayward, W7ZOI. It was published in the U.S. magazine ‘Ham Radio’ in April 1974. We currently have a project to build a receiver to go with a couple of QRP (low power) CW (= Morse) transmitters we knocked up a few months ago. Our attention span is usually very brief, but surprisingly, having started at Christmas, here it is the 7th January and we’re still going at it! The receiver will be simple – otherwise we’d never be able to make it – but attention is being paid to details, in order to get over the deficiencies simple receivers tend to have. Which are usually numerous. 8^) And, as we all know, it is much more difficult to home-brew receivers than transmitters.
One of the things we’ll certainly need is an audio frequency filter for listening to Morse code. In fact, an audio peak filter to emphasise the desired signal is an indispensable feature of any receiver, especially for CW. The main station here is an expensive Kenwood TS-590, and this is indeed equipped with means of selecting and also varying the pitch of the incoming signal, and amplifying the result, not to mention noise reduction, noise blanking &c., &c. The operation of these controls soon becomes more or less second nature, but I have found there is a tendency to keep fiddling with the settings to further optimise reception. However, this is all too often counter-productive. Because we usually operate low power (5 Watts or less), and work other low-power stations, fading (QSB) is a major problem. If one has become too selective when the incoming signal is loud, then when it fades down, it usually disappears altogether. Naturally, this always happens just before the other station is about to end their transmission and hand over to you. So you can’t be sure that they have actually done so. What usually happens, is that you wait until they must certainly have handed it back, and start sending to them. You send something like ‘TNX INFO DR OM’ – which of course really means “I have not been listening to what you just sent 8^) – but by then they have started calling you to see if you are still there. So you’re both transmitting at the same time. Result? Failure of the QSO, leading to:
Well, we don’t really want any of that, do we?
No. What we – I – need, is a simpler way of isolating the desired signal. Especially for my receiver project, because it will be of the Direct Conversion type. (More on this on a page to be written later.) Hence my search on-line for peak filters of that kind.
Wes Hayward, W7ZOI, has for many years been pre-eminent in designing highly-reasoned, efficient and often commendably simple amateur radio circuits for the home brewer. So when we found the above, it was a Must Try. Of course, it’s a retro circuit, being over 40 years old; but it’s simple, and uses still-available and extremely inexpensive parts, so we gave it a whirl.
Diagram, with acknowledgements, from website http://kambing.ui.ac.id/
Actually, W7ZOI’s circuit must have originally been from somewhere else, but hope it’s OK to copy the above here. The red box emphasises a dotted line, in which further identical stages may be added.
The red box contains the input circuit, while the following blue boxes are each stages which will put in a peak, and also amplify it. It is possible to have several in cascade, the final signal being taken via the 10µF electrolytic at the far right.
The one I made – in the photo. above – had three peak stages. Centred on 600 Hz, its performance was excellent – indeed, almost over-kill! Here’s a sample of it, on 7 MHz in this January afternoon, with the TS590 on full CW bandwidth, & RF gain about halfway:
An audio oscillator was taken, with its output at a lowish lever, and fed it into the filter in 13 steps. The output voltage was recorded on an ordinary multimeter. Of course, the x axis should be logarithmic, but we haven’t cracked Excel to that stage yet. 8^) Still, it’s clearly a superbly effective circuit.
There was only one slight drawback: the centre frequency of ~575 Hz was a little higher than I prefer. Back in the 1980s, one of my main receivers was the venerable (and gorgeous) HRO 5-T, and you could adjust the crystal filter to give you whatever pitch you liked for Morse. We soon decided on about 400 – 450 Hz as our preference. Well: it should be simple enough to lower this filter a little. In spite of all the graphs &c., we aren’t really all that technical, and it was not obvious to us which capacitor needed changing to lower the centre frequency. But after much googling, there was a distinct impression that it was the 6.8 nF from the base of the 2N3094s to deck.
To prove this, we built another, single stage circuit on a re-cycled bit of board…
… and plotted it with its original value of 6.8 nF:
Then, the 6.8 nF was changed, on spec., to a 10 nF:
Lo: Sod’s Law must have been temporarily in abeyance, as we had hit 450 Hz pretty well on the nose! It didn’t take long to swap the three 6.8 nFs to 10 nFs. Another test showed that the 3-section filter was still on 450 as near as dammit. We won’t bore you with the chart, even though I’ve discovered they’re quite fun to make – it reminds me of being back at Technical College, incalculable æons ago… Instead, here is another sample off 7 Mhz, this time 1000z, 8th January. (It’s true that in morning conditions, the TS-590s inbuilt filter is excellent with loud signals; but it’s set here wide open & with full RF gain, so only the W7ZOI filter is operating.)
It might cound a bit sepulchral to you, but it’s just the job for me. (See Appendix 1.) So there we are. Of course, there are times when signals come sailing in and one does not need a filter at all. So we can tidy this board up a bit, & put it in a nice box, with a switch & it will become a useful module for our receiver project. Thinks: I wonder how much current it draws, and whether it would run on 9 Volts? I don’t know; I’ll go and see… [5 minutes later]: Yes, it seems perfectly happy on 9 Volts at 10 milliamps. So, how long will a PP3 battery last? I don’t know; I’ll go and see… [5 minutes later]: Apparently a standard alkaline PP3 is rated at 550 mAh, so it wouldn’t be too bad. And it would save an extra lead sprawled over the operating position.
Very well, then. The board will go into a box with two phono sockets on it for in and out, powered by a PP3 battery. It’ll also need a pot. to control the input to the board, as it may be run from a headphone socket. Also, of course, provision must be made to switch in none, one, two or all three of the filter stages, as required. I wonder if it can be done with three miniature toggle switches rather than a rotary switch? And the first one of them can turn the power on too. Toggle switches are much easier to flip casually, rather then grab hold of a knob & turn it. Indeed, this easy flipping of toggle switches has been known in the U.S.A. for a very long time: their toggle switches are usually UP for on, and DOWN for off. That means, if you fidget about a lot and accidentally press down on a toggle switch, you will turn something OFF, rather than ON. Whereas in the U.K., they are the other way round. Look:
This is an escutcheon from on of my stock of new toggle switches. It is probably Japanese, so naturally observes the U.S. tradition as described above. In general, I think I have to agree with the U.S. approach. Though it all depends on statistics, doesn’t it? How many accidents have occurred when something was accidentally switched OFF, as opposed to how many have happened when something was accidentally switched ON. In theory, we would expect a deleterious event to be more likely when something was switched ON. Like a friendly neighbourhood mobile but psychotic chain-saw. OTOH, if you accidentally switched OFF somebody’s life support system, that might be difficult to live with. But I digress…
Appendix 1. Here write about the two different freqencies at wch the human ear seems to work best.
Page started 7th January 2016, but evidently never finished. Oh dear; sorry & all that!
Construction of the Classic 40 – a Direct Conversion
Receiver for the 7 MHz band.
Designed by Rick Campbell, KK7B.
Published in ‘QST’, August 1992.
8th January 2016. As mentioned on the adjacent web-page, since Christmas we have been engaged on a 7 MHz CW receiver project. It started innocently enough, with the wish to knock up a simple Direct Conversion receiver. We had made a couple in the late 1980s, and they were great fun. I don’t think we made more than a couple of contacts using them, though; because if you operated even a 2 or 3 Watt CW transmitter near them, being very primitive, they rolled up into a small ball in the corner of the shack, gibbering.
The Direct Conversion receiver – also (very occasionally) known as the Homodyne – is a very attractive thing to many of us. The principle is very simple indeed. An aerial brings signals into our shack. Hundreds of them mixed together, actually! However, our aerials are usually connected to an Aerial Tuning Unit (Aerial Matching Unit is the more precise term), so many of the unwanted signals are greatly reduced & got rid of. We are currently interested in the 7 MHz Amateur band, so if we have our ATU tuned to that band, the stuff getting to the receiver will be mostly around that frequency, especially if we put in a tuned band-pass filter – more on that later.
All we do is to mix that incoming signal – say it’s 7.03 MHz – with another signal of nearly the same frequency, generated locally in our shack. Suppose we had set our shack generator (it’s called a Local Oscillator, or LO for short) to 7.0305 MHz. What will happen when it’s mixed with the incoming 7.03 MHz?
Well, when you mix two waves together, you get a whole series of products. The main ones are the sum of the two frequencies, and the difference between them. There are lots of other, weaker products, but that’s not important right now. Actually, it’s only the difference between our two frequencies is really of interest to us. What is it? It’s
7,030,500 minus 7,030,000
Answer: 500 Hz. Which is really great, because 500 Hz is an audible frequency; you can listen to the signal right away. 500 Hz is a whisker higher than B above middle C on the piano – but that’s not important right now, either. 8^)
What is important, is that there has been a Direct Conversion of the radio wave to audio. You can hear it straight away. Marvellous, eh?
Better still, because of the way our audio has been derived, it has a beautifully clear and pure sound. The sound has been produced by a classically simple and elegant process. That’s why a lot of people who are into audio like it so much.
Now we come to the inevitable downsides – for you can never have something for nothing.
Firstly, the audio signal we have obtained is very small indeed – probably just a few millionths of a Volt. It will need to be amplified an immense amount to listen to it on a loudspeaker, or even headphones. This degree of audio amplification is difficult to obtain without various problems: distortion, white noise (hiss) generated within the amplifying devices themselves, the interaction of spurious, undesired products &c.
Secondly, as you tune across our chosen band – which is 7.0 to 7.04 MHz – each station will come in at a high audio pitch, which will fall as your LO approaches the frequency the station is transmitting on. When your LO is exactly on the same frequency, you won’t hear anything. Because then, there isn’t any difference between the ham and your LO signal. But as you continue to tune, that station will reappear & gradually increase in pitch until it goes out of audio range. So every signal appears twice. To be sure, this is a problem, but it’s not too difficult to get used to; and having a good peak audio filter is a great help.
The assorted ugly-construction boards at the top of this page have all been lashed up since Christmas as part of our tests & evaluations of circuits available on line. We’ve had great fun – plus a lot of frustration! – but eventually were able to receive some 7 MHz signals using mostly ramshackle stuff. Of course, we knew that a receiver will only work in an optimum fashion once it is rigidly installed in a metal box, and the LO properly screened &c., &c. But the performance of our lash-ups was so poor, it would certainly have remained dreadful no matter how nice a metal box we put it in!
A big, big re-think was necessary.
One circuit we had repeatedly come across during many hours of googling, was that of an advanced DC receiver designed by Rick Campbell, KK7B. It was published way back in the August 1992 ‘QST’ – the magazine of the ARRL, the national society of radio amateurs in the U.S.A. It looked quite complicated to us, so we sheered away from it – though not before borrowing the circuit of the simple tuned front-end bandpass filter for our project, whatever that was destined to be! But in time, we realised that if our project was to be worthwhile, we really had to bite the bullet & knock up something pretty good. The QST article is on-line as a .pdf, so we printed it out, read & studied it closely. You can do so yourself at:
It turned out that it wasn’t nearly as complicated as it looked from the circuit diagram. Which is NOT to minimise the years of work & study KK7B had put into it! Also, as the design was 25 years old, most of the components were quite normal analogue items that even I could understand. SO BE IT! We gradually bought in the components, taking care to find exactly the same ones as stipulated in the article. Happily, all were still available. Also, the circuit is laid out in logical modules, each of which could be prototyped and tested separately. A printed circuit board had been made available in 1992, and the resulting receiver was commendably small. But I have very big clumsy hands, so it was decided to employ ugly construction on rectangles of printed circuit board, just like at the top of this page; and though the receiver would come out quite large, I didn’t think that would much affect the performance. We shall see!
Rick Campbell includes no particulars for a VFO (LO) circuit, saying candidly that those have already been dealt with by consummate experts such as Les Hayward K7ZOI, Doug DeMaw W1FB, and Roy Lewallan W7EL; and that he had – as yet – not been able to exceed their attainments in that field. (At least, he hadn’t in 1992.) 8^)
Very well then. The final batch of condensers – sorry – capacitors, arrived this morning. At last the Quest could begin!
Here is the circuit of the first prototype board, plus the input filter. I’m sure Rick and the ARRL won’t mind me reproducing it bit by bit; after all, anyone can download the whole .pdf at the link above. I’ve actually been a bit clever and incorporated the Input bandpass filter from elsewhere in the article. (Hee hee.) It’s self-explanatory really. The aerial – no, I shan’t call it an antenna, so there! – goes into a tapped coil wound on a toroid, and goes via this symmetrical filter into the Magic Device contained in the Red Box. This is, straight away, is the very heart of the receiver. The SBL-1 has been made for many decades by Mini-Circuits™ of New York, and quite rightly so. It will take in any two signals between 0 Hz and 500 MHz, and mix them perfectly. While being the most expensive component in the receiver, its cost is surprisingly modest. Mini-Circuits™ themselves are happy to supply them at $9.50 apiece, in quantities of 10. http://22.214.171.124/pdfs/SBL-1+.pdf . The LO also goes into the SBL-1, as shown. (We’ll get to the LO later.)
The first rough prototype board itself. We haven’t shown the Input bandpass filter here, but the signal from it (the Radio Frequency or RF) goes into pin 1 of the SBL-1, as indicated. Of course, we needed to test this first board, in order to prevent creeping insanity. If it didn’t work, then there would be little point in making all the following stages, which even if they were OK, would not do their job, because this first board hadn’t. So we stuck the lead at bottom right into a nearby standard domestic audio amplifier, just to see. Or rather, hear. And behold: Morse signals, though very faint, could distinctly be heard. Oh joy! It was a pile-up in late afternoon. We are evidently on the right track. Here’s a sample. It’s alive – it’s ALIVE!
On to the next part of the circuit – the CW filter.
Values are also given in the article for a 3000 Hz bandwidth version for reception of SSB – whatever that may be… 8^)
This is a ‘7th-order Elliptical Low Pass Filter’. I have no idea what that really means, apart from it being a darn good low pass filter. That’s why I am a retired jazz musician who seldom got to play the sort of stuff he really wanted to, while KK7M was already in 1992, an academic at Michigan Technological University. Still, this low pass filter is a vital component of Rick’s design. You’ll notice of course that all the condens – capacitors we’re using on these prototype boards all have very long leads, so they look gawky. The reason is, of course, that these boards are merely prototypes. If we snipped off the leads to be neat and tidy, that may hamper us when we come to build the finished version, in which we will use the same caps. It’s not that these are the only capacitors we have; far from it. We bought them in 10 at a time to get them a bit cheaper, and that’s fine, because we can make other receivers later on with them. I suppose it’s because were old, and therefore somewhat parsimonious. Still, that doesn’t matter here; we’re ‘only’ dealing with audio frequencies, so three inches (7.5cm) of leads on a capacitor will have approximately zero effect on the performance. Of course, if we were dealing with UHF or something esoteric like that, I’m sure such long leads would be catastrophic! 8^)
Anyhow, it works great! There’s no audio sample, as 7MHz activity was at a low point when we finished it.
9th January 2016. We thought that today we would receive the LM387s we ordered a few days ago, along with the TIP29C & TIP30C transistors for the power amp. But the postman doesn’t come until about 1100z, so in the meantime attention was turned to our 7 MHz VFO. Yes, even I know you’re not supposed to have a VFO running as high as 7 MHz; but while googling for inspiration, I had found a great little circuit on the G-QRP-Club website, in a DC receiver called the ‘Direx’ – a datasheet of a constructional article by the Rev. George Dobbs G3RJV. Don’t think I’m name-dropping, because I am. (I first met George at 0009z on 1st January 1982 – we were each other’s first QSO on the 10.1 MHz band, which had been released for amateur use nine minutes before. He kindly enclosed a copy of SPRAT, the G-QRP-Club magazine, with his QSL card, and incredible vistas of amateur radio gradually unfolded for us.) (Since we wrote this, George has died. His life-long contribution to the promotion of amateur radio, especially in engaging the interest of younger people, and his tireless promotion of ‘Just build or make something, however simple it is!’ will have inspired hundreds, if not thousands, of recruits to electronics.)
This is a very simple VFO, which George says was ‘surprisingly stable’, and indeed it is. Plenty good enough to act as the VFO module in our prototype lash-ups. A 2N3904 was used instead of a BC109 – remember those? 8^) I have the impression that the stability might be something to do with the fact that the base is biased by two equal resistors (10 KΩ) and the collector & emitter resistors are also equal (470 Ω). I don’t, offhand, recall seeing transistors suspended so symmetrically between the voltage rail & deck, except in unity-gain circuits. Anyway, it had been working fine, so we tidied it up, provided some varicap diode tuning voltage from a 10-turn pot, and slung it into a die-cast box.
We had to adjust it a bit; it seemed surprised, even embarrassed; for a single-transistor VFO to be given its very own Eddystone die-cast box must be a fairly rare event, but eventually it settled down. (We made the mistake of putting the lid on after we’d just soldered stuff in place. It took ages to cool down & get more or less stable!)
Here it is: dear, sweet little VFO! By the way, the 25pF variable in the circuit above is actually a miniature air-spaced preset, rescued years ago from a piece of junk. It’s a beautiful little job, & we always knew it would come into its own eventually. You can see it next to the pot. Unfortunately, the postman did not bring us our LM387s. Grrh! So we made do with our existing LM386, which you see on the top right of the shot. There’s a contest on at the moment, and although it was late in the day, we picked up the following sample as the band closed. The DC rx is getting better, though we are far from the finishing point.
UC2K is a Club station in Kaliningrad, the Russian exclave on the Baltic Sea – about 900 miles from my QTH. The hum is due to everything (except the VFO, which did have its lid on) just lying on the bench, as you see above. The hiss is partly due to the modest LM386 audio chip.
Onward: ever onward!
Sunday 10th January. A cool, calm & sunny morning. We should really go out for a walk, instead of brooding over the continuing absence of the LM387s. The LM387 is not, as we imagined, simply an improved, low-noise version of the LM386. Well yes it is that, but it also contains two amplifiers instead of one. Here’s the next section of the receiver:
The components shown dotted (C21/R15 and C25/R22) are optional, and will reduce residual hiss, if this proves a problem. The 2N5457 FET in between the two halves of the LM387 will mute the receiver, when grounded, while you are transmitting. It is our intention to build this receiver exactly as it appeared in 1992, but other chips have doubtless appeared in the meantime that are better than the LM387. There are things we’ve come across like the OPA2134, used in hi-fi audio applications; we have ourselves used the NE5532 in a phono. pre-amp.
There was, however, nothing to stop us carrying on with the construction of the above stage in the absence of an LM387. We had intended to use a socket anyway. Accordingly, after a nice morning walk in local park…
For a very long time, we have been meaning to try feeding ducks, not with bread, but peas. Though bread has been traditionally used to feed ducks for many centuries, it’s not really appropriate, as it’s largely carbohydrate; apparently that is not normally a major constituent of the diet of ducks. Peas were advised as better, on a website I happened to visit. So I finally remembered to take a small plastic bag containing frozen peas to a local park. At first, the ducks didn’t seem to be too bothered. Also, because it was only about 4°C, the peas were still mostly frozen. But after a while, they got the idea, as did the black headed gulls, not to mention the many Canada Geese – though there are none of those in the shot above. The peas ended up being eaten greedily. Then we took our little walk, hoping that the rest of the peas would thaw by the time we got back. This was not the case; and worse still, somebody had emptied two carrier bags full of bread into the pond in the meantime, so all the birds were glutted. The experiment will just have to be repeated at a future date.
End of Interlude.
On our return home, work began in earnest.
Here is the stage completed. We always hold down our boards with insulating tape; there’s nothing so frustrating as chasing a board across the bench while trying to solder in a tiny metal film resistor. To solder from another angle, you just pull the board up & stick it back down at the angle required. You’ll see that we’ve re-wired the audio filter ‘properly’, to make more room on the board; also, we’re too clumsy – and impatient – to fix in ICs properly, so have just soldered an 8-pin holder to a piece of Veroboard. 8 short lengths of bare wire were soldered to the strips, and those in turn fixed to two strips of PCB each with 3 saw-cuts made with a razor saw, which gives 4 insulated pads.
The same board from another angle. Of course we won’t be able to test it until those pesky LM387s arrive!
The final part of the circuit is a power amplifier to drive a loudspeaker:
Wednesday 13th January 2016. Yesterday we reached the nadir of the project. By which I mean, the ruddy LM387s have still not arrived. So we checked, and found to our dismay they had been scheduled to arrive over a week ago. We have emailed the seller and asked them to check it out; also, we looked for another source of 387s. They really are pretty elusive these days. There are plenty on ebay, but the local ones are quite expensive. Then we found a cheap mail order source, believe it or not, in our own city, Birmingham! We’d missed it before, because they had used a generic image of a 14-pin IC, so we assumed it was something else. The supplier is, however, mail order only. We anxiously await their delivery.
So in the meantime, we carried on with the power amplifier. We managed to shoehorn it onto the same board, though we had no real reason to make it small. Just thought we’d have a go. It got quite cramped & fiddly towards the end, and expected it wouldn’t work, but it did – the power amp, that is. Audio signals of about 50 mV were injected & sounded fine from the loudspeaker. Whew!
Well, there we are, then; half-satisfied, but half-frustrated. Does the LM387 stage work? I suppose we might as well put it in the nice box we have waiting for it. It seems somehow wrong to drill & spray the box in our ‘house colours’ of red & black before the darn thing works, but what can one do? >8^(
LO & mixer/audio section all present and rarin’ to go – except for an LM387.
Thursday 14th January 2016. The LM387s did not arrive, nor have I yet heard back from the company I first ordered them from. Though we hoped we might get the other 5 we ordered locally, they didn’t come either.
Friday 15th January 2016. There is now no doubt in my mind that a Definite Conspiracy exists, probably involving Satan, Goblins, Ghosts, Ghouls, Malignant Sprites, Komodo Dragons, Fruit-bats &c.: the whole ruddy shebang! Not only is there still no word about my original order for two LM387s, but my second order for five, made to the local supplier, has been sent back with an apologetic refund; they obviously haven’t got any. Now I’m not getting paranoid, please believe me; but I am very nearly certain that the few thousand remaining LM387s have been carefully gathered together from all over the world, and locked away in a Sealed Storage Facility, probably in the middle of the Gobi Desert, just so’s I can’t get one. Yes, that’s what it looks like to me. Definitely. Yes, siree. Very well then: my Vengeance will be unleashed, and they all jolly well better LOOK OUT, that’s all I can say! In the early afternoon, we found a place in London that offered LM387s at great expense. We phoned them, diplomatically establishing that they did actually have some (three actually), and so ordered one. I would have had all three, but couldn’t afford that amount of bread. However, the guy was affable & efficient, so we made the order.
Saturday 16th January 2016. Hurrah! The LM387 arrived. It just looked like any other 8-pin IC, but we danced around the room, cradling it, singing to it, placating & extolling it in a crazy ritual of fulfilment. But would the receiver work? Trembling, we carefully bent the pins so that it would fit the socket. (Why do you always have to do this? Why don’t they come ready to plug in?) Eventually, in it went. All was set up, but a profound silence issued from the loudspeaker. Oh dear. A fresh mug of tea was prepared, and the Crucial Question ‘WHY?’ was asked. Since the entire circuit apart from the SBL-1 is at audio frequency, we injected an AF signal to the final PA. That was OK. The signal was injected into the LM387. That was OK. So was the 1000 Hz filter. So was the initial board; injecting the AF signal at the output of the SBL-1 worked fine, too. So why weren’t there any CW signals? After another mug of tea, we checked the frequency of the LO, and all became clear. Because it was a lash-up VFO, it had no buffer; and when we boxed it & set it up to tune 7.00 to 7.04 MHz, it wasn’t connected to anything. On connecting it to pin 8 of the SBL-1, it was pulled it down about 150 KHz to around 6.85 MHz. And there were no signals there at the time – hence the silence. The VFO was tweaked up to begin at ~7 MHz, and Lo! there were the CW signals, as clear as a bell. Whoopee! We were physically & mentally exhausted by this time, but came back later and recorded this little extract from a contest that was going on. Oddly, it doesn’t sound like a busy contest, though it was, because you can only hear about three stations; on an ordinary DC receiver, you can hear eight or ten; this, of course, is the Ultimate Tribute to Rick Campbell’s superb design.
This, at last, concludes our Christmas Project. If anyone is still reading this guff, I commend you for your persistence, and thank you for your patience. I think I might even make a YouTube video of it, using some of the stills. But now for a day or two off. Then, we shall have to use this receiver to make some contacts…
Page finished 16th January 2016. I have to confess, that we have never made a QSO using this receiver. I simply can’t handle two VFOs, is all…