A Popcorn QRP Regen Receiver and Lots More Air-Spaced Variable Capacitors

Many of us in the home-brewer community were really disappointed when Todd VE7BPO recently discontinued his popular and very through-provoking site “The QRP/SWL Homebuilder”. An archive of the site is available for download in pdf form and is a great resource. Though not quite the same as having the site, it’s great to have a copy of it for reference. Todd was having some issues with the site. He didn’t take it down due to lack of enthusiasm on his part and by way of proof, he’s back in style, with the new “Popcorn QRP – Scratch Homebrew Component-Level Radio Electronics” blog. It’s great – just like having his old site back. Todd combines circuit analysis and a theoretical approach with a strong leaning towards practical circuits that can be built by the home experimenter. Economy of design and performance are both considered, and from reading the accounts of Todd’s exploits, one suspects he is having a complete blast. If you haven’t visited Todd’s current blog or his previous site before, please note that the intent isn’t to provide the home-builder with complete step-by-step instructions on how to build a series of projects. This is circuit-level stuff, but if you’ve had a little experience at building circuits from schematics, all the circuits presented are tried and tested by our faithful protagonist, VE7BPO aka retired Professor Vasily Ivanenko.

All this is leading up to something, and that something is that our intrepid experimenter recently announced he had been revisiting the subject of regenerative receivers. He had built 4, and was sharing one of them with us on his blog. Happy, happy, joy, joy! Even better, this circuit was the same basic topology that I used in The Sproutie – a  circuit that separated the Q-multiplier and detector into separate stages. It’s an arrangement that is well-behaved and performs well. The regen control, a resisitive component, exhibits no hysteresis, and the addition of an RF preamp means that the circuit doesn’t suffer from common mode hum. If you build it well, it will be frequency-stable too, but that part is up to you 🙂

The blog-post is here. It is based on the regen circuit published by Makota 7N3WVM, whose website is a treasure trove of circuits for the home builder. You’ll notice great similarity to the circuit of Nicky’s TRF that first appeared in SPRAT Issue 70 in 1992, and N1BYT’s WBR from QST Aug 2001. What’s really interesting though, is that Mr Ivanenko has added a bipolar transistor (a 2N4401) to the J310 FET infinite impedance detector, to turn it into a hycas (hybrid cascode) pair in order to increase the level of audio from the detector, as well as the reverse isolation, which can never hurt, right? Great idea! His circuit includes a bipolar RF preamp in common base mode to add isolation of the oscillator from the antenna, as well as a little bit of gain, and a 2-transistor preamp after the detector that he has used in other “popcorn” designs. He also makes his argument for continued use of the LM386 as a final audio amp in this post. The description in the 2nd paragraph, of the folklore surrounding regens is mirth-inducing – and spot-on.

I know that at least a few builders are considering making their own Sproutie and if you are still at the planning stage, you might want to think about Todd’s version of the front end. I haven’t tried it (yet) but Todd ain’t no slouch 🙂  If I were to incorporate it into my next regen build, my current thinking is to use his front end, and feed it into a one-stage active audio filter using a 5532 op-amp or similar, and then into a nice, low-noise LM380 with it’s fixed amount of 34dB gain. If I ever do this, I can promise you two things –

a) it will be many months, maybe even a year before I do it, because I am very slow at these things and

b) I’ll show you the circuit of my AF stages so you can join it up with Todd’s (ahem, I mean Vasily Ivanenko’s) front end circuit

 

Todd just revealed 2 encouraging pieces of news –

a) He just ordered some black and red chicken-head knobs for his next project and

b) He has decided to spend another 2 weeks working on regens

If you don’t already follow Popcorn QRP, it’s well worth adding to your RSS reader, bookmarks bar or similar.

 

The rest of this post may be a little annoying to those who have limited bandwidth connections, though I imagine anyone who is still surfing the internet on a dial-up connection or a very slow mobile connection learned long ago not to come here 🙂 I recently acquired some more air-spaced variable capacitors and I’d like to share some photos of them with you for no other reason than short-term visual gratification! As always, my preferred brand is Hammarlund. Not only are they of high quality, but I happen to like the way they look as well. They are drop-dead gorgeous.

I’ll start with a piece that was acquired a few months earlier, before the current flurry of buying activity. This is an MCD-50-M and may well end up as the main tuning capacitor of my next regen, unless I happen to find another MCD-35-MX like the one that was used for tuning The Sproutie. MCD means that is is a double-ganged unit (unlike the single-ganged MC units), 50 is the maximum capacity of each section, and the M at the end refers to the fact that the offset rotor plates give it a “midline” capacity characteristic, keeping the rotation vs frequency characteristic reasonably linear. Nickel-plated brass vanes for a good temperature coefficient, and all mounted on a high-Q ceramic base. Perfection! Look at those bright and shining never-been-soldered-to-before terminals –

Leaping forward in time to a couple of weeks ago, this MC-200-M “midget condenser” came into my life sporting an older original box, in almost as good condition as the capacitor itself. I love the graphic design on these older boxes –

Then came my haul of just a few days ago. I bought them all from the same seller. The boxes are a bit beaten up, but the capacitors are in great condition. First off, here’s the group shot-

Among them were two of these HFA-100-A’s. If you’re as picky as I am, these are not ideal for maximum stability in a VFO or receiver tuning control, as the vanes are only supported at one end. It would be fine as an antenna trimmer where it’s placed in series with the antenna lead, or as a reaction control, in a regen where a variable capacitor is used for this function. I’m not sure what material the vanes are made of in these parts –

In the same vein (vane?), I also scored an HF140. What a great part. It offers two methods of mounting. If I possibly could, I’d use both of them for maximum stability. This variable cap could be mounted underneath a chassis so that the nut on the threaded shaft helps to hold the front panel to the chassis, while the mounting bracket was screwed to the underside of the main chassis for extra rigidity. Rigidity is a very good thing with regens 🙂 –

An MCD-100-M (2 x 100pF).  Nickel-plated brass vanes, a ceramic base (steatite actually), regular plate-spacing, and a “midline” capacity characteristic . Such a beautiful part. What else can I say? The US was once a manufacturing powerhouse –

Also in the haul were 3 x MC-20-S. I used one of these (from another buy) as the fine tuning control in The Sproutie –

Lastly, here’s an MC-50-MX. Single gang with capacitance swing of 10.5 – 53pF, midline capacity characteristic (for reasonably linear rotation-frequency relationship) and extra-wide plate spacing. This would be great for a VFO –

 

I also very recently acquired this National gear drive with 3 x 250pF variable capacitor attached. My relative lack of knowledge on National products caused me to misjudge. I discovered when it arrived, that the gearbox is a later model that is smaller than the older “classic” National gearbox drives. Also, it doesn’t have an eccentric bushing for driving the micrometer-style dial. I read somewhere that National stopped using the classic dial in their receivers in response to upgraded mil-specs that negated it’s use. It’s in good condition and turns freely, though the grease is old and it would probably benefit from a cleaning and re-greasing. I may well put this one up for sale –

There’s a little bit of surface dust on the plates and shaft but otherwise, it’s clean, and those terminals have never been soldered to. Amazing! –

No cracks in the insulators –

There’s only one detail that would require some attention from a builder, and that is that it looks as if the underside received a thwack at some point – either that, or it’s a manufacturing defect. One of the mounting holes is distorted and would need re-drilling and re-tapping if it is to be used. I think this is a solvable issue –

Another view showing this issue –

Old grease on the gears. If I were to use this, I’d want to clean out and re-lubricate, but that’s par for the course with these old yet still very serviceable gearboxes –

 

That’s it for now. Hope you didn’t mind me sharing all these pictures with you. As a parting thought, if you’re still planning to construct a regen and haven’t completely decided what to build, remember to take a look at VE7BPO’s Regen #4.

 

 

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My Ongoing Pre-Occupation With High Quality Air-Spaced Variable Capacitors

I’ve blogged before about air-spaced variable capacitors.  I’ve always liked ’em, but I think my understanding of what makes a good one is maturing a little more. I was the winning bidder on a really nice-looking specimen on eBay a few days ago.  Ever since placing the winning bid, I had been excitedly looking at the pictures of it posted by the seller. It looked great. How exciting when it arrived in the mail yesterday and I got a chance to see it “in person”, as it were!  I got it for $11.50 and I think I scored –

It is NOS (New Old Stock) meaning that while it is old, it has never been used. Surprisingly, there seems to be quite a few of these high-quality NOS caps still floating around. Here are the specs for this series of variable capacitors from Hammarlund –

The cap that I scored has nickel-plated brass vanes. Brass is good, as it expands and contracts with changes  in temperature less than aluminum does (the other main material from which variable capacitor rotors and stators are made.) Also good are the bearings on each end of the rotor shaft. I can’t see them, but I assume the bearings are hidden away. It gets better. This capacitor has wide-spaced plates, meaning less change in capacitance with temperature changes than a part with closer spacing. Oh – and this is all firmly mounted on a ceramic base. Ceramic is a great insulator and I’m thinking that this must also be good for the physical stability of the component with regards to changes in temperature.

I just noticed something. As you rotate the shaft clockwise, the capacitance increases. It’s normally the other way around. This must have been intended for use with a drive mechanism that translated the rotation of the tuning knob into rotation of the capacitor shaft in the other direction, or perhaps it’s from an era in which users expected to see wavelength increase with clockwise rotation, instead of frequency. I hope that the length of shaft protruding from the other end is enough for me to connect to, otherwise it might end up on the shelf for a few more decades! (EDIT – this wasn’t the case. It was used, very successfully, in The Sproutie Regen Receiver.)

One thing you may not appreciate from these photos is the feeling of solidity. This is a beautifully engineered part. See how the shaft is off-center? This makes for a non-linear relationship between the rotation of the shaft and the change in capacitance.  The change in capacitance occurs in such a way as to make the higher frequencies a little less cramped together, which is what happens with a capacitor where the relationship is strictly linear.

I mean, really – do variable capacitors get much better than this?  I don’t have definite plans yet for this little beauty but if my current interest in regens continues, I can see it paired up with the Jackson Brothers Dual Ratio Ball Drive and Dial I just ordered from the UK and used as the main tuning cap in a general coverage regen receiver – all built on a generously-sized aluminum chassis with front panel. (EDIT – unless I am able to connect the ball drive to the rear end of the shaft, this is not going to happen. Fingers crossed.)

Scroll back up to the top of the page and look at this fabulously engineered piece of American history sitting on top of it’s original box. That’s what it feels like to me – a piece of American history, and I got it for a few bucks. I will feel terribly privileged to be able to incorporate it into my own project at some point, though I’m going to hang onto that box.

Incidentally, while riding around Oakland, I noticed that this commercial space is up for lease.  It would be a good place for a ham-oriented business don’t you think? EDIT – It is now March 2014 and I recently noticed that this space has been turned into a coffee bar – the type that looks like it is part chemistry lab, with much glassware used in the brewing of the coffee.  Aah well – better than being left empty!