BITX Build Update #3

This weekend I built the VFO.   I used Doug DeMaw's FET Hartley, from his "QRP Notebook," page 50.

Getting a VFO into the desired frequency range always seems to involve a bit of "cut and try."  Not only do you have to get in the correct freq range, but you also have to try to get the tuning range of your variable cap (or coil) to match the band you want to cover.  In my case, thinking of 20 meter SSB, I need about 200 kHz of tuning range.  And sticking with my 9 MHz (for 20) and 13 MHz (for 17) plug-in filter plan, I need the VFO to be in the 5 MHz range. 

That main tuning cap you see above has more capacitance than necessary (19-148 pf).  One option would be to pluck out some of the rotor plates (been there, done that).  But that seems a bit barbaric, so instead I just reduced the overall capacitance by putting a smaller fixed capacitor (about 20 pf)  in series.      

Yesterday I had it percolating nicely.  Waveform looks beautiful on the Tek 465.   But it was drifting too much.  I had a cheap plastic trimmer cap in there.  This morning I replaced it with a more substantial ceramic trimmer.  This seems to have improved stability quite a bit.

I need to put some stabilizing substance on that toroid.   DeMaw prescribes Q-dope.  I'm all out.  What is the field expedient substitute?   Was it clear nail polish? 

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BITX Build: Update #2

Not much progress to report.  But I have been thinking about the filter frequency.  Here is my latest idea: 

Maybe I'll build the VFO in the 5 MHz range.  This would allow me to use the 9 MHz Yaesu filter (and associated crystals) that Steve Smith sent to me (see above).  With this I could be on 75 and 20 meters. 

I could build another filter at 13 MHz and, using the same VFO in the 5 Mhz range, get on our beloved 17 meter band.  I kind of like the idea of plug-in filters. 

You can see my ideas for the board layout.  I'm thinking of a Doug DeMaw Universal Hartley VFO inside the box shown above.   The tuning cap you see is out of an old Heath QF-1 Q Multiplier.  It has a very nice reduction drive built into the tuning shaft.  It measures 19 to 148 pf.     
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A Visit to the Arduino Factory (video)

Named for an Italian bar, produced by cool people in Ivrea who wield soldering irons and wear T-shirts that say "Make it Simple."  You gotta love Arduino! 

Today I will finish connecting my Arduino Uno to my Doug Demaw Lil' Slugger 10 Beacon rig.  Code (computer code!) by K6HX. 

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Video of my Barebones Superhet

I literally blew the dust off this thing last week.  I posted the schematic a few days ago (scroll down).   This morning I finished re-building the CW transmitter that went with it.   I am running out of rigs to re-build, so I suppose I will now have to start building some new ones.  Maybe a BITX-20?  Or a BITX-75/20? 

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DeMaw's Barebones Superhet

A recent e-mail got me thinking about Doug DeMaw's Barebones Superhet.  June 1982 QST.  Mostly 40673 Dual Gate MOSFETS (this one's for you Dino!).  Barebones indeed.  Check out the schematic.

I literally blew the dust off my version and fired it up on Sunday.  It sounds really great.   I had lowered the values of the caps in the ladder filter to widen it out for phone.   Also, I see that I used LM386 instead of the op amp AF Amp prescribed by W1FB.   I notice that my version has much better audio than another version of this RX (with the op amp) that I'm using on 17 meters.  Could the difference be the LM386 vs. op amp?  There is a lot more audio with the 386, and the AF response seems wider.  

I feel the urge to put this receiver to use.  I am rehabilitating the W1VD CW transmitter that used with it during the late nineties, but I'm a phone guy now, and I feel compelled to use this RX as part of an SSB rig.  Of course, I could build a standalone SSB TX, but how about a diode switching scheme to make use of the Barebones' filter, the VXO and the BFO in a transceiver?

Here is my article on the original build of this receiver: http://www.gadgeteer.us/HBHOME.HTM

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Richard's Rip-Van-Winkle Barbados Rig

Dear Bill,

Thank you for taking a few minutes to read my email. I recently found your blog and enjoyed reading the entries.  However, the most interesting and exciting article I discovered was your "Homebrew Radios in the Age of the Internet." The article reminded me of a project started 30 years ago this month and placed in a box in the attic to wait until I could return to it.
Thirty years of health care management work filled the interim.

The project is the 8P6 Hamcation rig by W1FB from QST of June and November 1982!

I searched through the cob webs in the attic and found that all the boards are completed and I even fabricated a case. The original QST's were there too! In response to a (somewhat strong) suggestion from my wife to "get a winter project going", the rig is back on my retirement work bench. Wahoo.

Progress to date has been instructive and fun. The boards are all cleaned, checked, and 95% wired together in the case and working. I attached 2 photo for your pleasure because you wrote specifically about the receiver in some blog posts.

This is where I am scratching my head. As you say in the article, "receivers are tough". I am confused and a bit frustrated about the correct procedure and sequence to correctly align the Barebones superhet. I can already hear stations and the noise level is acceptable. But I am not quite sure how to proceed in peaking up the receiver (hit a plateau?) Any suggestions for a newly retired ham now with the time to get back to the bench?

Thank you very much for your kind comments and interest in my project.

73's
Richard
WB2PEF
Cherry Valley NY

Wow, what a cool resurrection project!   I recognize both the receiver and transmitter boards.   The RX is clearly DeMaw's Barebones Superhet on a FAR Circuit board.  I am listening to one of those AS I TYPE.  Mine was put together by Dale, W4OP, and currently inhales on 17 meters.   The TX board is what became known as the VXO 6 Watter.   It was (I think) designed by W1VD, and appears in the ARRL book "QRP Classics."   This was my first really successful homebrew rig -- I built that transmitter in the Dominican Republic  in 1993.   I still have most of the board, and the 20 meter Barebones RX that I used with it.   Richard's message makes me want to put this old gear back on the air. 

I hope Richard will send us an update on his Barbados revival project.

I'm sure Doug DeMaw would be very pleased to know that his projects are still providing radio amateurs with a lot of fun and inspiration.


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VE7BPO on "Killing Q"

I'm still scratching my head a bit about HOW the resistor prescribed by Edgardo, LU1AR, cured the 250 kHz oscillation problem that was plaguing the JBOT amplifier in my 20 meter DSB rig.   Earlier I'd posted an excerpt from a CQ article in which Doug DeMaw talks about swamping and Q killing.   Last week I got a very thoughtful e-mail from esteemed homebrewer Todd,  VE7BPO.  Here is an excerpt :

Thoughts and Considerations
Let’s discuss squashing low frequency oscillations in a QRP transmitter; say at 200 KHz or so. A low value resistor across the coil (12t -- FT37-43) often works well to stop these.
Oscillations come from the transistor: gain versus frequency isn’t linear, nor is impedance at transistor ports. We’ll often add negative feedback and such to stabilize an amplifier towards unconditional status. In my Tx circuit that oscillated, no feedback was applied. 
 In the case of an inductor wound on a FT37-43 or FT50-43, the Q is already low (say 8- 15 or so). Obviously a resistor in parallel with such a coil is not going to lower Q since Q is already quite low. That R will reduce the inductor impedance and thus may serve to decrease the low frequency gain of the RF amplifier to stop any low frequency oscillations. This might not work so well with a way-high fT transistor where decoupling might be hampered if UHF oscillations are singing. 
Doug DeMaw often referred to the parallel resistor as a Q-killer. If we examine the equations describing parallel, or series resonant circuits -- if the Q of a tank is high enough, we can practically ignore the effect of resistance at resonance. Conversely if we add a resistance and make it high enough, we might even obliterate the resonant frequency or “kill the Q”. Engineers have long placed an R into a parallel-tuned circuit to drop Q and stop oscillations — they refer to it as damping. 1 example might be in old TV sets where a variable resistance was added to peaking coils to prevent a tank from ringing at a frequency determined by the coil L and distributed C. This applies to higher Q inductors and not our FT37-43 inductor. 
Decoupling
Our teacher, Wes, teaches us in EMRFD that coupling often occurs along the DC power supply lines.  Further, he’s taught us to decouple AC by placing high impedance in this path.  Often the impedance is a low-pass filter with series element(s) of a high Z and shunt element(s) with a low Z.  The filter must present a simple short circuit (or perhaps just a resistance) at low frequency so DC flows to the amplifier.    
Final 
Oscillations should likely be identified and treated according their frequency. This topic looks advanced and all RLC networks deserve more attention from us.
Todd, VE7BPO --- Feb 27, 2013  

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Swamping or Q-Killing

Sounds like some new TV series on the Discover Channel, doesn't it? Actually it refers to efforts to stabilize oscillators by reducing the Q of components that may be helping to turn the amplifier into an oscillator.   A 300 ohm resistor across the primary of the output transformer on my most recent version of the JBOT amplifier recently solved my instability problem.  As part of an effort to understand how and why I came across a CQ magazine article by Doug DeMaw.  The relevant passage appears above.  

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Putting 17 Meter SSB Station On the Air

Magnificent, don't you think? The sun is rising over Northern Virginia, and 17 meters is starting to come alive. Yesterday I finished the tweaking and peaking of the JBOT amplifier for the transmitter. It is nice and stable now. (And yes, Steve, it has a low-pass filter!) On top of the transmitter cabinet is the receiver. It is a Barebones Superhet designed by Doug DeMaw and built on a FAR circuit board by Dale Parfitt, W4OP. I changed it to 17 meters and broadened the filter response for use on SSB. Both the transmitter and the receiver use variable crystal oscillators, with two crystals in each (switch-able from the front panels). The frequency coverage of of the transmitter and the receiver match up fairly well (good thing!). Wish me luck! Today I will venture forth amidst the coronal mass ejections and try to make a few SSB contacts on good-ol' 17.

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Back to the Barebones (Receiver)

My rehabilitation of homebrew 17 meter gear from the last solar cycle continues. Following the same path that I followed in 2001-2002, I will now move from DSB to SSB. I pulled out the little receiver that I put together way back when. It is a version of Doug DeMaw's "Barebones Superhet" (aka "Barbados Superhet"). I bought it on the net. It had been put together by a skilled builder on a FAR Circuits board. The fellow who built it had changed the IF from Doug's original color burst freq (3.579 MHz) to 5 MHz. He had also put in a varactor controlled VFO using a DC voltage multiplier to get more voltage variation across the varactor. I also think he had it built for 20 meters.

I converted it to a 17 meter receiver. I put in a VXO, using two crystals controlled by a panel switch. I also changed the caps in the filter so as to broaden the response for SSB.

As I was going through all these modifications, I turned to the USENET for help and advice. Dale, W4OP, came to my assistance. Little did we know how DETAILED his familiarity with my RX was:

"Bill N2CQR MOHBR" ...@virgin.net> wrote in message

news:22f6e3ee.0503292244.1b9a1481@posting.google.com...

> Dale: Wow, another Barbados RX builder. That was my first successful
> superhet project. I now have the one I built (still on 20), and this
> morning
> I got another one (the one built by someone else on a factory-made
> board)going on 17 with a VXO. I have a THIRD partially built Barbados
> RX board. If this
> keeps up, I'll soon have a BBRX museum.
>Hi Bill,

Did I sell you mine years ago? I seem to recall using a temp stabilized
varicap in a shielded enclosure for main tuning. It was done on a factory
board. Or was that a 6M xverter I sold?

Dale

Newsgroups: rec.radio.amateur.homebrew From: meara.lon...@virgin.net (Bill N2CQR MOHBR) Date: 30 Mar 2005 22:33:57 -0800 Local: Thurs, Mar 31 2005 1:33 am Subject: Re: Homebrew projects Dale: Wow, small world! Yea I think that is the one I'm working on. I think you also had a DC-DC converter to bring the voltage to the varicaps up. Very nice enclosure for the oscillator. I now have it percolating nicely as a VXO around 23 Mhz (for the 17 meter band). Bill - Hide quoted text -

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Amateur radio at PAR (W4OP)

An e-mail exchange on QRP-L this morning about using ceramic resonators in 455 kHz filter circuits (great idea Grayson!) led me to the ham radio corner of the PAR electronics web site. PAR is the company run by Dale Parfitt, W4OP.
http://www.parelectronics.com/par-amateur-radio.php.
That's a nice looking 2-B Dale! Please send us the serial number! Even more impressive is Dale's award-wining homebrew solid state version of the 2-B (on the far right). Visit his web site for more info (on his site you can hover your mouse above the pieces of gear for more info).

I'm proud to say that I have a piece of gear in my shack that was built by Dale Parfitt. The story is told in SolderSmoke The Book: I'd built my own version of Doug DeMaw's Barebones Superhet and had liked it a lot. When I saw another one (this one built on a FAR circuits board) for sale on e-bay, I bought it. It stayed on the shelf for a while. Years later when I started working on it, I turned to QRP-L for help and this fellow named Dale Parfitt came to my rescue. It was only after a long series of e-mail exchanges did we realize that the receiver we were discussing had been built (and sold to me) by... Dale Parfitt.

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Steve "Snort Rosin" Smith: Silver-Tongued Devil! Dual-band "Vlad the Inhaler" RX

You'll see in the comments attached to my last blog post that our man on the left coast, Steve Smith, gave that cute little Doug DeMaw/Vlad Polyakov receiver a name that might set American-Russian ham relations back a bit: He called it "Vlad The Inhaler." Good one Steve! (But you might want to stay out of the diplo game!)

It occurred to me that with the installation of one little switch in the diode part of the circuit, we could turn this into a dual-band RX. Take a look here:

http://soldersmoke.blogspot.com/2010/03/polyakov-plus-dual-band-receiver-with.html

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Doug Demaw and Vlad Polyakov

I was looking through W1FB's Design Notebook the other day and I came across the above schematic for a direct conversion receiver (page 111). Note the Polyakov detector. One strange thing though: Doug was running the oscillator at the operating frequency. I thought the big advantage of the Polyakov design was that you ran the oscillator at half the operating frequency (that's why it is sometimes called a "subharmonic" detector). Any ideas on why Doug did it this way?

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The Last Roman Rig: New WSPR DSB Transceiver

They are coming to pack up my soldering iron next week, so I'm afraid this will be my last Roman creation. You can see the W3PM Colpitts oscillator in the center. The KA7EXM AF amp is in the lower left. The Softrock-based Manhattanized PA is in the upper right. To the right of the oscillator is a classic W1FB two diode balanced modulator. You see three big green relays. The top one switches the antenna and 12 volts during T/R switching. The middle one switches the audio amp input, the bottom one switches the audio amp output. (I use the same AF amp for receive and transmit, just switching around the input and output.)

It works fine and has sent and received WSPR sigs. I think it will also do PSK-31 with the FLDIGI program. I could use a one more stage of RF amplification between the balanced modulator and the PA driver amp. Also, the AF impedance match between the KA7EXM amp and the balanced modulator nees work: Roger's circuit was deisgned to drive high impedance phones. That balanced modulator circuit has about 50 ohms at each port. Ideas?

I was thinking of calling it the Achilles. But I think I will go with "L'Aquilone" (The Kite").

Faraday's Transformer Diagram

Looks familiar, doesn't it? Reminds me a lot of the toroidal transformer diagrams that you see in the Doug DeMaw books. This is from an 1831 lab notebook of Michael Faraday.

Dale Parfitt, W4OP, Homebrew Hero

Yesterday, in my search for pictures of the Drake 2B, I came across VE3MPG's excellent interview with Dale Parfitt, W4OP. Dale and I crossed paths years ago: After I built my first version of Doug DeMaw's Barebones Superhet, I went out on the net and found a couple of kit versions of this receiver for sale. A few years later, I was working on one of them, and turned (as you do) to QRP-L for some advice. Dale came to the rescue and we started exchanging e-mails. After a while he told me that the version I was working on sounded familiar. Sure enough, he was the original builder! The story appears in SolderSmoke -- The Book.

Here is further evidence that Dale and I have similar tastes in receivers: Pictured above is his solid state version of the Drake 2-B. FB! I note that the dial scales are the same as the hollow-state version.

Here is the VE3MPG interview:
http://ve3mpg.blogspot.com/2008/10/interview-with-dale-parfitt-par.html

Gil Cartoon of W7ZOI on the Mountaintop

Steve, WA0PWK, sent this one in. He found it in an old issue of "Hints and Kinks." Note the callsign on that FB HB portable rig. Wes tells me that this cartoon was actually a Doug DeMaw initiative. Doug took an older cartoon by Gil, W1CJD, and put "W7ZOI" on the rig. The drawing appeared next to a short article by Wes about an electronic keyer using a pair of 741 Op Amps. Wes also reports that his kids got a real kick out of it -- his son Ron was taking an elective printing course in Jr. High School at the time, and printed up a bunch of W7ZOI QSL cards featuring the modified Gil cartoon.

I thought that Steve, N0TU (now WG0AT !), would find this especially interesting.