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Saturday, June 11, 2022

Putting the "Mate for the Mighty Midget" Back to Work -- With a DX-100 on 40 Meter AM

After working on it for a while I got so fond of my old Hammarlund HQ-100 that I moved it from the AM/Boatanchors operating position over to a more convenient spot right next to my computer.  This left a big gap on the receive side of the AM station.  

I briefly put my HRO-ish solid state receiver above the DX-100, but I'm afraid that receiver needs some work.  More on that in due course. 

I thought about putting my SOLID STATE Lafayette HA-600A atop the thermatronic DX-100, but this just didn't seem right. The Radio Gods would NOT approve. 

So I turned my attention to the Mate for the Mighty Midget that I built in 1998 and have been poking at and "improving" ever since

This receiver worked, but not quite right. It received SSB stations well enough, but when I turned off the BFO I could no longer hear the band noise. I wasn't sure how well the RF amp's grid and plate tuned circuits tracked.  And I had serious doubts about the detector circuit that Lew McCoy put in there when he designed this thing back in 1966. 

As I started this latest round of MMMRX poking, I realized that I now have test gear that I didn't have in 1998:  I now have a decent oscilloscope.  I have an HP-8640B signal generator (thanks Steve Silverman and Dave Bamford).  I have an AADE LC meter. And I've learned a lot about building rigs. 


The MMRX has a tuned circuit in the grid of the RF amplifier, and another in the plate circuit of the RF amplifier.  There is a ganged capacitor that tunes them both.  They need to cover both 80/75 and 40 meters. And they need to "track" fairly well:  over the fairly broad range of 3.5 to 7.3 MHz they both need to be resonant at the same frequency.  

McCoy's article just called for "ten turns on a pill bottle" for the coils in these parallel LC circuits.  The link coils were 5 turns.  No data on inductance was given.  Armed now with an LC meter, I pulled these coils off the chassis and measured the inductances of the coils.  I just needed to make sure they were close in value.  They were: 

L1 was .858uH L2 was 2.709         L3 was .930uH  L4 was 2.672

Next I checked the ganged variable capacitors.  At first I found that one cap had a lot more capacitance than they other.  How could that be?  Then I remembered that I had installed trimmer caps across each of the ganged capacitors. Adjusting these trimmers (and leaving the caps connected to the grid of V1a and V2A, I adjusted the trimmers to get the caps close in value.  I think I ended up with them fairly close: 

C1: 63.77-532 pF          C2 64.81 -- 525.1 pF

I put the coils back in and checked the tracking on 40 and on 80/75.  While not perfect, it was close enough to stop messing with it.  


I've had my doubts about the detector circuit that Lew McCoy had in the MMMRX.  In his 1966 QST article he claimed that the circuit he used was a voltage doubler, and that this would boost signal strength.  But I built the thing in LT Spice and didn't notice any doubling.  And consider the capacitors he had at the input and output of the detector:  100 pF.  At 455 kHz 100 pF is about 3500 ohms.  At audio (1 kHz) it is 1.5 MILLION ohms. Ouch.  No wonder years ago I put a .1 uF cap across that output cap just to get the receiver working. 

Scott WA9WFA told me that by the time the MMMRX appeared in the 1969 ARRL handbook, the second "voltage doubling" diode was gone, as were the 100 pF caps.  Now it was just a diode, a .01 uF cap and a 470,000 ohm resistor.  I switched to the 1969 Handbook circuit (but I have not yet changed the 1 meg grid resister to 470k -- I don't think this will make much difference).  Foiled again by a faulty QST article, again by one of the League's luminaries. 

6U8s out, 6EA8s in 

We learned that the 6U8 tubes originally called for by Lew McCoy are getting old and not aging well.  So I switched all three to more youthful 6EA8s.  This seemed to perk the receiver up a bit. 

MUTING from the DX-100

My K2ZA DX-100 has a T/R relay mounted in a box on the back of the transmitter.  When the Plate switch goes up, it switches the antenna from receiver to transmitter.  The box also has a one pole double throw switch available for receiver muting.  I put the common connection to ground, the normally connected (receive position) connect the ground terminal of the AF output transformer to ground -- it is disconnected from ground on transmit.  The other connection (normally open) is connected to the antenna jack -- on transmit this connection ground the receiver RF input connection.  These two steps mutes the receiver very nicely. 

Replacing Reduction Drive

Over the years I have had several different reduction drives on the main tuning cap.  I had a kind of wonky Jackson brothers drive on there that needed to be replaced.  I put in a new one -- this smoothed out he tuning considerably. 

Ceramic Resonator

I never could get McCoy's 455 kc two crystal filter to work right.  So at first I made due with the two 455 kc IF cans.  This made for a very broad passband.  Then I put a CM filter in there.  This was more narrow, but with a lot of loss.  There may have been others.  But the filter spot is currently held by a 6 kHz wide ceramic filter.  This one is my favorite so far. 

Digital Readout

When I was running the DX-100 with the Hammarlund HQ-100 I built a little frequency readout box.  The box was from a Heath QF-1 Q multiplier (I am sorry about this).  The readouts are in Juliano Blue and come via e-bay from San Jian.  I now have it hooked up to the DX-100's oscillator.  I haven't tapped into the MMMRX's oscillator yet. 

Friday, June 10, 2022

SolderSmoke FDIM Interview: Adam K6ARK and his 2.6 gram Mini-Pixie SMD Transceiver

I've not bee a big fan of the super-simple Pixie transceiver, but Adam K6ARK could make me a believer. 

Our correspondent Bob Crane W8SX interviewed Adam at FDIM.  You can listen to the interview here: http://soldersmoke.com/2022 K6ARK.mp3

The video above shows Adam's tiny Pixie in action in the California desert.  His rig is about the size of a postage stamp and weighs about 2.6 grams. FB Adam. 

Adam did a lot better with his Pixie than I did with my far larger and more complicated SST transceiver. He also did better than I did on 40 when I was using my ET-2 (two FET) transceiver. 

I liked how Adam recorded in the field the CW from his rig, I also liked his key (!) and his EFHW antenna and "tuner."  Adam's ability to cope with no CW sidetone was also admirable.  

Adam's YouTube channel is here: https://www.youtube.com/c/K6ARKPortableRadio

Thanks to Adam and to Bob Crane.  

Wednesday, June 8, 2022

SolderSmoke FDIM Interview: Jack Purdum W8TEE on the Challenges of Decoding CW by Software


In his interview with SolderSmoke correspondent Bob Crane, Jack Purdum made some very interesting comments about the challenges of decoding CW with software. He notes that W1AW's code practice CW is perfect, but that below 18 wpm, they deliberately insert a "Farnsworth Delay" that increases the spacing between words -- this complicates automatic CW decoding.  

Jack also talked about the distinctiveness of different CW operators.  Jack  noted that W1AW has no real "fist" in this sense:  "It has the personality of a stick!" 

Jack mentioned that Pete Juliano had been reading book on SDR radios that Jack and Al Peter recently published:  https://www.amazon.com/Software-Defined-Radio-Transceiver-Construction/dp/B09WYP1ST8/ref=sr_1_1?crid=2KPYAMPOW5P6J&keywords=DR.+Jack+Purdum&qid=1654598559&sprefix=dr.+jack+purdum%2Caps%2C40&sr=8-1  

Here is our correspondent Bob Crane's interview at FDIM 2022 with Jack Purdum: http://soldersmoke.com/2022 W8TEE.mp3

Thanks Bob.  Thanks Jack.

Tuesday, June 7, 2022

Sticker Spotted Near the Old NYC Home of E. Howard Armstrong


Thanks Dave! 

SolderSmoke FDIM Interviews: A BRAVE HAM! Grayson Evans KJ7UM Presents a 50 Watt Amplifier to THE QRP GROUP!

Wow, talk about walking into the lions' den!  Grayson Evans, author of "Hollow State Design" and guru of all things thermatronic, went to FDIM and made a presentation TO THE QRP GROUP on how to build a 50 watt amplifier with a 6146 thermatron.  In New York that would have been called chutzpah.  The QRP ARCI guys seem to have tolerated this QRO-heresy; I'm not so sure the zealots over in G-QRP would have been quite so tolerant.

Grayson gave a nice shout out to SolderSmoke's Pete Juliano.  

And he offered some sage advice to those who live in fear of high voltage:  "Don't touch anything with high voltage on it."  Words to live by my friends.  He even managed to call those who shy away from high voltage "wimps."    This was all very reminiscent of the unforgettable safety advice he offered in his August 2021 interview on Ham Radio Workbench: "Try not to swallow anything, and don't sit on the thermatrons." I mean, who can argue with that? 

You can listen to Bob Crane's interview with Grayson here (about 6 minutes total): 

http://soldersmoke.com/2022 KJ7UM.mp3

Check out Grayson's  Hollow-State Design Book 3rd Edition: tinyurl.com/hollowstatedesign3

Check out Grayson's technical blog:kj7um.wordpress.com

Thanks Bob and thanks Grayson. 

Monday, June 6, 2022

SolderSmoke FDIM Interviews: Hans Summers G0UPL Talks about the QDX and His New Balloon Tracker


Thanks to Bob Crane W8SX for getting us this wonderful interview with Hans G0UPL.  Its really amazing to hear Hans talk about how many QDX rigs and Baloon Trackers have been sold by QRP Labs, and how quickly they sell.  Really great.  Hans's comments on the realities of the parts shortage was also very interesting. 

Listen here (about 7 minutes):  http://soldersmoke.com/2022 G0UPL.mp3

U4B Balloon Tracker 

SolderSmoke FDIM Interview with Farhan VU2ESE -- The sBITX is Coming!

Bob Crane W8SX --  our correspondent in Dayton/Xenia --  once again collected interview with FDIM presenters. Thanks Bob!  Here is his talk with our friend Farhan:  

http://soldersmoke.com/2022 VU2ESE.mp3

Here is a great post on the sBITX (May 30, 2022) from Farhan's web site: 


Here is Farhan's amazing presentation on the sBITX at the 2021 FDIM: 


Sunday, June 5, 2022

JFET (Junction Field Effect Transistor) Video -- Part 1

Good video.  I like how he related the real-world device to the Igs graph. Also, note the big variation in MPF-102 parameters.  Stay tuned for Part 2. 

Friday, June 3, 2022

Phase Noise and all that

Our friend Dave K8WPE has been listening to old podcasts.  He recently came across those in which Pete and I were talking about phase noise.   He asked for some resources on this topic.  Here is what I sent him: 

 Receiver performance expert Robert Sherwood explains it this way: 

Old radios (Collins, Drake, Hammarlund, National) used a VFO or PTO and crystal oscillators to tune the bands. Any noise in the local oscillator (LO) chain was minimal. When synthesized radios came along in the 70s, the LO had noise on it. It is caused by phase jitter in the circuit, and puts significant noise sidebands on the LO. This can mix with a strong signal outside the passband of the radio and put noise on top of the weak signal you are trying to copy. This is a significant problem in some cases: You have a neighboring ham close by, during Field Day when there are multiple transmitters at the same site, and certainly in a multi-multi contest station. You would like the number to be better that 130 dBc / Hz at 10 kHz. A non-synthesized radio, such as a Drake or Collins, has so little local oscillator noise the measurements were made closer-in between 2 and 5 kHz. 

Rhode and Schwarz have a good oversight video with great graphs that explain the fundamentals,  See above or here:  https://www.youtube.com/watch?v=hfgaEjf1154 

I think a lot of the fretting about advanced receiver performance measurements are really kind of over-the-top, and mostly of interest to advanced builders who want the very best performance from their receivers.  Most of the rest of us are happy if we can hear the band noise and separate the desired signals from the QRM.  But I must admit that as time goes on, I find myself getting more and more finicky.  I start to worry about gain distribution and dynamic range.  But I don't worry so much about phase noise because I am more of an LC oscillator guy and don't make much use of the PLL devices (like the Si5351) that do produce more phase noise. 

I've had many articles on the blog about about phase noise.  Here they are: 

Thursday, June 2, 2022

The Amazing Workshop and Test Gear of Tony Albus PE1ONS

This is almost too much.  This makes me want to go out and buy some more test gear, maybe another scope, or a spectrum analyzer, or at least another DMM.  I mean Tony has at least three of everything. 

This is really amazing.  Tony is obviously a test gear guy, but he also has a ham call PE1ONS.  He says he is not too active, but we should encourage him to get more involved with ham gear.  We need guys like him working on ham gear.  And he seems like such a happy person.  

Here is his QRZ.com page: https://www.qrz.com/db/PE1ONS

Here is his awesome YouTube channel: https://www.youtube.com/TonyAlbus
Designer: Douglas Bowman | Dimodifikasi oleh Abdul Munir Original Posting Rounders 3 Column