Video on the Strange Tuning of the Radio Shack DX-390 Receiver

I'm more of a single conversion guy myself, but in working with the DX-390 I came to appreciate the benefits (especially regarding image rejection) of the double conversion technique. 

While working on the DX-390, I discovered that the BFO control on the front panel DOES NOT change the BFO frequency.  It was fun to try to figure out why the designers did it this way.  It does make sense once you consider the limitation imposed by that PLL main tuning oscillator that only moves in 1 kHz steps. I hope the video explains things.  

Here is the drawing I used in the video: 

And here is a drawing that shows how a single conversion superhet with a fixed or switchable  (usually crystal-controlled) BFO works: 

Earlier this month  I did a blog post on my repair of a broken DX-390: 

https://soldersmoke.blogspot.com/2020/08/fixing-up-radio-shack-dx-390-aka.html

Mike WU2D's Great Drake TR-3 Refurb Video -- Part 2

Mike WU2D made another very nice video about his Drake TR-3 refurb. 

He cracked me up when he noticed that one of the calibration oscillators was 30 Hz off.  "Let's pretend we care," said Mike.  Indeed. But it was probably wise to tackle this problem, given how upset modern hams seem to get with 30 Hz discrepancies.  Mike noted that the problem was casued by "aging cystals." Yikes! Another thing to worry about, along with the Southern Magnetic Anomaly. 

Once again Mike has added a useful term to our lexicon:   "The Dribble Method" of signal injection (or extraction):  Just wrap a few turns of insulated wire around the tube or IF can and inject or measure away.

Neutralization! Now there's a blast from the Thermatron past.  I haven't done that in a long time. I liked Mike's "reverse neutralization" method. 

Mike's video featured some real Boatanchor eye candy.   That Heathkit HR-10 receiver caught my eye, as did that HP signal generator. 

Thanks Mike.  One hand behind your back OM.    

Very Cool Vosworx AZ-EL Satellite Tracker

When I was in the Dominican Republic working the RS satellites, MIR and the Space Shuttle, I didn't have an elevation rotator.  I could only move the antenna in the azimuth plane.  So I just aimed it about 45 degrees above the horizon, spun it around, and hoped for the best.  I also had no computer control of the azimuth heading -- I'd just watch the location of the satellite on my computer screen and operate the rotator control manually.  

This device is a big improvement. 

The Original Tuna Tin 2 (and the Mojo Transfer Ceremony with W1REX)

From the Amateur QRP Radio group on Facebook:‎

Brian Murrey‎to

 Amateur QRP Radio

The Original Tuna Tin II built by Doug DeMaw W1FB (SK)...Ed Hare W1RFI of the ARRL Lab, brought this to FDIM in 2001 and allowed us to borrow it for a night. It was so cool. The little guy was found at a hamfest somewhere on the east coast, and brought back to the ARRL. Bruce Muscolino W6TOY (SK) recieved it from the ARRL and he replaced missing parts and got the thing working again. It's in the museum at the ARRL HQ today. I think we made like 16 contacts using that for the transmitter, and a homebrew receiver from Diz W8DIZ that was part of the MultiPig Rig. Antenna was a dipole we had strung up out the window of the Ramada Inn (FDIM Central).
-----------------------------------
Here is my only encounter with the original Tuna Tin 2 -- In 2014 Rex W1REX conducted this solemn ceremony in which some of the TTT mojo was transferred to my homebrew BITX 17. 

The South Atlantic Anomaly in Earth's Magnetic Field

In case you've grown tired of worrying about infectious disease, here's another scientific problem for you to worry about:

https://www.youtube.com/watch?v=qpdQcw_52iM

SolderSmoke Lexicographer Worked on Earth-Moon-Earth Laser

Hi Bill

A ham, well known to us, actually me, was very involved in the very first laser reflector experiment.  I was a new EE graduate and was hired by the University of Maryland Physics department to design and build the control system for the laser experiment.

The first reflector was placed on the moon immediately after Apollo 11 landed.  And the equipment on Earth to use it was in place and ready to go to work.

Here is how it worked.  A very powerful laser was installed at the McDonald Observatory in west Texas.  It pointed into the viewing end of the telescope and had 5 Joules of energy.  When it was fired, a control system made from discrete logic chips was used to control an electronic shutter in front of a very sensitive photomultiplier tube.

In order to prevent stray photons from impacting the photomultiplier tube, the shutter was opened just microseconds before the reflected photons were expected to arrive from the moon, a round trip of about 2.5 seconds.  Then the photons were collected from the laser's pulse.  Only between 1 and 10 photons were collected from each shot and were statistically analyzed to get the best transit time to determine the distance to the moon.

Similar systems were set up on other continents in order to triangulate the distances between the telescopes to determine continental drift.  In addition, the outward shift of the lunar orbit was also determined.

Because the moon's orbit varies by about 10% and is easily predicted, the control system used thumbwheel switches to set the time of transit to open the shutter at the right time to keep out photons that were not from the laser.

The control system also sent the trigger pulse to the laser's gigantic capacitor bank to send trillions upon trillions of photons to the moon. So a 5 volt pulse triggered this massive release of stored energy into the lens of the telescope.

And here is the coolest part,  the astronauts could see the laser pulses from the telescopes when they were fired.

I had no idea that these reflectors were still in use and that their efficiency has degraded likely due to lunar dust. 

73

Steve

KB3SII ... .. ..

------------------------

See also: http://soldersmoke.blogspot.com/2020/08/earth-moon-earth-with-lasers.html
and
https://www.smithsonianmag.com/smart-news/scientists-shot-lasers-moon-decade-then-one-bounced-back-180975585/

WU2D's TR-3 -- Mike Refurbishes a Nice Old Drake Transceiver (PART 1) (Video)

There is a lot of Tribal Knowledge in this video.  And good discussion of the many moral issues faced by those of us who work on old gear. 

-- Mike seems apologetic about his blatant and blasphemous spray painting of the Drake copper chassis.  As well he should be. 

-- His stubborn replacement of the tube socket (to allow for shielding) seems wildly reckless to me.  The Radio Gods may retaliate with some unexpected instability in that circuit. 

-- He CORRECTLY refers to the rewiring of the final circuity (to accomodate 6146s) as "the evil thing."  Indeed. 

-- I love in the beginning how he is listening to some ham radio chatter and the guy is talking about the selection of COM PORTS. With old radios "we don't have COM PORTS -- we have an antenna connector."  Well put Mike. 

-- I was struck by how much the TR-3 innards look like my 2-B receiver.  But the TR-3 has no dial strings.  That is a major technological improvement. And it has a PTO.  Was this a case of Collins envy?

-- Mike adds a useful word to the lexicon:  "shotgunning" -- the indiscriminate replacement of entire categories of parts in old radios.    Now I don't mind shotgunning the electrolytics (some people bitterly oppose this).  But I agree with Mike on the wisdom of keeping the paper caps in there.    

I am looking forward to Part II.  These videos are like "This Old House"  but instead "This Old Rig." And I will go back and look at Mike's video on the Power Supply refurb.  Thanks Mike. 

Earth-Moon-Earth -- With Lasers

https://www.nytimes.com/2020/08/15/science/moon-lasers-dust.html

Thanks to Chuck KE5HPY for alerting us to this very interesting article. 

Fixing up a Radio Shack DX-390 (AKA Sangean ATS-818) While Suffering from Fat Finger Syndrome

I've had this Radio Shack DX-390 portable receiver since the early 1990s.  I bought it when I was in the Dominican Republic.  It accompanied me on some interesting trips to the Haitian border, and on one very memorable 1994 trip to the Haitian capital.  I have made some CW contacts with it serving at the inhaler.  

Click on the diagram for a better view. It is a dual conversion superhet.  First IF is at 55.845 MHz.  There is a big 90's era IC-based PLL oscillator that runs from 55.995 to 118.7 MHz -- The main tuning dial moves this oscillator.  Second IF is at 450 kHz.  There is an oscillator at 55.395 that takes the signal down to 450 kHz. Selectivity (not a lot) is provided by ceramic filters.  Finally there is a product detector and a 450 kHz  oscillator that produces the audio.   While there are many mystery chips in this receiver, there is also a lot of discrete-component analog circuitry in there -- it is kind of a pleasing mix. 

DX-390 Main Board.  Note kludged toroidal replacment for L10 (just above ferrite antenna) 

The old DX-390 suffered a lot of wear and tear.  The case is very beat up.  The most serious problem was that at some point, probably on a cold, dry, winter day in Virginia, static electricity took out the FET in the receiver's front end.  I made a half-hearted effort to fix it, but it never really worked properly.  

I occasionally found myself thinking of this receiver.   I spotted one on e-bay not long ago, and bought it.  This newer one was in very nice shape.  

But that old one was kind of staring at me from the corner of the shack.  "C'mon radio man," it seemed to say, "can't you fix a shortwave receiver?" So this week I took up the challenge.  

First the FET.  I had kludged an MPF102 in there, but that didn't seem to work well.  Internet fora seemed to think that a J310 would do better, so I installed one of them -- it did seem to work better.  (Note:  Pete Juliano likes J310s -- TRGHS.) 

Kludged in J310. And two sets of back to back diodes

During my earlier repair effort I had apparently destroyed the front end output transformer (L10) but I discovered that I had replaced this with a toroidal transformer.  It still worked, so I left well-enough alone. 

I was pleased that the old receiver was receiving OK, but there was a problem:   The "BFO" control wasn't working.  The BFO would come on, but turning the BFO control did not vary its frequency.  

At this point I discovered that while there are many copies of the DX-390 service manual and schematic on the internet, all of them have seriously degraded copy quality right around the parts of the circuitry that I needed to study.  Sometimes Murphy overpowers the Radio Gods. It took me a while to get a useful schematic of the BFO control mechanism. 

BFO is a bit of a misnomer here:  the control actually shifts the frequency of the 55.395 MHz oscillator that drives the second mixer.  See Block diagram above).   There is a varactor diode in the base circuit of a BLT oscillator circuit.  Turning the BFO control varies the voltage going to the varactor thus causing the oscillator frequency to slide up and down.  But mine wasn't moving.  And that was a problem. 

So I dove right in, trying to figure out why it was oscillating, but not shifting in frequency.  At this point I discovered that I too am afflicted with the disease that Pete Juliano suffers from: Fat Finger Syndrome.  That BFO control circuit has a nice big 100k pot, but all the fixed resistors and caps were surface mount and SMALL.  As I poked around trying to troubleshoot, I managed to make things worse.  It turned out that the lead carrying 6 volts to the BFO control circuitry had broken.  But before I discovered this, I managed to do all kinds of damage to the board.  I lifted two PC board  pads (I should have turned down the temperature on my soldering iron).  Then, when I tried to fix this, I managed to put a solder bridge across two parts of the circuit that definitely should not have been connected.  

This resulted in a bizarre BFO situation.  From the center position, turning the BFO to the left OR TO THE RIGHT would move the BFO in the same direction.  So I could tune in an SSB station by turning to the right, or by turning to the left.  That was just not right. 

Lifted solder pads.  And small wires that now bridge the gaps 

Uffff.  It took me a while to find that fault.  While trying to figure this out, I built the circuit in LTSpice just to see what it was SUPPOSED to be doing.   This helped.  Eventually, through careful inspection with magnifying goggles, I found a solder blob, and removed it.  Now all was right with the universe.  Even though I had caused most of the trouble, it was still quite satisfying to fix it. 

Some additional observations on the DX-390. 

-- It really is a Sangean ATS-818 in disguise.   Just look at the marking on the PLL board.  If you can't find a decent DX-390 schematic, just use an ATS-818 schematic. 

ATS 818 marking along the bottom (green) part of the PLL board

-- The service manuals on these receivers are quite good: the include bloc diagrams, detailed alignment instructions, and even voltage charts for all the chips and transistors.  Impressive and useful. 

-- The static discharge vulnerability is hard to understand.  There is so much cool circuitry in these receivers, why not add four simple diodes?   Not wanting to repeat this saga, I went in and put two sets of back-to-back small signal diodes in each receiver: one set on the telescoping antenna, and other at the input for the external antenna.  Curiously, on the newer receiver, it looks like a previous owner had gone in and tried to address this vulnerability -- but he did a very incomplete job.  He just put ONE diode between the external antenna input and ground.  I had always thought that two diodes back to back would give you good protection from static discharge.  And I don't think that single diode protects the front end in any way from discharge coming in from the telescoping antenna.  

This was a good project.  I got more familiar with general coverage dual-conversion receivers.  And I got reacquainted with an old receiver that I liked a lot.   Both receivers could probably use some alignment.  I'll take that up next. 

Tor LB4RG Builds an IRF-510 Amplifier

Click on the link to check out Tor's very FB and artistic video about his homebrew IRF-510 amplifier. 

Very Nice.   Also, check out Tor's FB SolderSmoke hat.  

https://gopro.com/v/bQ1JGOXRrRqrQ

And here is an update from Tor" 

https://gopro.com/v/gv19pDDqQnO1a