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. 

Geeks Without Borders and Random Hacks of Kindness

Phil, KD4IDH, sent me links to these two very interesting organizations. We sometimes joke about being "The International Brotherhood of Electronic Wizards." These folks seem to be taking that idea to a whole new level.

Geeks Without Borders +) GWOBorg is an international coalition of passionate problem solvers working together to assist people whose survival is threatened by lack of access to technology or communications due to violence, neglect, or catastrophe.

One of the things that GWOB does is called The Digital Mountaintop: This is a free, open communications hub, accessible via voice, text (SMS), plain old telephone (POTS), Skype, SIP, Google Voice, Twitter, Facebook, Email, Instant Messaging (IM)…and as many other networks we can connect to. In crisis situations, neighbors can ask for help simply by sending sending a message to the DMT, as long as they (or someone they know) can connect via any of the messaging or real-time communication methods it supports.

Random Hacks of Kindness (RHoK) is all about using technology to make the world a better place by building a community of innovation. RHoK brings software engineers together with disaster risk management experts to identify critical global challenges, and develop software to respond to them. A RHoK Hackathon event brings together the best and the brightest hackers from around the world, who volunteer their time to solve real-world problems.

Haiti: Dominican Hams Help

The video (from a phone patch the day after the quake) gives you a real sense of how bad it is. It was good to see that hams from the Dominican Republic were going over to help out. See below.

From: http://spectrum.ieee.org/telecom/wireless/hams-in-haiti/1
BY Anne-Marie Corley // January 2010

... Which leads to the other difference in Haiti: The country is home to very few amateur operators in the first place. Though there are around 100 to 120 ham licenses active for Haiti, according to Pitts, only about seven or eight operators were actually in Haiti as far as the ARRL can determine. According to Bill Pasternak, the president and cofounder of the Amateur Radio Newsline, which broadcast audio from one ham operator outside Port-au-Prince soon after the earthquake hit, most of the operators who have Haitian licenses aren’t even Haitian but rather missionaries and aid workers who travel in and out of the country.
Pitts says that the ARRL has heard from only a few operators, most from outside Port-au-Prince, though one operator did radio in from the city just to let the organization know he was alive. ”The ones that were there did all they could,” Pitts says, ”but we haven’t heard from all of them.” It is likely that some were killed, Pitts speculates. Others may have been concerned with safety, McPherson suggests, so hams in Haiti have been ”on and off the air,” he says.
To help out, hams from the neighboring Dominican Republic have traveled into Haiti several times to set up equipment, despite being attacked by looters last week. They set up a 2-meter analog repeater high on a mountain close to the Haiti–Dominican Republic border. The repeater takes in weak signals—even one from a clip-on radio putting out just 5 watts—and rebroadcasts those signals on a different frequency and at a higher power.
Dominican operators installed a second repeater near the airport in Port-au-Prince and were expecting a third to arrive from ARRL Tuesday, which they will likely put in the region southeast of the capital.

”They’re doing really good work,” Pitts says of the Dominican helpers, ”getting things where they need to be and coordinating with other teams.”
Pitts adds that the international nature of ham radio is well suited to emergency missions like this one. Hams in Argentina, Brazil, Cuba, Florida, and Puerto Rico, among others, were on the air and listening for any signals soon after the earthquake. ”Nobody was going to hiccup without being noticed,” Pitts says. And because they talk in radio code, language barriers don’t matter as much. ”We all have the same language,” Pitts says. ”We’re used to talking with each other.”
The embedded hams in Salvation Army recovery teams work, too, McPherson says, because they can tap into the entire amateur radio community. Nonofficial operators, for example, who may hear an embed trying to reach Haiti or to call out, may help relay a signal. ”It’s like [all the] amateur community is listening, standing by to help,” McPherson says.
The lesson to be learned, according to Pitts, is that ”in a situation or population where amateur radio is encouraged and present,” hams can provide better and faster information during a major disaster, which allows a faster response. ”That golden 48 hours is where the hams really can shine, if they’re there.”
So while cellular and Internet communication return ever so slowly to normal (or better than normal), what Haiti might also want to invest in is a few more homegrown radio operators.