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Sunday, August 16, 2020

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. 

4 comments:

  1. This is why tube circuits are attractive, everything is BIG..

    ReplyDelete
  2. I was given a DX-390 that was beyond repair. I'm thinking of using the 55.845 Mhz filter as the 1st IF for all-band HF rig....of course, I've be thinking for 2 years now...anyway, 2nd IF at 9 Mhz....I guess I should get cracking. Congrats on the repairs....73 de AC9JQ

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  3. Lots of nice parts in there Dean. Trimmer caps. Crystals. The filter might be similar to the 55.845 MHz model on this page: http://www.excequal.com/pro_mcf.asp
    That PLL unit could also be quite useful if it can be saved -- it will put out desired freq + 55.845 MHz across a broad freq range. Please let us know how this goes. 73 Bill

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