Tuesday, January 14, 2020

Mods for the Radio Shack DX-390 / Sangean ATS 818 Part 4 Panadapter

The next mod in this series is going to show you how to add an IF-Out Connector to your radio that you can interface a SDR receiver and use it as a panadapter! A panadapter is a device that gives you a visual display of the radio spectrum in a waterfall or line display and "capture" the audio from your radio at the same time. using a cheap SDR unit.

The benefits of using a panadapter are endless. You can get an idea of where activity is on your shortwave radio with a wider view of the radio spectrum. For example, you can tune to 6.060 MHz on the radio and see up to 2.8 MHz of the radio spectrum above and below that frequency. If you are chasing DX or following ham radio operators, then you can find out where the bands are active. Other benefits include digital mode decoding for DRM, RTTY, WeFax, JS8Call, Morse Code, PSK, and more.

A panadapter allows greater flexibility with digital signal processing giving the listener more filter options, decoders, and plugins. Many high end receivers and ham radio transceivers now offer an easy way to connect panadapters or rig interfaces to your favorite computer. You can also use the panadapter as a second audio processor/receiver by listening on two frequencies at the same time.



I recommend the RTL-SDR Blog V3 which includes direct sampling of HF frequencies below 24 MHZ if you choose to use it without the ATS-818/DX-390. These units will cost about $22-$25 or you can buy a complete kit with antenna for $30 from https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles You can also use the $8 DVB TV tuners to interface to your radio found on eBay. I tested both HDSDR and SDR# on my laptop and found that HDSDR works a bit better for this application because you can tune below the radio frequency. HDSDR can be found at www.hdsdr.de

What you need to perform this mod..
  • SDR as mentioned above
  • Necessary cables to connect the SDR to your radio (I used a pigtail cable with a female SMA connector and SMA male/male adapter) available on Amazon.
  • 1 k ohm resistor
  • 1 nf to 2.2 nf non polarized capacitor (I used a 2.2 nf film capacitor)
  • Drill/hand tools/soldering iron/heat shrink tubing

The DX-390 is a dual conversion, superhetrodyne receiver design. One thing that makes this radio great is hardware filtering. Looking at LW/AM/SW circuit in particular, there is a Murata crystal filter in the first IF stage and two Murata ceramic filters at the 2nd IF stage. The first IF is 55.845 MHz and the second IF is 450 kHz. We want to use the 1st IF stage for our panadapter.



For some reason, the second tuning coil (T2) was omitted in final production. We can use this for our IF tap.

Connecting your SDR directly to the radio could alter the IF stage and cause poor reception. It could also transfer unwanted DC voltage between the radio and your SDR. This is why we use a small value capacitor for blocking harmful voltage and resistor to prevent receiver sensitivity drain. Connect them in series to your radio.






Finding the correct tap point for a strong IF signal in this radio is fairly simple on the Sangean. We could tap between T1 andT3 or the output of Q3 and Q4 and receive a great signal. I decided to use the omitted T2 circuit where it connects to R39.




After cutting the cable to the correct length, I stripped and soldered the center conductor of the coax to the capacitor/resistor. I also cut the braid back since it isn't needed for the signal path and could induce noise into the IF output. You could install a ferrite core choke on the cable as well.



I decided to drill a hole in the side of the radio for easy access to the IF stage. A 1/2 inch bit was sufficient to get the connector through the radio body. It lines up perfectly with the other connectors.



Time to attach my cable to the radio.




Reassemble the radio and connect the SDR units! Both work fine for this application, however, the temp controlled oscillator is superior in the RTL-SDR V3 unit to prevent drift.


Fire up the software. Use HDSDR and install the correct ExtIO for your SDR. Files and instructions are available at www.hdsdr.de/hardware.html




Configure the bandwidth and AGC settings as shown in HDSDR. Choose 2400 or 2800 kbps for the sample rate.


 Setup your receiver in the RF Front End configuration menu. Set your SDR to receive the IF output and type 55845000 in the IF frequency box. Check the "mirror RF Spectrum in general" box.



After connecting all cables and running software, turn on the ATS-818/DX-390 and adjust the RF gain as appropriate. Tone, Volume, BFO, Narrow/Wide controls don't affect the signal to HDSDR. You can set those to your preference.  Now set a frequency on the radio in LW/AM/SW mode. An unused frequency is preferred somewhere within the band you would like to listen to. You will get 2.4 to 2.8 MHz of bandwidth to HDSDR that you can independently tune and listen. In HDSDR, you want to set your LO as the same frequency displayed on your radio. Right click to lock the LO frequency to prevent it from accidentally changing as you tune in the application.









73,
Jesse W9JES

Friday, January 10, 2020

Mods for the Radio Shack DX-390 / Sangean ATS 818 Part 3 Static Protection

Do you want to protect your radio from static discharge while using the whip antenna? This is a must-have mod for the radio because the existing design does not incorporate static protection on the antenna input circuit. Some folks have found their radios to be "deaf" on AM and shortwave bands because a static pulse could destroy the sensitive Q1 transistor. This usually occurs in low humidity conditions when a person touches the antenna to adjust it.

You will need (2) 1N4148 silicon (signal) diodes or equivalent and a short jumper wire to complete this mod.

Note: This does not protect the external antenna jack so I recommend using the same circuit between the 1/8" mono jack and your wire antenna.




Do you already suspect that your radio has a blown out transistor? Here are some tips from AA1LL on QSL.net to test and replace the transistor. 

"This relates to the front end burn-out phenomenon associated with the Radio Shack DX-390 multiband receiver. This happened to me with all the previously documented symptoms and I was able to repair it by replacing the input FET with a device which is available off the shelf from Radio Shack.
After taking the DX-390's plastic enclosure apart by removing all the black self-tapping screws (don't forget the one down in the battery compartment) and lifting up the single p.c. board by unscrewing all the silver colored screws which hold it down, you will find Q1 near the antenna connector which in my radio was a 2SK152. Remove the dead Q1 by sucking the solder off of its p.c. pads with solder wick and measure it with the DVM to make sure it is dead--mine looked like a 31-ohm resistor from drain to source. A good FET would look like a pair of diodes with their anodes connected together at the gate. The gate is the lower terminal looking at the device from the bottom with the flat side to your right; 

Paul Gili, AA1LL"


73,
Jesse, W9JES

Mods for the Radio Shack DX-390 / Sangean ATS 818 Part 2 Lights

WOW! Almost 10 years to the day and now it's time to provide an update to the trusty old ATS-818/DX-390 mods. Thanks for sticking around with me and being very patient as I was away taking care of life's impromptu moments.

It feels like things are winding down a bit and I've been more active in the SWL hobby once again. One of the things that sparked my interest was a great opportunity to pick up a Gundig G5 in excellent condition for $30 late last year. It was one of those situations where someone knew that I was a ham radio operator and said their friend had some old radios from a friend who recently passed. While I do like the size and sensitivity of the G5, it lacks some features on the DX-390 such as an excellent speaker for great sound and tone controls. The G5 goes with me on trips where size and weight are key factors.

Back to the 818/390. This project started out as a means of improvements to an already impressive radio. I liked most of the features on the radio, but wanted to squeeze out all of the potential it had. The next enhancement that I completed on the list was replacing the dim incandescent light and adding on-demand light control. For this task, I researched the radio schematic and attempted to make heads or tails of the  circuit. The 3 transistor circuit connects +6v to the lamp when the light button is shorted to ground. Capacitor C414 and C415 play a significant part in this circuit as they are responsible for triggering the transistors. Their values can be substituted with smaller or higher values to change the delay in trigger and duration. I replaced C415 and tested this theory with success. This circuit could also be rewired using the same components with some jumper wires and cutting traces to achieve my goal, but I chose to leave it as-is to avoid risk of permanent damage to the PCB.

Next, I looked at the Toshiba TC4S11F NAND gate (used in other radios) as a solution to my problem. This would work!



A little more digging and I stumbled across the KY002 Bi-Stable switch. Could it be that easy? Yes! The KY002 or KY002S variants use a trigger circuit to connect vcc to output 1 via a single trigger shorted to ground. The output is toggled on/off each time the momentary button is pressed. The KY002 can handle the voltage and amperage requirements for the light adequately.



Before wiring this module to the radio, I decided to replace the incandescent light with a frosted white, 5mm LED. The LED is brighter and drains half the current of the incandescent. I used a 1K Ohm resistor to suit my brightness tastes (the 2.2K resistor in the pic was swapped out because the LED was too dim).


Here is the radio schematic and wiring diagram to the KY002. Brown and red wires connecting the existing lamp circuit can be used to drive the KY002 and LED. This bypasses the old light circuit leaving it in place to revert it back to factory as desired. The only parts of the existing circuit in use are the light switch and ground pad of the old lamp.

Brown and Red wires connecting to old lamp circuit were cut and removed from here. The metal shield needs to be removed from the main PCB to gain access. Desolder or clip the wires closest to the board and solder them to the KY002.




The existing light switch needs to be connected to V1 of the KY002. There is a convenient thru-hole solder pad available next to R423 (104) that can be used.


Ground for the light switch can be found at the existing lamp location here. Lift up the tab that covers the old lamp. Connect this pad to GND of the KY002. Now is a good time to remove the old lamp and solder the short side of the LED to the same pad that you use for GND. I bent the leads to figure out best placement. You can use the white foam to support the led and a little hot glue to secure it.


I ran all the wires to the back of the board and attached the KY002 using double sided foam tape. Here you can see all 5 wires routed and soldered into place. I used scotch tape to hold down the wires, then reattached the metal RF shield.


The KY002 modules can be found on eBay or other Arduino parts retailers.. I paid about $2 per module.

Results!



So what's next for the 390? One possibility is an IF-Out mod and create a panadapter for my laptop/SDR dongle.



73,
Jesse W9JES