Add a Digital Display to an Old Communications Receiver

Introduction: Add a Digital Display to an Old Communications Receiver

One of the shortcomings of using an older communications gear is the fact that the analog dial isn't very accurate. You're always guessing at the frequency you're receiving. In the AM or FM bands, this generally isn't a problem because you usually know the frequencies of the local stations you're receiving. On shortwave or for AM DXing, you need to know the exact frequency you're on. I obtained this Realistic DX-160 recently for a good price and it was working well for a radio that was manufactured between 1975 and 1980. When it was new it sold for around $150.00. This radio was extremely popular in it's day and continues to have a following even now. In the mid 70's even the top receivers didn't have digital displays as the circuitry of a frequency counter was more complex and bulky as the radio itself. Today we are lucky, we can buy a complete digital display for $10.00 that can be added easily to a radio. In this Instructable I will show you how I added a digital display to the DX-160 and programmed it to give the right frequency the radio is tuned to.

Note: A display can be added to any radio as long as you can access the signal from the IF oscillator which will be where it's injected into the mixer. In a solid state receiver, the signal will probably be small enough to go directly into the frequency counter module (200 mV to 2 volts peak to peak). If you want to add a display to an old tube receiver, realize that the IF voltage will be much higher than this and a resistor attenuator network will be needed to reduce the voltage.

Supplies:

1) (1) Small project box (ebay or electronic parts store)

2) (2) digital display unit 1-1000 mHz. https://www.ebay.com/str/sensesmart: The item is called a 1 MHz-1.1GHz LED Frequency Counter Tester Measurement Module For Ham Radio.

3) (1) 4700 microfarad electrolytic capacitor (ebay or parts store)

4) (1) 1N4007 silicon diode (ebay or parts store

5) (1) 15 nF capacitor (ebay or parts store)

6) Different colors of stranded hookup wire. (ebay or parts store)

7) (1) Alligator clip (ebay or parts store)

8) Electronic solder (hardware store)

9) Soldering iron (hardware store)

10) Hot melt glue and gun (hardware or craft store)

11) (2) pieces of Vector or Perf board (ebay or electronic parts store)

12) Electric drill (hardware or home improvement store)

13) Sharp knife (hardware store)

14) (6) small tie wraps (black) (craft store)

15) (6) large tie wraps (white) (craft store)

16) Long nosed pliers, (hardware or craft store)

17) Assorted screwdrivers (slot, phillips) (hardware store)

18) wire strippers (hardware store)

19) double sided tape (hardware store)

Step 1: Theory

A communications radio is a radio that's designed with more sensitivity and selectivity than an ordinary radio receiver. The DX-160 covers approximately 150 kHz to 30 mHz in five bands that are switched on the front panel. This radio was $150.00 when it was new and was of good quality for the money and was aimed at young DXers and young Hams getting into the hobby. Nevertheless, it's a fun radio to play around with and I like the retro look of it.

This radio is called a"single conversion superhetrodyne" which means that every frequency tuned and received is converted to 455 kHz and put into the Intermediate Frequency amplifier before being detected and turned into audio. Higher priced receivers can have double or triple conversion which makes the receiver more selective and adds better image rejection. One of the annoying problems that single conversion receivers have on shortwave are what are known as "images". For example, if you are receiving WWV on 5 mHz, you might hear it 455 kHz below or above the real frequency. Having a digital display lets you know the exact frequency you are on and makes it easier to determine if you are receiving an image or the real signal. In most of the bands, you won't notice this problem, but sometimes if you aren't aware of it, it can confusing. This radio has a number of different features that an ordinary AM/FM radio won't have such as an AM/SSB switch, BFO pitch control, RF gain control, antenna trimmer, ANL switch, AVC switch (fast or slow) and a switch that allowed the radio to be put into standby if the radio was being used with a ham radio transmitter.

The reason I used 2 displays is because bands A,B,C and D have a the IF oscillator tracking 455 kHz above the tuned frequency and band E has the IF oscillator tracking 455 kHz below the tuned frequency. These displays each need to have their offset programmed separately. Since these displays don't like displaying below 1 mHz, the first band doesn't display it's digital frequency. In North America, there's just navigational beacons on the frequencies from 150 kHz to 500 kHz anyway. Only in Europe, Asia and Africa is longwave use for broadcasts.

Step 2: Looking at the Digital Display Units

Looking at the first picture, you see the back of the digital display unit. There are two connectors provided, power which is 9-12 volts and the signal input which has an input and ground. I found that only the red wire on the signal input is needed for it to work. If you look closely at the pictures, you will notice that the red and black wires are on opposite sides so that you won't plug the signal input into the power plug and vice versa. The two buttons are for programming the display unit. One controls the decimal place and the other one is the "mode" control which allows you to program either the IF + to be displayed or IF - to be displayed. In this radio I programmed it to display 455 kHz + to be displayed on one unit and 455 kHz - to be displayed on the other. The displays always are 910 kHz apart.

Step 3: Mounting the Display Unit in the Project Box.

I cut out two openings in the project box for the displays. The openings were cut a little bigger than the glass portion of the displays because they didn't sit straight. Once the right size holes were cut, the two displays were held in place with the plastic pieces that were cut out of the front and hot melt glue. Little dabs of hot melt glue were put at various strategic points of the displays to hold them in place. Make note that the bottom edge of the board where the power connector sits is the side that should be the bottom of the box. This will ensure that the display is right side up.

Step 4: Making Power Board and Isolation Capacitor Board and Installing in Box.

Make the power supply board as in the picture because there is very little room for it to fit between the display boards sideways. The capacitor can be mounted as shown in the picture and both are held in place with hot melt glue. Tie both red wires from displays together and do the same with the black. The black ground signal wires can be tied together and taped off. In my installation, I found that it wasn't needed for shielding. The red signal wires are tied together inside the box and connect to one side of the 15 nF capacitor, the other side of the capacitor runs out of the box through a wire that can be connected directly into the radio with a small alligator clip. The input of the power supply board is 6 VAC which comes from the dial lamp supply of the radio which is very easy to access inside the radio.

Step 5: Wiring Digital Display Into Radio

Cut three wires long enough to go from the counter to the radio through the back panel which has a number of air circulation holes drilled into it. Once you have run the wires through the back panel, (give lots of slack) and connect the two power wires and solder them onto the connector inside the radio as shown. This is the 6 VAC supply for the dial lights. Take the end of the signal wire and attach a small alligator clip as shown in pictures. Take this alligator clip and attach it to the end of R12 where it comes together with C12 to attach to Q4. This is the point where the IF oscillator signal is injected into the mixer FET. I chose to use an alligator clip because I plan to do a replacement of all of the electrolytic caps of the radio in the near future. If you want to make a permanent solder connection, this can also be done. You can use the larger tie wraps to make a tight connection between the wires and the back of the radio. The smaller tie wraps can be used to keep the wires neat as they cross the top of the radio. Put the double sided tape on the bottom of the display and mount it on the top front of the radio.

Step 6: Check Everything Out.

Once everything is connected and the displays programmed to give the right offsets, you can put the back on the display unit. During programming, you can set it up to display to display to 3 or 4 decimal places as shown in the photos.

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    2 Comments

    0
    partrc02
    partrc02

    Question 11 months ago on Introduction

    Hi..!
    Been looking for a lockdown project. I have a National SW-54, 1955 vintage, that I think the frequency pointer could be 'improved' upon by using the digital freq display ..!
    But as your article says "Today we are lucky, we can buy a complete digital display for $10.00 that can be added easily to a radio" . From where ? Can you give me, and some of my work colleagues, guidance.
    Thanking you in advance.
    Kris Partridge. G8AUU
    Senior Location Engineer
    BBC Newsgathering