SWL Modifications

SWL Modifications || AM Detector

Mixer Injection Level Pot || Modifications to the VFO Switching || VFO Modifications and Setup Information
Crystal Filter Modifications || AM Detector || Splitting the Boards Apart

International Short-Wave Broadcast Bands

Mhz Meters
2.30 - 2.55 120
3.15 - 3.45 90
3.85 - 4.05 75
4.70 - 5.15 60
5.75 - 6.30 49
7.10 - 7.40 41
9.40 - 10.05 31
11.50 - 12.20 25
13.50 - 13.90 22
15.10 - 15.70 19
17.40 - 18.00 16
21.30 - 21.95 13
25.60 - 26.10 11

If you are buying the receiver for SWL only, go to Instructions for the SWL Version of the Kit. These instructions show how to make the modifications as you build the kit for SWL reception, simplifying the building process.

It is not recommended to try frequencies below 4.000 MHz, because of the IF frequencies at 3.547 and 4.000 MHz. Converters should be used to upconvert to some band that is set up on the receiver.

The following information shows how to modify a receiver previously built for ham band reception. Four of the most popular SWL bands (41, 31, 19 & 16 meters) are set up using the same "dual image" technique the ham band version uses.

Mixer Injection Level Pot

The Mixer Injection Level Pot is located at the first VFO amplifier. (Picture) It is a black PC mount 100K potentiometer.

A 100K panel mount potentiometer is mounted at the front of the receiver so taht the injection level can be easily adjusted.

A bypass cap (.01) should be placed on the pot between the ground lead and the Gate 2 lead. The PC mount pot is removed and wires run from the holes to the panel mouned potentiometer. Shielded cable would be best.

This pot controls the VFO injection level to the first mixer. The LEDs at the mixer will indicate the VFO injection level by their brightness.

Since SWL stations have megawatt outputs the receiver can easily overload. The Mixer Level Injection Pot is used to prevent overload of the receiver.

If a popping noise is heard while adjusting the Bandpass potentiometer the injection level needs to be lowered until the popping is gone. Finding the band(s) will be much easier. The popping noise is caused by the megawatt stations overloading the first mixer.

Using the same antenna for all the bands will require use of the Mixer Injection Level Pot. The bands that are resonant with the antenna will come blasting into the receiver with a need to lower the injection level. Bands not resonate with the antenna will require more injection level.

The Mixer Injection Level Pot varies the gain of the mixer. Higher levels give greater gain but less dynamic range. Lower injection levels will lower the gain and give more dynamic range.

Experiments with the prototypes have found that running the LEDs at the mixer where they are just barely lit works best with the bands that come in very strong. With the weaker bands the LEDs are run almost at full brightness.

Modifications to the VFO Switching

The VFO IR switching needs to be disabled and replaced with two SPST switches. The imaging for the SWL bands does not follow the same combinations as the Ham Band version, so all the switching will be manual.

Two miniature SPST switches are mounted between a couple of spaces that are mounted between the VFO board and the Input board. (Picture) One side is connected to 12 Volts at one of the 12 Volt boxes. (Picture) The other side of the switch ties into the ungrounded end of the 100K resistors connected to the gate of the VN0106N3's on the VFO board. (Picture)

Heat shrink tubing is used to cover up the lens of the phototransistor and photodiode to disable them. No parts are removed and no traces are modified. The IREDs do not need to be touched.

This allows easy selection of the proper VFO with the proper Bandpass filter to receive the SWL bands that will be set up below.

VFO Modifications and Setup Information

31 and 16 Meters

The most popular SWL band is 31 meters. By changing the highest frequency of the VFO to 13.5 MHz, both the 31 meter and 16 meter band can be received. The VFO should tune from 13.5 MHz to 14 MHz. This is a wide bandwidth for the tuning capacitor without a reduction drive, but with AM stations, the tuning will not be as critical as trying to copy CW and SSB.

To move the VFO to 13.5 MHz, parallel a 39pf NPO capacitor to the 6.8pf capacitor going from the VFO to the main tuning cap. (Picture) This will enable the 14.068 tuning capacitor to reach 13.460 MHz (approximately), to 13.900 MHz. If the bandwidth is set 13.505 to 14.017 MHz, WWV at 10 MHz can be received.

Subtract the 4 MHz crystal filter frequency, and you are receiving the 31 Meter band, 9.500 to 9.980 MHz [13.500 - 4.000 = 9.500]. Add and you will receive the 16 Meter band, 17.500 to 17.900 MHz [13.500 + 4.000 = 17.500].

To receive the 31 Meter band, select the 4.000 MHz Crystal Filter. The Bandpass switch is on the 40/30 filter, and the Bandpass pot is almost mid range. Tuning is very sharp.

To receive the 16 Meter band, the Bandpass Switch is on the 20/17 filter. The Bandpass pot is almost all the way to one end (clockwise if wired like the pictures in the instructions).

Both LEDs are off at the VFO, indicating that the VFO frequency is running at 13.500 MHz. Both VFO switches are off.

19 Meter Band

First, remove the 68pf NPO capacitor next to the 10.545 yellow trim cap. (Picture)

Setting the second frequency, switched on by the 10.545 relay, to 11.453 MHz will give you the 19 meter band, 15.000 to 15.710 MHz. (11.453 + 3.547 = 15.000 MHz) With the 400kHz bandspread of the main tuning cap, the coverage will be 15.000 to 15.400 MHz.

To receive the upper section of the band, select the 4.000 MHz crystal filter. This will give 11.453 + 4.000 = 15.453 MHz to 15.853 MHz (11.453 + 4.000 = 15.453 MHz).

This leaves a gap in the middle of the band which is not received, 53 KHz, but if this section is important, the VFO could be lowered in frequency 53 KHz (11.400 MHz) to get that portion of the band using the 4.000 MHz Crystal Filter.

To receive the 19 Meter band, select the 3.547 MHz Crystal Filter for the lower portion of the band, or the 4.000 MHz crystal filter for the upper portion of the band. The 10.545 LED is on at the VFO (one VFO switch on, one off), and the Bandpass switch is set to the 20/17 Bandpass Filter. Slight adjustment of the Bandpass pot may be needed when switching between the upper and lower sections of the 19 Meter band.

41 Meter Band

If the third frequency is set to 10.847 MHz, you can receive the 41 meter band. The 10.455 trim cap will reach 10.847 MHz with no changes to the board. (Picture)

When setting the VFO, both LEDs at the VFO should be on. The 41 Meter band is 7.100 to 7.500 MHz. The 40 meter amateur radio band is 7.100 to 7.300, which is not covered. The coverage with the receiver for the 41 meter SWL band is 7.300 to 7.600 MHz.

Since the 40 Meter ham band ends at 7.300 MHz, setting it up for 7.300 to 7.500 means 7.300 + 3.547 = 10.847 MHz for the VFO frequency.

If coverage of the low end of the 41 meter band is desired, solder a 20pf NPO capacitor across the 10.455 yellow trim capacitor (underneath the board), and adjust the VFO to 10.647, which will tune the 41 meter band starting at 7.100 MHz.

To receive the 41 Meter band, first select the 3.547 MHz Crystal Filter. Both LEDs on the VFO will be on (Both VFO switches on), and the Bandpass switch is set for the 40/30 Bandpass Filter. The Bandpass pot will be clockwise left of center position.

The signals at night will be very strong, so remember, if popping occurs while tuning the Bandpass pot, turn down the VFO Injection level.

This, by no means, are your only options. Just take the band you want to receive in MHz, and add or subtract 3.547 MHz (or 4.000 MHz) to see where the VFO frequency needs to be to bring in the band you're interested in receiving. There will be little difference, whether you add or subtract to get to the band you want, in receiver performance.

Other combinations may not yield as many bands, but you can certainly get the bands which are your favorites.

Crystal Filter Modifications

To receive AM, the crystal filter bandwidth needs to be widened. A good starting point is to remove two crystals, leaving a one crystal filter. This has been done with good results. A wire is used to jumper between the first crystal and the output transformer. The capacitor going to ground on the output side of the crystal is also removed. (Picture)

AM Detector

The receiver has a product detector, which does not do well on AM. You can zero beat the station and probably get clear copy, but it is not ideal. The BFO can be turned off by turning the "Gain Adjust" pot on the BFO all the way clockwise, till the LED on the amplifier goes completely out, but AM will not automatically come out.

An AM detector can be switched in place of the product detector to receive AM. Check out the AM Detector page for AM detectors that have worked well with the receiver.

The simplest one, which works very well, needs only four steps to complete.

1. Remove the 3.3pf coupling capacitor between the BFO and BFO amplifier.
2. Take the output of the 455 KHz filter and connect it to the input of the BFO amplifier.
3. Lift off one of the diodes going to ground from ground (one of the two on the right side above or below the T1-1T transformer).
4. Add a 10K resistor across the first .05 mfd capacitor in the post-product detector filter.

For pictures and instructions see the AM Detector page.

Splitting the Boards Apart

Each of the boards for the receiver is basically a receiver in itself. By adding very surprisingly little circuitry, you can have two SWL receivers for the price of one.

Board 1

By just the simple addition of a regenerative detector at the end of the first board will give you an SWL receiver that will just about receive any type of modulation, AM, FM, CW, and SSB. This has been tried with a very early prototype of the receiver and it worked quite well.

The regen is set up to receive the IF frequencies of the first board. For example, the IF's of the first board are 3.5467 and 4.000MHz. A regen that can receive those frequencies can be tuned to the IF frequency you want, and then the VFO tunes the bands.

You don't have AGC anymore, but the Mixer Injection Level pot can be used as an RF Gain control.

Kitchen's regen was used in the prototype and it worked very well.

The following interface circuit was used to match the output of the crystal filter to Kitchen's regen.

regen matching circuit

The AGC connection comes from a pot that varies the voltage between 9 to 0 volts. The FET is turned on full at approximately 5 volts. The pot could be set to run the voltage between 5 to 0 volts if a sensitive adjustment is necessary. The adjustment was not that critical in the prototype.

Another regen that looked like a perfect match for this setup was the regen article in the August 2001 issue of QST, "The WBR Receiver", by Dan Wissell, N1BYT, page 34. (WBR means Wein-Bridge Regen).

However, initial tests with this regen didn't work. More tests are needed to find out why, but the initial impression was that some gain was needed between the crystal filter and this regen.

Two other regens on the internet that are as good are at http://www.aa5tb.com/regen.html and at http://harry.pmp3.net/. The AA5TB site has a great list of links to regenerative receiver projects.

This opens up the possibility of changing the crystal filter frequencies to any that you want. Using higher frequency crystal filters will allow lower the VFO frequencies for a very stable receiver. Going further, by using a bank of crystal filters (using inexpensive surplus crystals), several bands could be received with only one VFO frequency. Tune the regen to the crystal filter frequency.

Experimenting with the coupling between the output of the crystal filter and the regen is encouraged. The circuit above is only a starting point. The Mixer Injection Level pot can be an important control; the level is set to prevent overloading the regen. I have found with some regens that the lighter the loading, the better the regen works.

Depending on how well shielded the regen is, you can still receive broadcast stations when tuning the regen. It is amazing just how sensitive a regen is!

Another post receiver for the first board can be an 80 meter direct conversion receiver. Michael Hopkins, AB5L, was the first to try this setup and it worked quite well. No interface circuit was needed and he commented that the receiver was very quiet, better than the 455 IF strip.

A direct conversion receiver only decodes SSB/CW, but it can be used for SSB SWL stations or the ham bands. Michael like it so well that he abandoned the second board and kept the 1st board/direct conversion setup as his favorite way to enjoy the receiver.

Board 2

By adding a VFO, an AM loopstick, and a 365pf tuning cap, Board 2 can be made into an AM Band DX receiver. The filter at the end of the 455 KHz IF Strip sets the bandwidth of the receiver, and with the filters supplied with the kit, is about 6 KHz.

It is very easy to see that this board is a receiver with a 455 KHz IF. The VFO will be 455 KHz off the desired receive frequency. This makes a great low band receiver, as the receiver will have image problems at the higher frequencies.

See the AM Detector page for instructions on modifying the product detector to receiver AM.

To set up the VFO, disconnect the crystal oscillator from 12 Volts, by removing the 100 ohm resistor that supplies power to the oscillator, remove the 39pf coupling cap to the crystal oscillator amplifier, and tie the VFO into the input to the amplifier.

To increase the sensitivity of the mixer, remove the 470 ohm resistors at G1 on the mixer MOSFETs, and replace them with 100K resistors. The G1 locations are marked on the PCB.

Since you can use the amplifier on the PCB for a VFO amplifier, all that needs to be built is the VFO itself. The amplifier should raise the output to the level needed by the mixer.

The mixer LEDs show injection levels, and setting them moderately bright will be the proper injection level. You will get more gain out of the mixer the brighter you get the LEDs.

Replacing the G2 100K resistor to ground at the MOSFET amplifier, with a 100K variable pot, will give give you a variable injection control just like on Board 1, at the first VFO amplifier. If you have any questions, look at the setup on Board 1 to see how it is done.

With the high noise level of the lower bands, it is unlikely that you will need an RF amplifier.

More will be posted here as we do our own experimenting on setting up each board as a separate receiver.

SWL Modifications || AM Detector

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Last Update: 7/14/08
Web Author: David White, WN5Y