WB6BYU 80M Receiver Construction Notes

 

Printed Circuit Board (PCB)

L4
When using the variable inductor from Dan's Small Parts (as included in the kit), a jumper is needed between ground and the open connection of the inductor.

T1
Both Red and Green wire are provided in this kit. Enough green (or red) wire is provided to wind the primary winding, and to make the bifilar winding. When making the bifilar winding, use 4" of each color to identify the two wires and twist them at about 2 -3 turns per inch.. When winding the coil, it counts as one turn each time a wire passes through the center. Wind 16 turns for the primary and 4 turns for the bifilar winding. The end result will be green wire for the primary and the Bifilar Secondary will have green at one end, green and red for the center tap, and red at the other end.

Note: Figure 6 in the QST article shows 17 turns for the primary winding and 5 turns for the secondary winding.

The insulation used is Solderable Polyester Nylon (SPN) that will melt under soldering temperatures. The 700 degree soldering iron with a ball of solder on the tip took about 4 seconds before the solder started to strip the wire. Tinning the wires before mounting T1 to the PCB is highly recommended to avoid damage to the PCB.

After the receiver has been assembled and tested, gluing T1 to the PCB will help reduce the microphonics.

Resistors
The resistor hole spacing is 0.4" with the exception of R5 (100 ohms) which is 0.5". A wooden stick is provided with the resistors as a convenient bending jig to get the 0.4" lead spacing if you don’t have a bending jig.

Electrolytic Capacitors
The longer lead on the capacitors is the positive side. The negative side (shorter lead) is also identified by a black strip on the capacitor body.

The PCB hole spacing varies between 0.090" and 0.150" for the electrolytic capacitors. Use the two larger diameter caps for C5 (220 uF) and C20 (47 uF) as they will both fit the .150" PCB spacing.

Don’t force C7 (4.7 uF) or C27 (4.7 uF) flat against the board or you risk damaging them since the hole spacing is 0.150" and the capacitor lead spacing is 0.080".

C13/C14 Capacitors
It will be convenient to put C13 (220 pF mica) into the location for C14, and C14 (10 - 120 pF trimmer) into the location for C13. Because the trimmer will then cover one of the center holes for L1, both center wires of L1 will then need to go in the outside hole on the PCB. You may need to slightly enlarge the holes at the C13 position to allow the C14 trimmer to fit easily into place.

C23 Capacitor
Note that the orientation of C23 is correct on the PCB and wrong on the schematic. The positive side of C23 should go to Q4.

Varactor Diodes
The MV209 varactor diodes are inserted with the flat of D6 facing towards L4 and the flat of D7 facing away from L4.

3 Conductor Shielded Cable (4" and 6")
Strip off approximately 1 inch of the covering from both ends of the two shielded cables. On the side that connects to the pot, cut off the shield wire. The white wire is the center conductor, the black wire goes to ground (closest to the top of the PCB), and the red wire goes to the plus side of the circuit (closest to the bottom of the PCB.). Tin the shield and solder it to ground near the edge of the PCB. The 4" length is used for the tuning pot and the 6" piece is used for the gain control

2 Conductor Shielded Cable (4")
The two conductor shielded cable is used for the audio jack connection to the printed circuit board. Prepare it the same as the 3 conductor shielded cable. Solder the 3.5mm audio jack to the end of these wires. Make SURE the same ground wire on the PCB is soldered to the ground connection (goes through the center of the jack) on the jack to avoid shorting out the audio.

Other Wires
The two conductor unshielded cable is used for the sense switch connection to the printed circuit board. When soldering, the solid color/red wire goes in the sense switch hole towards the bottom of the PCB. Solder the pushbutton sense switch to the end of these wires.

The 4" piece of black wire should be soldered to the sense position on the PCB and left loose. It will be attached to the sense rod during final assembly.

Make sure the black wire of the battery connector is soldered to the (-) connection on the PCB (ground plane.)

Final Check
When the PCB is fully assembled, you will have the populated PCB with six wires/cables soldered in place for the battery, sense switch cable, sense antenna wire, audio jack cable, tuning potcable, and gain control cable. The wires for the sense switch will have the sense switch on the other end and the audio cable will have the audio jack connected to the other end. Inspect the board CAREFULLY at this point for shorts, good solder joints, and wires connected to the proper locations!!!

 

Loop Antenna

A John Guest fitting is used to connect the ends of the plastic tubing. To remove the tubing from the fitting, pull the tubing out while pushing the end of the fitting in towards the center.

The red tape on one side of the tubing is used to identify the direction of the strongest signal when the sense switch is pressed. Strip one end of the yellow wire to identify that end and put it in the tubing so it comes out nearest the red sense marker tape.

The loop wires go through the drilled hole in the John Guest fitting before going into the plastic loop tubing. . I’ve found it easiest to fold the long black wire (L1) in half and insert each end in opposite directions through the center hole in the John Guest fitting. Feed the wires through the tubing and back through the John Guest fitting until you have two turns in each direction leaving the fold to identify the center of the L1. You might find it a bit easier to put the wires though the ends of the tubing if the tubing is left loose from the John Guest fitting until the wires have all been pulled through

 

Final Assembly

Clean the decal with alcohol and place the clear label material over the decal after removing the back protective paper. Cut the decal using a razor blade just outside the black line. Clean the enclosure front with alcohol or soapy water before applying the decal. If using water, make sure the enclosure is dry before applying the decal. With an X-acto blade, cut out the label material covering the enclosure holes.

Note: If the decal is cut much larger than the black outline, the edges of the decal will go over the round enclosure corners and probably lift up. Trimming the decal close to the black line helps prevent this from happening.

When fastening the four standoffs to the enclosure, hold the screws and tighten the standoffs to avoid distorting the decal.

Install the Pots with the 100K near the top and the 50K in the middle. Install the On/Off switch with the contacts nearest the bottom of the case.

Solder the 3 conductor shielded wires to the pots so that red goes towards the top of the receiver, white in the middle, and black towards the battery.

At this point, you can test the receiver. Before applying power, double check the PCB for solder shorts and make sure the semiconductors are all inserted in the correct direction. After checking, you can put the PCB on the standoffs and test the receiver by 1) plugging in a set of earphones, and 2) plugging in a 9V alkaline battery. You should hear a short burst of noise, silence for about a second or so, and a return of the noise. Turning the gain control should results in a clear change in volume level. You probably won’t notice any change by turning the tuning control. A click in the audio is normal when pressing the sense switch.

Insert the two rubber grommets into their respective holes on the top of the enclosure. Put the loop wires though the 1/2" piece of 3/8" tubing from the John Guest fitting and insert through the 1/2" rubber grommet before attaching the loop antenna assembly to the enclosure. For right-handed people, the red tape on the tubing will be on the left side of the receiver looking from the front.

When soldering the sense loop wires (L3), the yellow wire coming through the loop nearest the red tape gets soldered into the hole closest to the sense wire connection. This makes the sense direction (when the sense switch is pushed) on the side of the loop with the red tape.

Be careful when fastening the PCB to the standoffs so that the coax cable is not between the pots and the PCB.

Put the rubber cap on one end of the 1/8" aluminum sense rod and insert into the small rubber grommet on the enclosure. Use the two tie-wraps to fasten the rod to the loop at the top of the loop. Strip about 3/8" from the 4" black wire and fasten to the other end of the sense rod using the orange wirenut.

DO NOT use a metal allen wrench to adjust L1; use only a plastic wrench. A metal wrench can more easily break the center ferrite as well as making it more difficult to tune. With the capacitor values included with the kit, moving the core out about 3 - 4 turns will set the frequency to about 3.5 MHz with the pot turned fully counterclockwise.

When tuning the sense antenna with R23, the correct tuning will be somewhere around 1/4 turn starting from the fully counterclockwise position.

 

Calibration

The procedure described in the QST article works just fine. I find it more convenient to use the MFJ 209B Antenna Analyzer with the Grid Dip Meter optional coils. The procedure I use is:
1) Adjust C14 to bring the loop to resonsance at 3.6 MHz using the Grid Dip option for the MFJ209B.
2) Set the MFJ209B to 3.5 MHz, turn the tuning control fully counterclockwise, and adjust L4 until the signal from the dip meter can be heard. The tuning range will be approximately 3.5 – 3.65 MHz.
3) Initially set R23 to ¼ turn from the fully counterclockwise position. Using an 80M transmitter into a vertical antenna, I check that the signal level is distinctly higher when the red tape side of the loop is facing the antenna vs when the red tape side is facing away from the antenna. Any adjustment to R23 shouldn’t need to be more than 1/8 turn from the initial position.

Troubleshooting

CHECK THE BOARD FOR SOLDER SHORTS AND BAD SOLDER JOINTS.

No audio
1) Check that power is getting to the board.
2) Remove the audio jack from the enclosure and test again. If the audio is okay, the connections are reversed to the audio jack.
3) Check that the audio output leads are connected to the proper tabs on the audio jack.
4) Check that the LM386 was inserted in the proper direction into the PCB. If it was installed backwards, it will most likely need to be replaced.

Receiver oscillates when turned on
1) Make sure you are using an alkaline 9V battery.
2) The value of R19 may be too small.

No tuning
1) Make sure a jumper is in place from the open connection of L4 (variable inductor) to ground.
2) Check wiring to the tuning pot.
3) Check for solder shorts.

Tuning knob works backwards
1) Make sure both varactor diodes are inserted with the proper polarity.
2) Check wiring to the tuning pot.

No change in volume
1) Check wiring to gain control.
2) Check that the leads to T1 have been properly stripped and soldered to the PCB.

Sense switch has no effect
1) Check that the wires are connected to the proper locations on the PCB.
2) Make sure a signal is present when testing.


Updated Tuesday, April 28, 2009 at 22:20 PDT