TelePost LP-100, Manual

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Assembly cont’d
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Solder the 12 SMT resistors onto the attenuator board as shown. Don’t be afraid of these parts. These are VERY big parts as SMT
goes. The resistor values are printed on the resistors. Use a fine tip on the soldering iron, and tin ONE of the PCB pads for each
resistor with a small amount of solder before attempting to solder the resistors. Hold the resistors in place with a tweezers, and
apply a little heat to the edge between each pad and the board until the solder flows between the resistor and pad. It is helpful to
slide the resistor over the pad as it melts onto the solder drop, so that the other end exposes a little of the pad on the other side.
Solder the other side in place by applying heat and solder to the edge where the resistor sits on the pad. Then go back to the
tacked pad and touch up if necessary. Applying a little flux to the board ahead of time will help to hold the parts in place and aid in
solder flow. To verify the proper installation of the resistors, use an ohmmeter to check the resistance of each bare stripline
connection to ground. The mounting hole is grounded, as well as the long strip along the top edge. Each point should be about 83
to 84 ohms. If not, check your soldering.
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Install the PCB onto the side of the coupler above the BNCs as
shown. The board mounts with the hole near the bottom edge.
Bend and solder the pigtails from the BNCs to the two striplines
near the mounting hole. Use 4-40 x ¼” (6.35mm) machine screw,
the small split lockwasher and small hex nut to mount the board.
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Solder one the short piece of RG-316U prepared earlier to the
solder terminal on the bottom of the coupler, with the coax shield
connecting to the grounded lug, and the center to the lug that
connects to the current sampling xfmr.
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Solder the center conductor from the other end of the coax to the remaining PCB stripline pad, and the shield to the ground lug on
the center-most BNC.
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Slide the current sampling transformer over the short piece of RG-142 as shown in the diagrams, being careful to position the
windings and the coax shield as shown. This is a tight fit, but if you take your time and rotate the coax as you press it into place,
you shouldn’t have any trouble. There seems to be a little variation in the diameter of the RG-142, so you may find that you need to
file the inside of the bushing a little to allow a good fit. This can be done with a small rat tail file, a rolled piece of sandpaper or a
reamer. An alternative, suggested by K8SIX is to use a #10 drill bit to drill the hole out a little.
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The xfmr should be oriented level, with the windings facing up before soldering. Solder the coax on the shield end into the SO-239
connector, and the shield wire to the solder lug on the XMTR connector. Cut the wire from the outside left of the transformer
secondary to length and solder it to the lug on the XMTR connector. The other end of the coax will be soldered along with the long
wire from the voltage xfmr in an upcoming step.
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Install the 2-lug terminal strip, then cut and solder the wire coming from the inside right side of the xfmr to the insulated lug on the
terminal strip.
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Prepare the voltage xfmr as shown in the photos on the bottom of page 13, using the 0.625” (1.59cm) standoff, 4-40 x 0.75”
(1.91cm) screw, lockwasher and solder lug. The long lead should exit the core on the side with the standoff as shown in the
overhead picture of the coupler. After approx. serial #350, a lockwasher was added between the standoff and nylon bushing.
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Bend the solder lug out at close to a 90 degree angle, and solder a small length of discarded component lead to it.
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Attach the assembly to the side of the coupler using 4-40 x 3/8” (9.53mm) hardware. The solder lug and pigtail should be facing the
PCB. It is important that this assembly be attached firmly or you will see erratic operation. A lockwasher should be placed between
the coupler wall and standoff, and one between the screw head and outside of the coupler.
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Prepare and solder the short end of the toroid winding so that it connects to the solder lug on the right-most BNC. If the core is
mounted correctly, this wire should come off the right side of the core from the inside. Solder the pigtail from the standoff to the
stripline pad on the end of the PCB. Leave a small bend in this lead to allow for flexing when the top is attached to the coupler and
the walls are pulled apart.
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The long wire coming off the outside left side of the core goes to the output SO-239 as
shown. The end of the wire should be placed inside the SO-239 center connector
alongside the RG-142 center conductor, or looped around the SO-239 center
conductor as shown in the photos. Be sure to scrape the end to allow good soldering in
either case. Before soldering these wires to the SO-239, the walls of the coupler need
to be pre-tensioned so that there won’t be any stress on the RG-142 when the top is
attached to the coupler. To do this, I use a 2” (5.08cm) long standoff placed between
the walls above the xfmr to separate the walls slightly. A suitable substitute would be a
2” (5.08cm) long piece of wood dowel. The wire should be routed about 1/8” (3.18mm)
from the current xfmr as shown. For best phase accuracy at 50 MHz, a little coupling to
the current xfmr secondary is desirable. Remove the spacer.