Assembly guide for the 1.8-54 MHz 1KW RF Deck using the MRF1K50N or the 1K50H
Note: the MRF1K50N is pictured here with its blue PC boards; these kit instructions also apply to the MRF1K50H and its green PC boards
Here's the schematic for this RF deck:
I
Begin with the output board, and install all of the .01uf RF capacitors to their positions at C9 through C20, and at C22 and C23. On the input board, install them at C7 and C8.
Put the input board aside for now.
Install the 240pf RF capacitors on the output board at C25 and C26. The board
is not marked for these, but they will go from drain trace to ground trace in
line horizontally with C19 and C22.
Locate the 36 inch length of TC18 coax supplied with your kit and trim off two 15-inch pieces; set aside the remaining 6 inches, it will be used later.
Remove 12mm of outer insulation from each end of those 15 inch pieces.
Trim away all but 5mm of the shield and tin it with solder to hold the strands in place.
Remove half of the remaining inner insulation to expose the center conductor.
Construct the two matching transformers.
Locate a large ferrite core and one of the prepared TC18 coax pieces.
Pass the end of the coax through the core, leaving about 1.5 inches extending out the
left end (see photo).
Fold the end away from you over the edge of the core to keep it in place while you wind the remaining turns.
Pull the coax toward you and up against the side of the core. Pass it through the core to the right of the previous turn and repeat until all turns are in place. Fold the end over the edge of the core away from you (see photo). The two ends should be the same length; if not, make an adjustment and rewind until they are.
Make the second transformer the same way you made the first.
Install
the first transformer by soldering one end to the traces as
shown, center conductor to the bottom trace, shield to the top, making certain
the insulated part of the center conductor is all that bridges the gap.
Solder
the other end of the transformer across the vertical gap between the bottom two
traces, center conductor to the right side, shield to the left.
Install
the second transformer to the right side in mirror image style in the same
manner as the first (see photo).
Make the drain chokes; locate the two #14 tinned magnet wires, straighten them out, and pass each through a ferrite core (Laird 28b1000-000) as shown here. One end should extend out the end of a core by about an inch, the other by almost 2 inches. The wire should pass through the center of the core 9 times.
This
photo shows how and where to mount these chokes. Leave as much space as possible
between them, about 1/8 inch is enough.
Construct
the output balun.
The RG402 coax supplied should be trimmed to a length of 15.5 inches (395mm). Begin making the balun by passing the coax through the core from left to right, leaving about 1 inch coming out of the core on the left side. After winding the coax so it passes through the center of the core 4 times, you'll have about 2.25 inches (57mm) coming out of the right side.
For the single-band 6m kit, the balun does not use the ferrite core and the
coax is cut to a length of 36.5cm.
Practice these next steps on the short piece of RG402 left over from trimming; proceed with trimming the balun ends and output capacitor only when you feel you've mastered the technique.
For the balun, remove 1/2 inch (12mm) insulation from each end, exposing the outer
conductor. Roll the coax under a knife blade to 'score' it, at 5mm away from the
insulation, taking care not to cut completely through the shield (outer).
Using
a pliers, bend the shield end back and forth, breaking it away at the scored
location. Once it is broken, it can be removed with a pair of diagonal cutters,
or just pulled away from the end of the cable. This needs to be done at both
ends of the balun.
On
both ends, remove all but 2 or 3mm of insulation from around the center
conductor, taking care not to cut into the center conductor itself. The balun is
now ready to be installed.
Beginning with the longer lead, form the center conductor so it will lie flat on the trace in plane with the outer, and solder it in place across the gap as shown; center conductor soldered to the left side, outer soldered to the right.
In
this step, you'll need to bend the coax down and then level again as it
exits the core, and secure the ends as shown. Solder the outer conductor first,
then bend the center conductor down onto its trace and solder it in place.
Just one last thing before the output board is finished; trim the remaining piece of TC18 coax (6 inches is left over) to 128mm in length, it will be used to fabricate a 30pf coaxial capacitor. This capacitor improves output and efficiency on the higher bands (6m in particular). Fabricate it from the coax like this:
The center conductor is soldered to the output trace and the shield to ground; the other end is open and can be secured with a loop (as shown here) so it does not contact other components or short to ground.
The capacitor is made from Teflon coax to enable it to withstand very high voltages; when the rf deck is used with a reflective LPF, harmonic energy returned to this location on some bands created voltages high enough to destroy even 3kv capacitors.
Lastly, install the 220uf capacitor, positive lead to the VDD trace, negative
to the ground foil.
The output board is finished.
Continuing now with assembling the input board; install all components
(except T1) as shown in their locations below. C5 is to the left of R14 and
under the R13 marking.
Cut
12 inches of RG316 and remove 5mm of insulation from the center,
exposing the shield in that location. Tin the shield there.
Remove
4.25 inches insulation from each end, exposing the shield. This is an old photo
showing only 2.25 inches removed, but you get the idea....
Holding
the shield in place with your left hand, push the right end of the shield to the
left, forming a mushroom bulge about 5mm away from the insulation on the left.
Tin the 5mm of shield at the spot referenced by the red arrow.
Using
a pair of diagonal cutters with the cutting blades parallel to the coax, trim
away the mushroom bulge and slide the loose shield off of and away from the
insulated center conductor.
Install
the coax into the smaller ferrite core in criss-cross fashion as shown.
Position the transformer as shown, routing the center conductor of the coax on the left across the core to the right, then through the core from right to left. Solder the exposed shield to the left gate trace, taking care not to bridge the gap between the traces.
Do the same with the coax on the right; routing the center conductor of the coax across the core to the left, then through the core from left to right. Solder the exposed shield to the right gate trace, taking care not to bridge the gap between the traces.
Repeat the crossings to add one additional pass through the core on each side.
Pass the coax on the left across the core to the right, then through the core from right to left.
Pass the coax on the right across the core to the left, then through the core
to the right.
Taking
care not to cut into the center conductor, trim insulation away from the
ends and tin them.
Solder the coax center tap to the trace below it (one end of R10 is also connected to this trace).
Solder the center conductor on the left to the trace at R14 and the input trace.
Solder the center conductor on the right to the ground foil to the right of
C7
The
input board is finished.
Your
heat sink should be drilled/tapped for 40-40 machine screws (or your metric
equivalent) using the
pattern in this template.
Position your copper spreader (with LDMOS attached) over the drilled pattern in your heat sink.
The recommended way to attach your LDMOS to the copper spreader is to flow-solder it as you see it pictured here. Machined spreaders with LDMOS attached using this process are available on the parts page here.
Slide the boards into place under the transistor tabs
Secure the board and spreader to the heat sink with 4-40 x 5/8 machine screws (or your metric equivalent) using flat washers under the screw heads. Once the boards are in proper position, tighten the screws.
Solder the 4 transistor tabs to the PC boards.
Turn on the 50v main supply voltage, but not the bias; there should be no current drawn
Turn on the bias and note the idling current drawn from the 50v supply. Adjust IDQ for 2 amps. Note: the current drawn by the bias supply (usually12v) is not what you are measuring here...you must measure the idling current (IDQ) the LDMOS draws from the 50v supply.
Shut off the power supply, and remove current limiting.
