Elecraft KPA100UPKT - Manual (Page 9)

9
Reassembly and Test
Locate the thermal pads for Q1 and Q2. You may use the pads you removed if they are not damaged. New pads are
provided with the kit. Remove the small punch-outs at the mounting holes in each pad, if still present in the new pads.
Position the pads on the heat sink with the holes in the pads aligned with the holes in the heat sink. Note that these pads
have no adhesive to hold them in place. Do not omit these pads: the power transistors will be quickly destroyed without
them.
Ensure the bottom of both Q1 and Q2 are clean bare metal with no bits of the thermal pads or other material stuck to
them.
i
Do not use lock washers between the heat sink and the PC board when attaching the board in the next step.
Install the PC board/rear panel assembly onto the heat sink. Press the board down until it is flat against all seven
standoffs on the heat sink, including the two machined standoffs on the pedestal.
Adjust the locations of the thermal pads for Q1 and Q2, if necessary, so that they are still aligned with the mounting
holes. Insert 4-40 x 1/4" (6 mm) pan head screws and split lock washers into the mounting holes for Q1 and Q2 and start the
threads to ensure the thermal pads are held in place. Do not tighten these screws yet. Only screw them in far enough to
engage the threads in the heat sink.
Secure the rear panel to the heat sink using three 4-40 x 3/16" (4.8 mm) black pan head screws.
Tighten the screws to secure Q1 and Q2 to the heat sink Use a small-handled Phillips screw driver to fasten the
hardware tightly without risk of over-tightening or stripping out the screw heads.
Secure Q3 and Q4 to the heat sink using 4-40 x 1/4" (6 mm) pan head screws and split lock washers. Use a small-
handled driver as you did with Q1 and Q2.
Look between the PC board and heat sink to verify that no component leads or bottom-mounted components are
touching the heat sink or associated hardware.
Place a #4 split lock washer over each of the seven set screws.
Install the shorter 1/2" (12.5 mm) standoffs on the two set screws at the front edge of the board. One is near the
"SPKR" and "GND" pads, and the other is near the "AF" and "GND" pads. Do not over tighten.
Replace the remaining 5/8" (16 mm) standoffs on the remaining five set screws.
Re-solder the speaker wires to the loudspeaker terminals. The silver wire connecting to the SPKR pad on the KPA100
goes to the + terminal on the speaker.