[Elecraft] KPA100 assembly note: Mistaking the FT50-43 core for the T50-10 black core.

[Eric Swartz WA6HHQ - Elecraft]

KPA100 assembly note: Mistaking the FT50-43 core for the T50-10 black core.
By Gary Surrency ( support@elecraft.com )

Symptom:

If the KPA100 works OK on all bands except one or two, be sure you did not use
the FT50-43 dull gray core in place of one of the T50-10 black cores in the low
pass filter (LPF). Two or three bands can be affected since the black T50-10
cores are used at several locations and are shared in some cases with more than
one band: 30/40m; 17/20m; 10/12/15m.

The red T50-2 cores are less likely to be confused, but check for their proper
locations too.


Result:

This error will cause very low RF output on the band(s) where the FT50-43 core
is mistakenly installed. Both QRP or high power modes will be affected since the
LPF's on the KPA100 are in the signal path at either power level when the KPA100
is connected.

Do not operate the radio in high power for more than a few seconds if this
condition exists. Troubleshoot the KPA100 in QRP mode and insure no low output
problems exist on any band before switching to high power. Keep the high current
KPA100 supply off, or disable the PA in the Secondary Menu entry.

The SWR detector will not work properly with the wrong core. Incorrect FWD and
REFL power outputs from an incorrect core at T4 will make it difficult or
impossible to get the FWD and REFL cal pots set properly. The C1 SWR null
adjustment will also behave correctly with the wrong core.

What to do:

It is fairly hard to find this problem unless you recognize the symptom and
notice the difference between the color of the two core types. The windings on
the core makes this more difficult to see.

Also do not confuse the smaller FT37-43 core with the larger FT50-43 core. The
smaller core is used at T3, and the larger core is used for T4. Be certain the
correct winding direction (sense) is used on the cores, so the PCB pads are
lined up with the wires they connect to.

The winding direction for SWR sensor T4 is critical to its operation. Follow the
manual pictorial on page 27 for correct construction. Make sure the enamel is
completely removed from the wires on all toroids and that they are pre tinned,
bright, shiny, and clean right to the edge of the core before installation.