Elecraft KPA100 assembly note #1: 

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

June 26, 2002

If the KPA100 works OK on all bands except two or three, 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 tor three bands would be affected since the black T50-10 cores are used at several locations and are shared on 2 or 3 bands: 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.

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.

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