[Elecraft] KPA100 SWR Bridge Issue

[Don Wilhelm]

Curt,

You do have a wattmeter problem, and likely if you fix that, all will 
settle down.

You can work on the wattmeter problem at low power only - at low power, 
you do not have to mount the board to the heatsink.  Just power the base 
K2 and do not plug any power into the KPA100 board.

First set the two trimpots R26 and R27 close to mid-range.  Then measure 
the resistance to ground from U5 pin 3 and U5 pin 5.  You should read 
something between 40k and 50k ohms.
If vastly other than that, first check diodes D16 and D17.  Make sure 
the diodes are oriented correctly - look at the parts placement diagram 
near the back of the manual.
The next thing is T4 - it must be wound *exactly* as shown in the 
diagram in the manual - including the direction of the winding - and the 
leads must be in the proper holes.
The "1 turn link" provided by the red wire and the bare wire soldered to 
the SO-239 center must also be in the right places and well soldered.

Did you put the correct trimmer capacitor at C1?  If this is a new kit, 
the proper trimmer has a white body (not the yellow one).

I have a suspicion that you may have wound T4 in the wrong direction or 
have the leads not in the right holes.

73,
Don W3FPR

On 6/22/2016 9:18 PM, Curt Milton via Elecraft wrote:
> Before I dismantle this thing one more time, curious if any sage info is available --
> I noticed that with a dummy load plugged in, and operating at 5 watts -- I get a reasonable power reading but SWR is around 10:1.  It does not change at all as I tweak C1.  both outputs from the op amp are reading around 1 volt more or less.  both diodes are reading okay with my DVM.  both pots have been adjusted for 43k ohms per the instruction.
>
> maybe I damaged C1 is disassembly, or I don't have the windings connected to the PC board - but DVM affirms conductivity so at least one is attached.  yes I realize not much in this network, yet something is astray.  (no surprise that sensing a high SWR that the KPA won't do high power?)
>
> 73 Curt
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