[Elecraft] K2 KPA100 SWR Bridge Readings Way Too High

[Dick Lamb]

Don,

I found the problem:  the short wire that connects the ungrounded side of
C1 to the SO259 jack had a poor solder connection.

I figured that since C1 wouldn't tune, there had to be a problem in that
bridge circuit prior to U5.   I started tracing each component's
connections with my Ohmmeter, and I finally discovered that the ungrounded
solder pad for C1 didn't connect.

Thanks for your help, Don.  You're a terrific resource and your
extraordinary efforts and patience in helping bewildered folks like me are
really appreciated.

73
Dick, K0KK



On Tue, Oct 3, 2017 at 3:10 PM, Don Wilhelm <donwilh at embarqmail.com> wrote:

> Dick,
>
> That is mystifying.   With the diode "backwards", you get a positive
> voltage at U5 pin 3 - that should be the correct orientation for D17.
>
> For the power indication, check the polarity of the voltage at U5 pin 5 -
> if that is negative, then D16 is also backwards.
>
> You may have a situation where T4 is not wound with the correct sense,
> although the orientation of the diodes should still produce a positive
> voltage at U5.
> Something is strange here.
>
> 73,
> Don W3FPR
>
> On 10/3/2017 3:44 PM, Dick Lamb wrote:
>
>> Yes, the voltage is negative on U5 Pin 3, but D17 is not installed
>> backward--i.e., the cathode band is on the side away from the coax
>> connector, as shown in the manual.  I tried reversing the diodes (cathode
>> band on the side of the the coax connector), and Pin 3 voltage does drop to
>> millivolt positive range, and the SWR shows 1:1, but the power output
>> doesn't change regardless of the "power" knob setting.  I verified that the
>> diodes are banded correctly with a semiconductor tester. Dick, K0KK
>>
>> On Tue, Oct 3, 2017 at 1:20 PM, Don Wilhelm <donwilh at embarqmail.com
>> <mailto:donwilh at embarqmail.com>> wrote:
>>
>>     Dick,
>>
>>     Is that voltage at U5 pin 3 actually negative?  If so diode D17 is
>>     backwards - the voltage should be positive in all cases.  Yes, C1
>>     will not adjust if the D17 polatity is wrong.
>>     You can refer to the parts placement diagram at the back of the
>>     KPA100 manual if you cannot see the silkscreen clearly.
>>
>>     After putting D17 in correctly, recheck the 43k initial setting of
>>     R27 and go from there.
>>
>>
>