[Elecraft] Re: KPA100 SWR bridge adjustment 160m & 80m

[Koert Wilmink]

Dear Don, Geof and Gary,

I did receive the diodes and did replace them today. The problem is 
solved!! SWR is ok on all bands now (with the dummyload).
I want to thank you all for the support!!! I'm in the US next week, see 
what I can order from Elecraft :-)

73 Koert PA1KW

Don Wilhelm wrote:
> Koert,
>
> A Great 2008 to you and yours as well.
>
> The only other suggestion at this time is that you make a practice of 
> disconnecting antennas when not actually in use.  A small amount of 
> lightning surge or even 'snow or rain static' can produce enough peak 
> voltage to zap those diodes.
>
> 73,
> Don W3FPR
>
> Koert Wilmink wrote:
>> Dear Don and Geoff,
>>
>> First of all I want to wish you a great and healthy 2008!!I did cut both
>> diodes D16 and D17 and I'm able to set R26 and R27 to 43K!! I will ask
>> Elecraft to send me 2 new diodes. Any other suggestions?
>>
>> 73 Koert PA1KW
>>
>> Don Wilhelm wrote:
>>   
>>> Koert,
>>>
>>> Yes, that would be a good and informative step - do it that way.
>>> If the trimmers then adjust over the 0 to 100k range without the
>>> diodes in place, just set the trimmers at 43K and replace the diodes
>>> with new ones - then try the balance adjustment (C1) again before
>>> changing anything else - if it nulls, then all the rest is OK and you
>>> can move forward to the calibration of the forward power..
>>>
>>> 73,
>>> Don W3FPR
>>>
>>> Koert Wilmink wrote:
>>>     
>>>> What do you both suggest? First take out the diodes and see if  I can
>>>> adjust R26 and R27 to 43K? That will exclude the LM358 as the course of
>>>> failure?
>>>>
>>>> 73 Koert PA1KW
>>>>
>>>> Geoffrey Mackenzie-Kennedy wrote:
>>>>   
>>>>       
>>>>> Don and Koert.
>>>>>  
>>>>> Please have patience with me while I get our 18 year old daughter
>>>>> safely to one of her friend's house in Northern 'darkest Scotland',
>>>>> the sun is setting as I write :-)  I will be back in a few hours time.
>>>>>  
>>>>> Koert, if you do start to replace components may I suggest that you
>>>>> change only one or two at a time, if you replace several at one
>>>>> time before checking the circuit's performance it might not be
>>>>> possible to determine which is the troublemaker unless a component is
>>>>> obviously faulty e.g a shorted diode.
>>>>>  
>>>>> Don, yes I note the path through R29 which made me wonder about the
>>>>> type of meter Koert is using to measure resistance.
>>>>>  
>>>>>  In haste,
>>>>>  
>>>>> 73,
>>>>> Geoff
>>>>> GM4ESD
>>>>>
>>>>>     Geoff and Koert,
>>>>>
>>>>>     I am not yet convinced that a valid null was measured during the
>>>>>     initial setup.  At a power level of 10 watts, I typically want to
>>>>>     see a voltage of less than 20 mV at TP4.  Something has been awry
>>>>>     all the time based on the information that I have seen so far.
>>>>>
>>>>>     Perhaps I am getting confused by all the pieces, but so far, the
>>>>>     resistance readings at the R26 and R27 wipers are not correct and
>>>>>     are an indication of a problem - that should be resolved before
>>>>>     taking other steps.  It may be that reversing the leads of the
>>>>>     meter will give better readings, but if not, then the diodes are
>>>>>     suspect.  Notice that with shorted diodes, there is a path
>>>>>     straight to ground through R29  - its 3.3k value is not
>>>>>     significant compared to 25k.
>>>>>
>>>>>     Koert, you are in the midst of a typical troubleshooting process -
>>>>>     find a potential problem (it may not be the source), and then
>>>>>     verify it is OK or determine the cause of the improper readings -
>>>>>     after that is done, move on to the next situation.  Bear with us
>>>>>     while several possible causes are explored - and we need your help
>>>>>     to take the measurements and report them so that we all understand
>>>>>     the conditions under which those measurements were made.  We will
>>>>>     get to the end of this problem, but it may take some time but we
>>>>>     will go down some pathways that lead to nothing fruitful during
>>>>>     the process.  Troubleshooting is as much a process of eliminating
>>>>>     possibilities as it is in taking steps toward the problem source.
>>>>>
>>>>>     73,
>>>>>     Don W3FPR
>>>>>
>>>>>     Geoffrey Mackenzie-Kennedy wrote:
>>>>>     
>>>>>         
>>>>>>     Don and Koert,
>>>>>>      
>>>>>>     Koert, what type of ohmeter are you using to measure the
>>>>>>     resistance between each potentiometer wiper (R26 and R27) and
>>>>>>     ground? If it is an Avo or something similar which 
>>>>>>     could cause a diode to conduct, the maximum value of resistance
>>>>>>     between wiper and ground would be found when the wiper is close
>>>>>>     to the mid point of its travel if the meter forces enough
>>>>>>     current. If you are using a meter similar to an Avo swapping
>>>>>>     leads would be worthwhile. However this type of meter could upset
>>>>>>     the op amp's, adding further error to the resistance measurements
>>>>>>      
>>>>>>     Don,  if the diodes were shorted I don't think that it would be
>>>>>>     possible for Koert to find a null on the higher frequency bands,
>>>>>>     or am I missing something?
>>>>>>      
>>>>>>     73,
>>>>>>     Geoff
>>>>>>     GM4ESD
>>>>>>      
>>>>>>      
>>>>>>     Don Wilhelm wrote:
>>>>>>
>>>>>>         Koert,
>>>>>>
>>>>>>         Doing a bit more thinking about your resistance measurements,
>>>>>>         I believe your diodes D16 and D17 are shorted.  There is no
>>>>>>         other way that I can see for the maximum resistance of the
>>>>>>         wipers of R26 and R27 to indicate 25k ohms to ground.
>>>>>>
>>>>>>         73,
>>>>>>         Don W3FPR
>>>>>>
>>>>>>       
>>>>>>           
>>>>   
>>>>       
>>
>>
>>