[Elecraft] KPA100 D16/D17 still blowing

[Don Wilhelm]

Bill,

Unfortunately, the new C31 value has not yet been included in the 
KPA100UPKT BOM.
Send a request to parts at elecraft.com and ask for one.  It is important 
to install it - in some updated KPA100s, the old C31 value is not great 
enough to act as an effective bypass and the KPA100 will self-oscillate 
(often at full power) at a frequency somewhere between 600 and 800 kHz.  
The oscillation will usually start when there is a high SWR present (and 
that situation does occur when the KAT100 is searching for a match).

73,
Don W3FPR

Bill Carpenter wrote:
> My amp is the older design shipped in August, 2006 but the KPA100UPKT 
> was installed.  If the new C31 was part of that kit it's in there but 
> if not I don't think it is.  Question - would it make sense to tune at 
> low power until I figure out the problem?  I'm thinking that would 
> avoid blowing the diodes and still allow me to use my new KAT100.
> ----- Original Message ----- From: "Don Wilhelm" <w3fpr at embarqmail.com>
> To: "Bill Carpenter" <nz0t at cox.net>
> Cc: "elecraft list" <elecraft at mailman.qth.net>
> Sent: Monday, January 07, 2008 8:19 PM
> Subject: Re: [Elecraft] KPA100 D16/D17 still blowing
>
>
>> Bill,
>>
>> That is not normal - BUT -
>> Before I can offer a single suggestion, can you say whether your 
>> KPA100 is the older T/R switch design or is it a KPA100 that shipped 
>> after Nov 2006? If it is the older design, has it been updated with 
>> the KPA100UPKT - and if it has been updated, was C31 also changed to 
>> 0.22 uf?
>>
>> *If* you have the older T/R switch design, there is a possibility of 
>> a parasitic that can show up during tuning, particularly on 40 
>> meters.  With the new design, *if* C31 was not changed to 0.22 uf, 
>> there is a possibility of a low frequency oscillation that shows up 
>> when operating on 160 or 80 and sometimes on 40 meters.  It might be 
>> possible for an oscillation to create a sufficiently high RF voltage 
>> to zap the diodes during a tuning cycle of the KAT100.  (Note that 
>> there are many 'ifs' here, thus the reason for the questions to be 
>> answered - it depends on the level of your KPA100).
>>
>> If your KPA100 is the older design, I suggest the KPA100UPKT may cure 
>> the diode zapping, but if you already have the update (including the 
>> new C31), then there is some other problem.
>>
>> 73,
>> Don W3FPR
>>
>> Bill Carpenter wrote:
>>> Hi all,
>>>
>>> Since adding a KAT100 to my K2/100 about a month ago D16 and D17 in 
>>> the KPA100 have blown 3 times.  The first time I thought it was 
>>> caused by antenna static buildup during a blizzard but the last 2 
>>> times it happened as the tuner was attempting to tune an antenna.  
>>> Could there be problem with the tuner that is causing this?  
>>> Everything is well grounded and the antennas are not bedsprings but 
>>> a G5RV and twin sloper.
>>>
>>> I'm getting real tired of replacing those diodes!
>>>
>>> 73,
>>>
>>> Bill NZ0T
>>>
>>
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