[Elecraft] [K2] KPA100 problem

[Andrew UK]

Hi Jerry,

I did not try to isolate as the track went straight to the max chip pin 8, I
just removed the chip. C38 was the cap that was cracked, so instead of 10nF
it was a few 100 pF!!! Or less.

I bought my KP100 some 3 years ago, and since then it has undergone a lot of
improvement from Elecraft, if yours is less than a year old then you will
have the latest revision. There are changes to the TR and switching and bias
circuits.

I had around 150V to 200V at times, if pin 8 is low, then check the
components to the right of C37. The high voltage supply is only bias and
supplies very low current. Check Q6 as if this has a very small leak, it
will drag the bias voltage down and check D11 to 14.

Q6 has failed a few times in my rig prior to the modifications, however,
this was due to me leaving the aerial connected when high winds and storms
were about.

73's

Andrew 


Don Wilhelm-4 wrote:
> 
> Jerry,
> 
> That answer depends on which set of "specifications" you want to believe.
> 
> I am saying that the Absolute Maximum Specification for the Maxim 
> MAX1406 for Vss (pin 8) is -14 volts - you can check the data sheet for 
> yourself.  Yes, many KPA100s exceed that value - note that the KPA100 DC 
> voltage chart lists the nominal voltage at U4 pin 8 at -22 volts.
> Whether that is a cause for failure of the chip is reason for 
> speculation, there is not enough data to say yeah or nay. .
> What I have offered is a way to reduce the Vss voltage to the MAX1406 
> without also reducing the bias voltage to the KPA100 T/R switch.  That 
> involves removing D8, and D8 alone.  The data sheet Electrical 
> Characteristics current for pin 8 (Iss) is typical 185 uA with a maximum 
> of 500 uA. for a Vss of -13.2 volts Minimum to -10.8 Maximum - that is 
> not a lot of current draw on that pin.
> 
> I am stating that a Vss (U4 pin 8) voltage less than -14 volts exceeds 
> the Absolute Maximum Voltage rating per the MAX1406 datasheet - nothing 
> more and nothing less.
> 
> 73,
> Don W3FPR
> 
> n0jrn wrote:
>> Don:
>>
>> Just to clarify !
>>
>> The -28V's I'm seeing at pin 8 of U4 is measured without the IC
>> installed.
>>
>> Lets say for the sake of conversation that I install a good MAX1406 and
>> it 
>> draws on the rectifier circuit a minimal amount.
>>
>> Would / could that bring the -28V's back into specs for this chip???.   (
>> I 
>> have to figure Wayne designed the circuit taking this draw into 
>> consideration )
>>
>> OR      is my -28V's so abnormal that I need to be looking at other
>> issues 
>> in my rectifier pack to bring this negative voltage back into spec ???
>>
>> Could the negative voltage I'm getting be responsible for damaging this
>> chip 
>> ???
>>
>> I have 2 new chips coming from Mouser.   I'm not installing them until
>> I'm 
>> satisfied that I've checked and double checked   EVERYTHING   involved.
>>
>> Thanks again and happy holidays to the group:
>>
>> 72             Jerry             N0JRN
>> ----- Original Message ----- 
>> From: "Don Wilhelm" <don at w3fpr.com>
>> To: "n0jrn" <n0jrn at mchsi.com>
>> Sent: Monday, December 14, 2009 6:22 AM
>> Subject: Re: [Elecraft] KPA100 problem
>>
>>
>>   
>>> Jerry,
>>>
>>> I will take back what I said about U4 - it is faulty.
>>> That negative voltage is there only to allow the RS-232 drivers to
>>> produce 
>>> a negative level, the current draw should be minimal.
>>> The only thing U4 does is convert RS-232 levels to TTL levels and 
>>> vice-versa.
>>>
>>> Actually -28 volts is a bit much and does exceed the maximum speced 
>>> voltage for the chip.  -12 volts is more than sufficient and even -5
>>>     
>>
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