[Elecraft] KPA100 problem

[Don Wilhelm]

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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