[Elecraft] K2/KPA100: External Amplifier Keying Test (PA Key) Problem

[Don Wilhelm]

Hideki,

The only other thing that could be wrong is high resistance between the 
drain of Q9 and the center of the RCA jack.  Check both those places for 
good soldering.

Look at the schematic and you will see the circuit is dead simple.  Q9 
has its source connected to ground, and the gate is connected to U1 pin 
16 through R1.  The drain of Q9 connects directly to the RCA jack.
There is nothing more to the circuit.
In operation, when U1 pin 16 goes to a high level (5 volts or so), the 
voltage at the gate of Q9 is also raised - and will likely be close to 5 
volts.  In any case, that should allow Q9 to conduct (with very low 
resistance) from the drain to the source.  With the source connected to 
ground, and the drain connected to the center of the RCA jack, if you do 
not have a VERY low resistance between the RCA jack and ground, either 
there is a bad solder connection or Q9 is faulty.

Check your ohmmeter - if it is reading milliohms instead of ohms, the 
350 milliohms would be a normal resistance when TUNE is activated.

73,
Don W3FPR

JF1KGX wrote:
> Don,
>
> I have checked what you advised.
> But envrything looks fine.
> I don't believe that Q9 itself is something wromg.
>
> Thank you,
> Hideki
> JF1KGX
>
> ----- Original Message ----- 
> From: "Don Wilhelm" <w3fpr at embarqmail.com>
> To: "JF1KGX" <jf1kgx at jarl.com>
> Cc: <elecraft at mailman.qth.net>
> Sent: Wednesday, July 29, 2009 6:47 AM
> Subject: Re: [Elecraft] K2/KPA100: External Amplifier Keying Test (PA Key) 
> Problem
>
>
>   
>> Hideki,
>>
>> With power off, check the resistance from the source of Q9 to ground.  If 
>> you can check from the lead of Q9 rather than the solder connection.  If 
>> it is not zero ohms, you have a soldering problem there.
>>
>> If the source of Q9 is well grounded, next check the voltage at the Q9 
>> gate during TUNE - it should be in the 3 to 5 volt range. - if lower than 
>> that, check the soldering and value of R1 and the soldering at U1 pin 16 
>> (pin 16 of U1 should be very near 5 volts during TUNE).
>>
>> If all is good with those checks, Q9 itself is the prime suspect (but I 
>> have never hear of one being a problem).
>>
>> 73,
>> Don W3FPR
>>
>> JF1KGX wrote:
>>     
>>> Somebody has asked the same before. But nothing has been replied.
>>> Now I have the same problem.
>>>
>>> Page 49 of the KPA100 instructions calls for testing the resistance 
>>> between
>>> ground and the tab of Q9 on the KPA100 board (or center conductor of the 
>>> PA KEY
>>> output jack, J7).
>>>
>>> According to the instructions, the resistance should drop to less than 3 
>>> ohms
>>> when the K2 is put into TUNE mode.  Mine only drops to about 270 ohms. 
>>> The
>>> troubleshooting in the KPA100 manual doesn't even include any information 
>>> for
>>> what to do if the resistance doesn't drop to less than 3 ohms.
>>>
>>> So far everything else seems to be working correctly with the KPA100.
>>> My serial number is 06617.
>>>
>>> Please advise me.
>>>
>>> Thanks
>>>
>>> Hideki
>>> JF1KGX
>>>
>>>
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