[Elecraft] Dead K2 After nearby power surge

[Don Wilhelm]

Fred,

We are 'doubling' with overlapping posts here along with fast paced 
responses.

 From that brief measurement, I think Q3 is the problem - I base that on 
the fact that you measure 8 volts on both D6 and D7.  Just remove Q3 to 
check that theory.  The K2 will receive without Q3 installed, but will 
get mixed up if you try to transmit.
If you get receive and a display with Q3 removed, then a new 2N7000 
should fix it.

BTW, you cannot check a transistor or FET for short/open by making 
resistance measurements - it is just not reliable nor predictable.

73,
Don W3FPR

KC9QQ wrote:
>
> KC9QQ wrote:
>   
>>
>> Don Wilhelm-4 wrote:
>>     
>>> Fred,
>>>
>>> In cases like yours, first step is to connect a dummy load to the K2 
>>> just in case it is trying to transmit - it is not good to enter transmit 
>>> state without a load on the PA transistors.  If it is a K2/100, remove 
>>> the KPA100 and connect the dummy load to the BNC antenna jack on the 
>>> lower rear panel.
>>>
>>> First thing is to check the state of the 8T and 8R voltage rails. Those 
>>> are easily checked at the anodes of D6 and D7.  If D7 anode has 8 volts 
>>> on it while D6 anode has zero volts, then the K2 is in a transmit 
>>> state.  If both D7 and D6 anodes have 8 volts present, power it only 
>>> briefly while making further checks lest some components overheat.
>>>
>>> If both 8T and 8R are present, you need to check the state of the RX 
>>> signal at the gate of control board Q4 as well as the TX signal at the 
>>> gate of CB Q3 - one should be on (about 5 volts) while the other should 
>>> be off (near zero volts).  If both of these are on, the microprocessor 
>>> was zapped with the power surge, but if only one is on, the problem is a 
>>> shorted Q3 or Q4 on the control board.
>>>
>>> 73,
>>> Don W3FPR
>>>
>>>
>>>
>>>
>>>       
>> Don,
>>
>> I pulled the control board again to reset the relays and then reinstalled
>> it to make the measurements on D6 and D7.  When I powered the unit,
>> Elecraft was again shown and the frequency of 7100 was briefly displayed. 
>> When then display went out I measured the anodes and cathodes of both d6
>> and d7 and measured nearly zero volts on both. 
>>
>>     
>
> I tested the voltages on D6 and D7 anodes again when I powered up.  I had to
> power up twice, but each showed around 7 volts before they fell to near
> zero.  I have found that I can let the unit sit for a few minutes before I
> re-power and I don't have to pull the control board to reset the relays. 
> This only gives me about 2 seconds to make the measurement.
> The voltage on the gate of Q4 is 5 volts.  The voltage on the Gate of Q3 is
> close to zero for the brief period I have to make the measurement.  After a
> few moments the gate of Q4 drops to zero.
>
>  How do I tell which is shorted?  I made the following measurements to
> resistance measurements referenced to ground.
>
> Q3
>   S  - 0.4 M
>   G  - 5.4 M
>   D  - 8.1 M
>
> Q4
>   S  -  0.3 ohms 
>   G  -  3.9 K ohms
>   D  -  8.1 K ohms
>
> I am assuming the culprit is Q4.  Am I correct?  Is it possible the
> something else is fried that may take out Q4 if I install a replacement?  I
> still have 4 boards that I have not built (SSB, DSP2, AT and NB).   I
> believe at least one of these has a 2N7000 I can rob for a temporary
> replacement.  I just don't want to install it until I am sure another
> failure somewhere else upstream won't cause it to fail.  Are there any other
> items I should check before I replace the failed transistor?  
>
> Thanks,
>
> Fred
>
>
>   
>