[Elecraft] [K2] T/R switching

[Don Wilhelm]

Sorry for the blank post, it was a "slip of the finger" problem.

The 2N7000 is a great switching FET, but as you noticed is susceptible 
to ESD damage until it is mounted in its circuit, and then the static 
susceptibility is contained.

As for your fix and substitution, measure the output of the 8 volt 
regulator - that is a 10% device, so anything between 7.2 and 8.8 volts 
is acceptable.  Your 8T and 8R voltages should be within the range of 
7.2 and 8.8 volts if all is well.

I suspect that your substitution for the 2N7000 will work just fine in 
this particular case.

73,
Don W3FPR

On 3/15/2012 6:27 PM, Don Wilhelm wrote:
>
>
> On 3/15/2012 12:26 PM, picmaster at mail.bg wrote:
>> Hello fellow hams,
>>
>> One more K2 (#7293) was born this week in Europe - thanks to the 
>> Elecraft team for designing this kit and making it available for the 
>> ham crowd.
>>
>> The radio is now assembled, but I have small issues with the testing. 
>> During the initial testing on 40m reception the radio was almost deaf 
>> (even 7.3MHz AM stations were faint), and I found out that both 8R 
>> and 8T control lines were activated at the same time (thus enabling 
>> all attenuators). It turned out that Q3 (2N7000) on the Control Board 
>> was zapped and was pulling the base of Q1 low even without applied 
>> G-S voltage, thus activating erroneously the 8T line. I didn't had 
>> any spare 2N7000 parts around, so I had to invent a BJT-replacement 
>> for this low-power MOSFET, based on BC550C and 33K base resistor (you 
>> know, it's hard to buy spares in the night). The replacement worked 
>> OK, but I was wondering whether my circuit modification has changed 
>> the T/R control circuit voltages. I did a comparison between Q4's 
>> drain voltage (the healthy MOSFET) when ON and Q3's (now BC550C) 
>> collector voltage, and the results were 5mV on Q4's drain and 46mV on 
>> Q3's collector. To me even with the higher (45mV vs 5mV) saturation 
>> C-E voltage of BC550C, Q1 and Q2 switches will still be able to 
>> saturate properly when enabled.
>>
>> So my 1st question is why originally 2N7000 were used to control T/R 
>> switches (especially taking into account that these MOSFETs are very 
>> sensitive to ESD)? I'll also appreciate comments whether my T/R small 
>> modification looks acceptable.
>>
>> My 2nd question is what's the acceptable voltage on 8R&  8T control 
>> lines? I'm measuring around 7.5V on those control lines, while the K2 
>> manual (rev.H1, p.114) says that the voltage should be 8V. While PIN 
>> attenuators and MOSFET switches should be rather insensitive to the 
>> 0.5V difference between the theoretical and practical values, there's 
>> a possibility for changes in the PA bias (although the current mirror 
>> and current steering circuit seems to keep the bias rather stable, at 
>> least on the schematic).
>>
>> Thanks in advance for your comments&  responses.
>>
>> Regards,
>> Nikolay (LZ1NRD)
>>
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