[Elecraft] K2 10W PA problem

[Don Wilhelm]

Guido,

I would like to see the base bias voltage a little bit lower - 0.63 
maximum, .62 nominal.  I recommend replacing Q11 and Q13.
Yes, the current draw without Q7 and Q8 installed is within the normal 
range.

73,
Don W3FPR

Guido Tedeschi wrote:
> I removed the PA transistors and measured the bias, which is 0.64V and 
> this should be fine...
> Probably, it was only an heatsinking problem, unless the TX current is 
> wrong: without the PA transistors, with RF Power set to 5.0W (with any 
> value greater than 0.1W is the same), the TUNE current is 0.62A (0.24A 
> in RX)
> Is it ok 0.62A?
> Thanks and 73
> Guido, ik2bcp
>
> Don Wilhelm wrote:
>   
>> Guido,
>>
>> Since the normal base to emitter voltage on the PA transistors self 
>> limits at about 0.63 volts, the best way to check the bias voltage is 
>> to remove the PA transistors and measure it.  With the PA transistors 
>> out, if the base voltage during a TUNE is between  0.6 and 0.63 volts, 
>> then Q11 and Q13 are doing their job and you have faulty PA transistors.
>>
>> For proper heatsinking, yes the PA transistors must fit flat against 
>> the heatsink.  Before soldering the PA transistors in place, I 
>> position them on their mounting hardware and secure them temporarily 
>> with a nut (no lockwasher, no thermal pad).  That places the tab 
>> parallel with the RF board.
>> Solder the transistors and remove the nuts - add the thermal pads and 
>> mount the heatsink.
>>
>> 73,
>> Don W3FPR
>>     
>
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