[Elecraft] K2 10W PA problem

[Don Wilhelm]

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

Guido Tedeschi wrote:
> Tom W8JI wrote:
>   
>> This is almost certainly thermal drift from heat.
>>
>> Poor heatsinking of transistors will cause this, so make sure he used 
>> the CORRECT very light amount of any heatsink compound and that the 
>> transistors are clean underneath and bolted to correct tightness.
>>
>> Also a very small upward drift in bias voltage will cause this. This 
>> can be because of leaky series pass bias transistors or bias pass 
>> transistor heating from inadequate heatsinking.
>>
>>     
> Many thanks to Tom and the others: I've noticed that PA transistors are 
> not installed perfectly horizontal and it is possible that the 
> mechanical contact with the cover is not so good.
> This poor heatsinking can explain the drift but, probably, not the 1.2A 
> with only 0.5W... Maybe, also the two transistors used for bias voltage 
> are not so healty...
>
>