[Elecraft] K2 RF Q6 2SC5739 too hot

[Don Wilhelm]

Lennart,

Where did you find data that indicated the normal voltage on the Q6 
emitter was 0.23 to 0.30 volts?
The 0.42 volts that you are measuring is normal and consistent with the 
DC Voltage tables.

The base voltage should be close to 0.6 volts higher than the emitter, 
so your measured value of 1.075 volts is very close to what it should be.

You may want to check the Q6 emitter resistor (R50 on the bottom of the 
board), or even replace it.  That will control the DC gain of Q6 more 
than any other element.  You may even want to consider replacing R50 on 
the speculation that it may be changing resistance as it heats from the 
current through it.

I don't know why extra heat sinking of the transistor body would be 
required, it is not in normal circumstances.  You may have a Q6 that is 
acting strangely.  It may be wise to replace both Q6 and R50.

73,
Don W3FPR


Lennart wrote:
> My Driver Q6 has too high voltage on the emitter, 0.42 volts instead of
> 0.23-0.30 volts. It gets very warm and current through it kept increasing
> until I lifted it off the PCB and installed a separate cooler on it. Now
> current is stable (and I constantly get very good audio reports on SSB.)
>
> Transmit current on SSB no audio is 0.60 amp.
>
> Can I reduce the drive current through R47-R48 adjustments, or should I
> suspects something to be broken? Q6 base is 1.075 volts which is normal.
> Seems like the 2SC5739 has too high gain?
>
> Regards,
> Lennart - SM6KNL
>   
>