K2 RF Q6 2SC5739 too hot

[Lennart]

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

[Don Wilhelm-4]

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
>  
>  

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[Lennart]

I had a to replace a final transistor earlier this year, and after that I checked most of the compeonents and voltages around Q6-8. That is when I noticed that the Q6 had an unhealthy temperature. R50 looked normal and measured 1.5 ohms. The Q6 got glowing hot and the emitter voltage kept rising.

I got this from Elecraft support:
"Normally with the 2SC2166 Q6, there is very close to 1.0VDC on the base in transmit, and the emitter is 0.25 to 0.3VDC when in transmit. Q6 does get pretty warm under normal operation too
The 2SC5739 has a  little higher gain, so it may be producing a great emitter voltage.
I have not yet seen a K2 that has the 2SC5739, so I am not sure if this is the case."

After that I put some better cooling on Q6 and the radio operates normal, but I still want the Q6 back to normal temperature. I probably need to test replacing Q6 and see what happens.

Regards,
Lennart