Re: Another #20m #qcx with low TX Power #qcx #20m


scot forshaw
 

Is the key shaping transistor getting hot? 


On 23 Apr 2020, at 21:15, DL2MAN@... wrote:

Hi there,

this is my first Post into this group.
I´ve built a 40m QCX with absolutely no issues. 
After that, I´ve built 20m QCX, and this one lacks -as many others- TX PWR.

History:
- I wound L1-3 according to manual.
- I measured abt. 1,5 Watts out, so I unsoldered coupling c29, and measured Low-Pass with Nano VNA.
- Lowest SWR of Filter was around 11MHz, and damping at 14MHz was around 3 dB - Way too High.
- I unmounted L1-3, measured inductance, and reduced windings, until I matched the documented µH values.
- While trying to measure Power before Coupling C29, I smelled something going wrong....
- I exchanged Q1-Q3 and Q6.
- Now Filter has Lowest SWR at 14MHz, but still a much too high pass through damping of 1,23 dB
- For Comparison Low Pass of QCX 7MHz has below 0,5 dB
- Power OUT is now around 2,5W @ 13,8V vs 4W of my 40m QCX
- So instead of ranlomly exchanging stuff, I thought I might do a thermal photo while TXing
and Bingo:

C25 and C26 get hotter and faster hot then Q1-Q3, So there goes my Power....

-Already checked them, they´re the right ones for 20m: 390 pF, labeled 391.
-Another interesting Overservation: L4 gets hot as well when transmitting, and I have no clue why.....

Since Hans made very clear, that he´s using high quality components, I´d like to know what is happening there, why,
and how could I fix that. 

As a first step, I would exchange C25 and C26 to "Glimmer" Type C´s.

What´s your opinion ? Any suggestions ?

73 de Manuel; DL2MAN
Sorry for tyops. Not my native language, and technical english is even harder for me.....

<img_thermal_1587666118008.jpg>

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