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QDX TX power was almost zero.

 

Hello. everyone.
My QDX(order #50315) arrived in Japan on November 8th.
I assembled it immediately.
And I checked the reception performance.
The reception sensitivity is good for each band.
Next, I connected a dummy load to the QDX and measured the transmit power.
The output from QDX was almost zero.
20m has more output than 80m, but it is still only a few mW.
I confirmed the soldering of the toroids etc.,
?but there is no abnormality.
I saw the signal on an oscilloscope.
And I found that the signal was attenuated at both ends of C2 (2.2uF).
When I measure the capacity of C2, it is only around 100PF.?
It was this capacitor that caused the output to drop.
I soldered a 0.47uF ceramic capacitor instead of C2.
(Because I didn't have a 2.2uF ceramic capacitor.)
As a result, each band has an output of 5W at a power supply voltage of 12V. (T1 is 3:2)
This C2 was originally 2.2uF, but it seems to have been degraded by heat during the replacement of L1.
I recommend attaching it to the back of D3 instead of C2 when replacing L1.
?
My current info:
1 - Windows 11 Pro 21H2 22000.282? ?WSJT-X v2.5.2?
2 - QDX version v1_01b
3 - Built for 12 volts
4 - Power supply 3to14V 1.5A Linear Mode type
5 - Rev 1 PCBs
?
73? syuji ota? JH3AQQ
 

syuji ota, in the years i have been reading all of the posts to [QRPLabs], yours is the most carefully constructed, informative and clear description of a problem and its solution that I have seen.

If English happens to be your *second* (or other) language, then WOW.

Thank you.

--
MC
VK1MC
 

Dear Syuji,

Thanks for this post, I to have low power on 80M 40M, 30M , but 20M is near normal, if the capacity of C2 had gone down this would explain it, I will test mine and let you know. (I have not disturbed L1 yet, I wish to measure harmonics before and after the inductor replacements)

Very Best Regards

Andrew

PS the Elecraft K3 10 Watt version suffered from this, the 100 Watt PA bypass capacitor used to go low and give the same low power fault on low bands, this was hardly ever spotted, as every Americans always bought he 100 Watt version! some caps to not like 10Watt, if they are going into an ATU when the aerial is mismatched.
 

Hi just measured my C2 and it 1.67??F so 25% down, so will replace.

73

Andrew G8UUG

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