QCX 20m - Low power (~2.5W) with oscilloscope capture of L1, L2 and L3 nets #qcx #lpf


Hello everyone, I am new to this group and I have just finished building a Version 3 20m QCX kit. It was a lot of fun.


I have low power output (2.3W) though. I connected a 50 ohm dummy load on the RF output, and I probed around the following points on the circuit (#1, 2, 3 and 4):

Anyone know why I am getting 78.4V peak-to-peak at #2? That seems so much higher than #1 and #3 points!


Cheers and best 73



Ham Radio

Your RF voltage measurement are probably correct but the impedance at points 2 and 3 is not 50 ohms which means the computed power is not accurate.  I assume you used a 10:1 probe?  The voltage output from the 3 FETs from the Class E PA may a bit low based on your scope pic number 4 (hard to tell).  Not sure if the impedance at test point 4 is really 50 ohms.

What is the current draw in TX mode at the voltage you are using? It should be around 500 ma with a 13.8 volt power supply; if much less than that value, you may have one dud FET in the class E amp.  To get 5 Watts out, the PA should draw about 5.6 Watts based on what I have read. You may be losing power in the Low Pass filter, but first confirm what power the PA is consuming...

73, Bernie, VE3FWF


Yes I think you are right that the impedance at point 2 and 3 are not 50 ohms. Thanks for the explanation. I was using a 10:1 probe.
I didn't measure current draw. I will do that next.

BTW, since then I've bought the Peak LCR meter and have desoldered L1, L2, L3. They are measured to be higher then what they should be. So now I've got rid of a few turns, now they measure:
L1 = 0.78 uH (instruction says it should be 0.77 uH)
L2 = 0.84 uH (............................................. 0.90 uH)
L3 = 0.75 uH (............................................. 0.77 uH)

But the output is still low, at only 30Vpp (2.25W). The supply voltage after the reverse voltage diode is 14.15V.  By the way, that's worse that before I rewound the toroids.
Here are the measurements now at Net #1 to #4 (as per my image):
#1: 30    Vpp
#2: 52.8 Vpp
#3: 39.6 Vpp
#4: 46    Vpp

What to do next, 

Alan G3XAQ


If you search back through the messages on here you will see a lot (I mean a LOT) of builders have found they were supplied with dud capacitors for C25 and C26. They are 390pF. Go buy some decent quality ones from any of the major suppliers. You need C0G dielectric 100v working. Either leaded or SMD are fine, whatever you are comfortable with.

73, Alan G3XAQ


Oh thanks for the guidance. I got the turquoise-coloured 390pF, so I think you are right. OK I will go and find some and change them now.

Cheers and 73