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Any idea what the questionable capacitors read on an LCR meter? They may be mismarked.
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On Oct 9, 2017, at 7:43 AM, Andrew Lenton <a@...
I had the same issue with the 390 pF capacitors and when I replaced them all was good, I have also had issue with the 560pF capacitors on the 30M BPF again changing them restored the correct response.
I now have a set LPF that work, so can move on to installing the 5 Watt Amp
See enclosed plots.
From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of Glenn S
Sent: 03 October 2017 11:18
Subject: Re: [QRPLabs] QCX on WSPR
I found out why the power was slowly rising on the 20m QCX!
As suspected, it turned out to be capacitors in the low pass filter. One by one the capacitors were replaced and the RF output behaviour checked.
There was no change when the outer 2 x 180pF (C27+C28) caps were changed, but a noticeable change when one of the inner 390pF (C25) was changed and then perfect when the other 390pF (C26) was changed.
I changed each cap with a 250V ATC ceramic chip cap.
The output power is now very stable and the supply current has dropped. Output is 3.5W with 13.8V supply. Some squeezing of the turns may improve this.
I also experimented with the 30pF capacitor from the drains of the finals to ground. The efficiency went up slightly without it. I put a 30pF trimmer in its place and found best efficiency at almost no mesh of the trimmer.
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<test board 30 M plus markers LPF.png>
<QRP Labs 30 M LPF.png>
<QRP LABS 20 M LPF With maplin 390 pF disc ceramics.png>
<QRP LABS 20 M LPF.png>