New build Tx stopped :(

[m0knc]

Hi,
I built an official kit (Classic bands) yesterday and it all worked first time , when I finished it last night.

I connected a dummy load and tested the TX powered by usb and set the frequency and it was txing on all bands , low power & low(ish) efficiency. The job today was to adjust the the coils on the RF board, so I connected the 12volts and the dummy load (Kept the bottom of the case on too!) and running up the bands I was getting , from memory, 4 watts, on 80, 3, on 40, 3 on 20, 3 on 15 , and 2 on 10m . - all with efficiency less than 50%

I had a fiddle with the coil on 80 and brought the output to about 4.8 Watts, then I jumped to 10m coil and when I tx'd I watched (in horror) as the tx power slowly dropped to zero I was doing nothing at the time, but I DID have my fingers all over the place.

Now it gives Zero power and "nan" as an efficiency % .. I went back an re-soldered all my coils etc, ( (thinking maybe the choke or the swr coils were to blame but to no avail.

This kit uses a single FET (FDT86256) as the final stage, so I am guessing it is this FET that has gone - I see no damage on any components at all.. ( I have ordered a few FDT86256's - they are like £1.20 each, but £12 freight LOL)

do you agree that the FET has gone? I do still see some output on the tuned frequency on local receivers BTW, & the Rx seems to be OK

Thanks in advance
Neil

[dl6sez]

Hello OM,

yes the FDT86256.
Why did'nt you tune the harmonic notch before with a VNA?
If you have read on Classic and High band this is a MUST to do before adjusting with pressing or widening the turns of Lx1 for highest output power....
I don't understand why you did it without adjustment before and why you used than also 12V from the beginning....
Could have been saved if you had done what should be done a simple thing to follow the suggestions of the developper.

73 de Chris

[m0knc]

Thank for your friendly reply...

I am sure that you have never ever made a mistake ... Obviously I knew I had made a mistake, so I didn't need it rubbed in, I just needed to be advised if it was likely to be the FET from someone who may have the experience. One LEARNS form one's mistakes.. LOL
I would like to rephrase your reply in a way that is not so aggressive, if I may...

"Yes it will be the FDT86256
If you have a read up on the Classic & High band, you will find that you MUST tune the harmonic notch before widening the turns or adjusting the coils. Also you can do it with 5v and then apply 12volts.
That is why you blew it up, so that was a bit silly of you, next time you will know."

for the record, the reason I didn't do the VNA test FIRST is becasue I don't have another SMA connector and it is the Easter holidays..

Thanks
Neil

[dl6sez]

Hello Neil,

no because you can read this 100 times here and it is okay if you recognize it was ....

73 de Chris