No Power Output anymore

[DL1SER]

I built up the High Band TruSDX . It all worked good. On 10m also DXing with 0.5 Watt. After I supplied FT8 with 12.6 V At first 2 Watt Output on 10m . After a Minute no Output in FT8 anymore. What hapened ?
Also with USB Supply no Output. Did I kill the FDT86256 Transistor ? How can I check ? SWR will display 9 .99. Between Drain and Gate...shortcut . I don´t see anything, who went in smoke .

Receiving is ok. and I can hear a Signal in my commercial Transceiver.

[dl6sez]

Hello OM,
On the high band with FDT86256 you must adjust your toroids and begin with harmonic notch.
Yes sure your FET is dead and no it will not blow away it silently became dead
You are not using high power so the FET killing is normally not very spectaculous.

73 de Chris

[DL1SER]

Does that mean, I killed the transistor only using high power on the wrong frequency?
How can I measure the built in filters? I have a Nano vna, but how can I measure the filters, when built in? Can I remove the transistor easily. I o la have a soldering pen, suction pump an a fluid Flux.

Is is a problem to work on 10m Ft8, CW abd SSB? The range is very wide on this band. It happened on 10m that the PA had the failure.

[dl6sez]

Hello OM,

i see you didn't have a look on any of the videos at Manuel's Youtube channel...
This forum is not to explain everybody again and again what and how it should be done
So please go to DL2MAN's youtube channel and look how tuning low pass filters is done, as i said 100% a must for high band version!
If you don't exactly know how to replace the FDT86256 please ask a Ham collegue in your near to help you.
Think what you do with PA at FT8, SSB and CW in comparance. Where are the FETs most heated up and have to do the hardest work....

73 de Chris

[DL1SER]

Hi Chris,
Thanks for the info. Normally you are only looking for things you ar searching for.
I didn't know, that the high Band version is more sensitive than the lower band. I watched the video of Manuel now. If I had known before, .
I ordered a new transistor, hoping that only this one was damaged. And.... Operating in FT8 has always the Danger of burning the p a. Anyway, I'll use this trx more for CW and SSB. Ft8 is just like wspr to see, how are the condx.

Does a heatsink for the FDT86256 help to prevent from "overload" or is a heatsink only useful for classic transistors ?
I´m no specialist in electronics, so that´s the reason for my noob question.

[dl6sez]

Hello OM,

normally the FDT86256 has a cooling heatsink due to the soldered Drain, the largest soldered pad.
There is no heatsink for the Low band version with the 3x BS170, it is not necessary it is not getting really hot when everything is tuned.
The problem of the semiconductor in the FETs giving up is not very often a thermal associated fault. More often is too high induced voltage or too high current and the semiconductor will breakdown or burn so fast that a heatsink could not get the thermal power out of the semiconductor die
So leave it as it is. Extra Heatsink can only make problems here due to capacitive coupling.

73 de Chris