Lost Tx Power after 6 months of good use

[JohnJEY]

I have the Standard Lo Band pre-built from uellyqh via eBay - with 3-21 T2 update.

After 6 months of good use on FT8, with 12v and 13.8v supplies, the radio died while in use. I was able to re-power it from 5v and after a quick review of the internal components I tried the 12v supply again and it also powered the radio. Back on FT8 and it appeared to be doing well, but getting no contacts and no Spots. Had a closer look at the FETs and saw one was obviously blown - changed it - only managed one very short Spot, so replaced the other 2 FETs. It seemed to work but the Spots were only about half my normal distance and only lasted about 30 mins, then no more new ones. I went to CW and checked the power out and got 0.0 with SWR of 1.0-1.1

I have looked here at similar examples. I have a VNA and LCR meter, but no Transistor tester or Scope.

If anyone can point to other components that I should be checking before I trash more transistors, that would be appreciated. Many Thanks - John

[JohnJEY]

I could use some guidance - following my lost Tx power, I ordered a Hi Band board kit, came in a week (thanks Sunny). I put it together, as per DL2MAN guide, including the 3:21 T2 and cleaned up all the contacts and flux - nice job! Tried it with my questionable Main board on just 5v USB power and 20m. Worked great, FT8 was buzzing and SPOTS coming back from all over the US. No contacts and the SPOTS stopped updating after about 10-15 mins. Tried the other Frequencies - All were also very busy on the input, but I couldn't get out, even though the radio was showing Transmit and I could hear the audio.
I switched to CW and checked the output - it showed 12.6 watts and 99.9% Eff - does that make sense from 5v USB (using Audio cables)?
So, I have 2 different RF boards (Lo + Hi) with the same issue (no Tx) and one Main board - Is there something on the main board that could be affecting the loss of transmission - given that it worked for a while, between the first one breaking and the second one breaking?
Thanks
John

[Chip]

I think you blew your fet. What was the swr on that antenna?

[JohnJEY]

Thanks for the reply Chip.
The first time, I think the fets blew due to the 13.8v regulator dying. I'm not sure if that would take out any other components, such as Capacitors and make subsequent use more vulnerable??
My SWR was 1.0-1.1 on the 20m band when the second board stopped transmitting.
I only use FT8, and it has performed well at swr higher than 7, and I was only using the 5v input the second time, so not sure how it gets to read 12.6 watts - is that a reflection of dead fets??

I am happy to focus on the FETs again, but can anyone say if I should also be looking at other components?
Thanks again, John

[Chip]

You can look at your fets and see if they appear blown but usually with that power output it is most definitely the fet.