Maybe I played too much with the RF board?

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iw0dfd
 

Posts: 3
Joined: 06 May 2023, 09:06

Maybe I played too much with the RF board?

Post by iw0dfd »

After about 30 years, I took the soldering iron back on an RF project. In recent years I have designed and implemented many electronic projects, but not in the RF field. I ordered the (tr)uSDX kit from Up Tech Store (Aliexpress) and started. I was very excited to make my first HF transceiver with my own hands. I followed Manuel's procedure for adjusting the notch filters using a nanoVNA with no problems. Everything worked, but I wasn't able to get more than 2.5W on each band. I tried to add or remove one or more turns from the toroids with no success. I suspected one or more of the mosfets was faulty and as I had some spare BS170, I replaced them all. I made a little progress - a maximum of 3.5W, and in one case almost 3W at 75% efficiency - but no more. It seemed enough to me, but since I was testing, I had soldered the mosfets with long leads. I decided to shorten them. From that moment on, I couldn't get the board to work again: all the mosfets gone, one way or another, especially Q1. Actually, even the very first mosfet mounted in Q1 position was getting very hot during TX tests, even when I pressed PTT for just one or two seconds. Tried to replace them multiple times, frying them each time. I consider myself pretty good at soldering, so I don't think I've broken mosfets with heat, at least not all the times. I also always used a good 50 ohm dummy load during tests. Yesterday I turned on the device one more time: never touched the PTT, but the two mosfets left on the board suddenly burst and smoke came out, presumably from the other side of the RF board. Is it possible that I damaged the PCB so much to make some short circuits between tracks near the mosfets? Or maybe I misplaced the fets? I'm giving up, but I'm still grateful for the time spent assembling the kit. I suppose I have a lesson to learn from this story, but what is it?
73 de Giuseppe
73 de Giuseppe IW0DFD, JN61LM from Sermoneta, Italy
dl6sez
 

Posts: 399
Joined: 30 Dec 2021, 22:54

Re: Maybe I played too much with the RF board?

Post by dl6sez »

Hello OM,

from my point of few it sounds like you should get real BS170s replacement FETs from Mouser or a good wellknown supplier wich delivers real Onsemi (was in former years Fairchild now Onsemi).
All other FETs called BS170 are not real BS170 and it makes a difference, because we us a simple on/off FET for small power DC switching as a very fast switching RF FET.
I don't believe you have a different problem as the one above :)

If you want to check all before:
You can remove all 3 BS170 now by using a wire cutter and test your TX mode in cw and measure at the Drive pin on mainboard, first without rf board connected. Use an oscilloscope and check if you see a near square wave signal. Than your TX path on mainboard is okay.
Than connect rf board too and measure without the 3 BS170-FETs at gate pin of one BS170 again with oscilloscope tx in cw. If you see also there nearly the same square wave on tx as before,
then you know 100% it is not a PCB or soldering error.
Replace the BS170 with good known real ones and readjust all Lx1 for max output power with 5V than go to 12V and see if it is okay. I would leave it at 3,5W this is not bad.

73 de Chris
Chris DL6SEZ, JN48XL near Ulm, Southern Germany
iw0dfd
 

Posts: 3
Joined: 06 May 2023, 09:06

Re: Maybe I played too much with the RF board?

Post by iw0dfd »

Hi Chris,
I removed all the mosfet and already found out that the Drive signal is ok up to the gate terminals. I found a 9V pp square wave. First lesson learned: be rational and not emotional! :D
Thanks a lot for your advices. I will thoroughly follow them.

73 de Giuseppe
73 de Giuseppe IW0DFD, JN61LM from Sermoneta, Italy
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