(tr)uSDX Forum

Hi, I see some chatter on using a diode or 2 in the PA circuit to protect the finals on the uSDX forum.Is there a proven method of doing this in our true uSDX?I am using a resonant antenna for this reason and have ordered a manual tuner. Just would like to be extra safe. No magic smoke!

I´m not a fan of this, as Zener Diodes act as capaciance diodes and interfere with the Class E Filter behind it. This created undefined states.

- For extra safety: Only tune / check SWR while on USB Power Supply (Max output power @5V is 1W = A lot of safety margin) Once checked and confirmed apply main power supply....

If you want to have even more headroom (while tuning or determining SWR):
- Try to get a handful of BS170´s, get a cheap component tester, and sort them for equal Values (Cg, Vt and Rdson). If you solder in 3x BS170s that are as equal as possible, you will have a lot more safety margin. I transmitted for over 20 Minutes (using CQ loop) into SWR of 1:9 and worse at full 5W Out, and the PA is still OK.



73 Manuel; DL2MAN

madmike wrote: ↑12 Dec 2022, 13:08 Hi, I see some chatter on using a diode or 2 in the PA circuit to protect the finals on the uSDX forum.Is there a proven method of doing this in our true uSDX?I am using a resonant antenna for this reason and have ordered a manual tuner. Just would like to be extra safe. No magic smoke!

What tuner? How is it working for you? You are using a doublet antenna, right?

The tuner is the Emtech ZM2. For portable use random wire. No portable doublet, although it will tune a balanced antenna with open wire/ladder line or coax. I bought the pre built one, the diy kit is about $30 less. Unfortunately, I haven't had a chance to try it yet due to cold weather here in the northeast...Massachusetts. I also need to figure out a way to use Guido's USB 1 watt power suggestion, while maintaining my rather small "shack in a box" setup, this is to avoid the risk of output PA damage. He also mentioned using 3 ??? devices, but I'll need to read a bit more about that.

I meant Manuel, not Guido, that had made the 3 mosfet suggestion. Thanks

Is ther a write up on how to install the 3 bs170 to make the PA less susceptible to higher SWR as Manuel suggested. Also, not sure on the matching process and device to use. Thanks

uhm, there might have been a misunderstanding:
I was assuming, you had the Lo Bands RF Board with 3xBS170.

If you have High or classical Band Kit, you should NOT put in 3xBS170. There is no writeup for that, as it´s not recommended, exept you know how to compentaste for the different shunt capacitance.

If you have that single FDT86256, you should be pretty safe for bad SWR. It should be able to handle at least 3-4W of reflected Power without releasing the magic smoke. That should be enough for even SWR 10:1
Try not to push it, but the FDT86256 was chosen for a reason... It can handle more abuse....
Running of USB Power supply it is virtually impossible to destroy the FDT86256..... AT 12V Power Supply, you need really bad SWR over longer period of time to make it smoke.

73 Manuel; DL2MAN

Thanks Manuel. I was about to order new finals to keep "just in case" If I were to do that, Is/are the final (s) plug in or soldered in? Obviously I haven't opened the case nor built mine. Recommended source? Recommended brand? Classical band unit. 73 Happy New Year

It is soldered behind, it is SMD SOT-223.
I already burned one

Hi, I'm a little lost here. I ordered the classical bands kit from Suny which did not come with any PA FETS. I ordered three BS170's and have just noticed in this thread that the classical band kit should use a FDT86256. Oops!

When I looked closely on the single page of plans that come with the kit there it is on the top right - FT86256.

Damn, time to order more FETs.

Are you sure there is no fet installed?
Fdt is a smd device and should be presoldered, it is just on the opposite side of the pcb…

S57L

Thanks so much. You are correct! I was confused by the Q1, Q2, Q3 places on the other side of the board. Anyway this is good news.

I feel I am missing a whole thread or video on the differences between the classic RF board and the low band board I have right now. I must do some more searching.

73 va7rlx

madmike wrote: ↑21 Dec 2022, 02:22 I also need to figure out a way to use Guido's USB 1 watt power suggestion, while maintaining my rather small "shack in a box" setup,

I use this pack as it gives both 12v and 5v USB output in one small package and also a on off switch to prevent plugging and unplugging the dc power cable all the time.

https://www.amazon.com/dp/B01M7Z9Z1N/ref=emc_b_5_mob_t

I added a power switch in line w my battery supply. I then found there is an on off feature. Turn Dow the volume and hold the button

Hello OMs,

but it still draws a current... not really possible to switch the (tr)uSDX of without disconnecting the power. There is no semiconductor switch in the circuit.
If you don't believe me have a look at the schematics

73 de Chris