0.2W and 10% Eff on (tr)uSDX Hi Band

[DO8THW]

I've had my (tr)uSDX as a Hi Band version for a few days now and couldn't get it to work after putting it together. With my laboratory power supply it draws 70mA when receiving and 110mA at 13.8V when sending. I re-wound the two transformers twice and paid attention to the winding direction, re-soldered all the soldering points about 3 times and still I have an SWR of 1.0 on a dummy load but only an output power of 0.2W and between 5% and 10% efficiency. I don't know what else to do since I've basically rebuilt the RF board several times.

[KE0QQQ]

Welcome,

This is frustrating, I hope we can summon the best here, Till then, let me try

The high band version has the single FET for output. My assumption says it has failed. Both versions can fail

I never apply any voltage anywhere unless the antenna is connected or has a dummy load.
Test all points of the SMA connector to the antenna. Tiny wires can back out. The barrel connector can loosen. Adapters get unseated... those are the mistakes I have made using this radio.

When I see 0.2W on the display, that is the tiny FET that drives the big final making that wattage and the big final has failed

It will work, but .2w does not go far. That is why I carry more than one radio until I searched the word zener on this forum and learned how to save my FET's

That is my assumptions based on my use this cool cool little radio

It sounds like you might have some bench tools? a scope maybe?

To everyone: What are the Pins to check for proper voltage ? 5v here .. RF voltage there?

[DO8THW]

I measured 0.4 ohms between gate and drain on the FET with a multimeter and continuity tester, so the dummy load was probably not in the right place. I measured with my oscilloscope in parallel to the dummy load and had 2V pp and a clean sine signal at 28 MHz, so I would now say that only the FET has left us

[DO8THW]

So here i am again, i ordered some FDT86256 Mosfets and replaiced the broken one. Now i have a new problem, i tried sending on the 10m band but the screen of the trusdx turned black and my power supply shows that it draws around 3 watts. Turns out that it killed my mosfet again in like 2 seconds of pushing the PTT button. I first thought the efficency was too bad but the trusdx showed around 68% efficency and i checked the rf board with my Nanovna, it looks okay to me, does anyone have an idea?

73 DO8THW

[DL2MAN]

Hi,

first of all:
When you´re tuning, you should not run the (tr)uSDX on unlimited power supply.
Either limit current or even better: Run it un USB Power supply. This way it´s virtually impossible to blow a fet.
second:
Your insertion loss on the passband is pretty high. It´s not supposed to be 0dB but usually in the range of low single digits. In my case usually under -1dB. Your NanoVNA measures -11,5dB ! That´s unwanted attenuation !

73 Manuel; DL2MAN

[DO8THW]

Thx for the response, i limited the current to 600mA @13,8V but i'll try it with usb. What can i do when the attentuation is too high? I've tried to separate or compress the windings but i didnt get any closer to -1dB. I'm no expert but can it be that the impedance mismatch between Nanovna and trusdx has something to do with not showing the correct readings and then mismatching?

73 DO8THW