Comparing 3-board and 2-board uSDX

[S52CQ]

Hi guys,

firstly I congratultate Manuel for making this little wonder. I've done the first - 3 PCB version and it works great. I've made it work with CAT control via USB adapter, also it worked FT8 with USB sound card.

Now I've decided to buy from Sunny Lo band uSDX, because it offers USB interface and CAT control.
So when setting FT8 parameters I've blown output FETs. I thought that I was not careful, so changed them and decided not to bother with FT8. While setting the radio for N1MM then I've blown output FETs again.

Now the question is: how come the old - 3 board version never gave me problems with output FETs on the same antenna, while this new radio keeps blowing output FETs? Is there any dramatic difference in PCBs or maybe in the SW that new radio gives me problems?

On old uSDX I have set TX Drive = 5, PA Bias MAX=76, while at new uSDX TX Drive = 4, PA Bias MAX = 128

Are these settings causing my FETs blowing that fast?

Best 73 de S52CQ, Jure

[dl6sez]

Hello OM,

i think you eventually forgot to switch to the right Band version.
Was it on LO band?
TX drive value can not kill the FETs, but PA bias max could, but value 128 is the only we use today for 3x BS170.
There are many "BS170" on the market not being real OnSemi Semiconductor BS170!
So maybe your replacement is not real BS170.
This device is copied 1000times because normally only used as simple switch FET for low frequency,
but we use it for rf switching, so original spec has to be met.
There are also "BS170" on the market with switched pinout, these will do a dead short in the circuit when installed.

73 de Chris

[S52CQ]

Hi Chris,

nope, band was set correctly to Lo, BS170 were purchased from Farnell, original OnSemi.

If at 3-PCB version PA Bias Max was set at 128 it caused output power of 10W (yes, I could not believe my eyes!), so I have reduced the power to 5W which corresponds to value 76.

So I will investigate toroids and compare filters to see if there is any difference between these two.

In the mean time radio works just fine at 10V, sure output power dropped a little but I was heard across the EU on RBN. Actually there is no practical difference from 5W or 3W

Best 73s de S52CQ, Jure