Re: QDX arrived but Q4 is damaged
2N7002 may work well as Q4 but it has a higher turn-on Vgs threshold than BSS123.
From Onsemi data sheets:
BSS123: VGS(th) = max 2V at VDS = 2V for 1mA drain current
2N7002: VGS(th) = max 3V at VDS = 3V for 1mA drain current
Q4 will have to sink quite a lot of current to ground when the 40m LPF is selected, about 77mA in case of 12V QDX supply. The gate drive is only 3.3V from the processor and this may not be enough to saturate the transistor. Anyway, it could be worth a try. Tune to a 40m frequency and measure the voltage over Q4 (between Q4 drain and ground). If it's less than a volt or so, you are good to go.
Steve Z wrote:
Hmm, I just realized that I do have some extra SOT-23 sized 2n7002 transistors left over from repairing the keyline out on my KX3. Does anyone know if I can use the 2n7002 in place of the BSS123 for Q4 in the QDX. Seems like it is just a switching transistor for the low pass filters, no?