On 10/19/2019 8:45 PM, nm0s_qrp via Groups.Io wrote:
I did a quick check of the power transistors on the main PCB, since the SMT parts are too small to probe safely. With 12.6v supplied, the transmitter keyed, and no modulation:
Thanks for that data, but after comparing with my non-working Nouveau 75 I wonder if you could confirm one of those voltages. If the drain of Q12 is truly at 0 volts with the PTT keyed, there would be zero resting carrier output, and I'm sure that's not right (even controlled carrier rigs have some resting carrier). I measure 10.5 volts on Q12's drain with the PTT keyed, which produces a resting carrier of 3.25 watts and that seems to be in the right ballpark.
The problem I have is that Q11 gets very hot very fast when any modulation is applied and inadequate modulation - something isn't right in the class G modulator. Here's what those voltages measure on my Nouveau 75:
G D S
Q3 9.6 10.5 13 FQP7P06
Q11 7.5 0 12.7 FQP7P06
Q12 3.8 10.5 .2 IRF510
This modulator never has worked right. After the first Q11 melted down, I went ahead and replaced all the big FETs, but that did not fix what's wrong. The BJTs in the modulator have junctions at least, I don't know of a good way to test the small FETs but at least they aren't shorted.
It looks like something is messing up the bias of Q11 - if the above voltages help you to point me toward the right area of the modulator circuit, I can make additional measurements and do SMT repair. I just haven't yet had time to build a spice model for that bootstrapped class G modulator to help me understand how it works.
Thanks and 73,