Re: nouveau 75 Dave Cripe/Voltage Checks


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:

            G    D    S
Q3    7.9  9.8  11.7    FQP7P06
Q11   12   0    12      FQP7P06
Q12   5.0  0    0       IRF510

A large deviation from these voltages would indicate damage to the board, or defective component.

Hi Dave,

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,


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