On Wed, Oct 28, 2020 at 09:29 PM, David R. Hassall WA5DJJ wrote:

  I also changed the two 10K resistors R1 and R2 with a pair of 1% 10K resistors.   That brought the voltage up to 1.98VDC but not to the 2.5 Volts I was expecting.   Looking at the circuit the only load for that voltage divider is the Quadrature Sampling Detector IC4 a surface mounted FST3253 of which I do not have. 

There lies the issue.  IF R1/R2 is not 2.5V then T1 and U4 are suspect OR ther eis a leaky path between
them and maybe ground.  C6 could have a weak (high resistance) short between it and ground.
That can be a result of the cooked traces and or board.

U4  Can have a input or output that in blown up (partial high leakage) due to ESD
at assembly time.   if the input votlages match the output something under or over
the board has caused a sneak path.

Also it can be as simple as ta resistor that was stuffed almost correctly save for one lead goes
though a VIA rather than a correct hole.


No matter what if Top of R2 is 5V (+-10%) then the middle must be half that (nominal 2.5V).
If that is not the case then stop there and find out why.  Going further down the amp just
amplifies the problem that starts there.

Allison
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