Looking around inside the FSS field box I found that some of the resistors had been replaced with the wrong values on the main board .

• R7 was 470 Ω instead of 100 Ω.
• R1 and R2 were 470 Ω intead of 453 Ω (knew this, before I didn't have exact values).  
• R3 is a 100 Ω radial metal film radial packaged resistor on long janky leads instead of a 124 Ω surface mount. Maybe someone in the past wanted to be able to switch this out regularly.

R1 and R2 were ok values, but the solder job I did when I replaced them from 50 Ω resistors was not so great.

I've now switch these values to the ones correct for the schematic.  The R7 is the most liklely to make a difference here.  Its not clear why 470 Ω was used.