In a message dated 10/02/2008, lconklin@............ writes:

Thanks  for your suggestions.  I haven't yet made any serious attempt to 
work  the current manifestation of this problem, mostly for lack of new 
ideas  for something new to try.



Hi Larry,
 
    You have to approach fault finding step by  step.
 
    1 Check the PSU lines for DC level and AC noise  first.
    2 Visually check all solder joints with a  magnifying glass. These are 
the commonest problems.
    3 When the system is noisy, disconnect the  oscillator drive, observe any 
change in the trace and then connect it again.  Also measure the DC level on 
the TP output of the first opamp. 
    4 Check the two sensors for operation.
    5 Assuming that here is no significant change,  disconnect the drive 
again, short the input and observe the output and DC level  changes. Try 
unplugging the sensors in sequence
    6 If you can't use a freezer can, try pushing /  tapping components with 
a plastic rod.
    7 Definitely check for crevice corrosion under  solder joints.
    8 Clean and put vaseline on the input plugs. Nickel  and particularly 
chrome plugs develop tough oxide coatings in the damp.
    9 You can brush coat the circuit tracks with  polyurethane single pack 
varnish. You can solder through it if necessary.
    
    Because you have changed the opamp does not mean  that the new opamp is 
good! When I change opamps, I usually fit a plug in  holder. You can wreck an 
opamp by overheating it during soldering.
 
    Regards,
 
    Chris Chapman
 



   






In a message dated 10/02/2008, lconklin@............ writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>Thanks=20
  for your suggestions.  I haven't yet made any serious attempt to 
=
work=20
  the current manifestation of this problem, mostly for lack of new 
idea=
s=20
  for something new to try.


Hi Larry,
 
    You have to approach fault finding step by=20
step.
 
    1 Check the PSU lines for DC level and AC noise=
=20
first.
    2 Visually check all solder joints with a=20
magnifying glass. These are the commonest problems.
    3 When the system is noisy, disconnect the=20
oscillator drive, observe any change in the trace and then connect it again.=
=20
Also measure the DC level on the TP output of the first opamp. 
    4 Check the two sensors for operation.
    5 Assuming that here is no significant change,=20
disconnect the drive again, short the input and observe the output and DC le=
vel=20
changes. Try unplugging the sensors in sequence
    6 If you can't use a freezer can, try pushing /=
=20
tapping components with a plastic rod.
    7 Definitely check for crevice corrosion under=20
solder joints.
    8 Clean and put vaseline on the input plugs. Ni=
ckel=20
and particularly chrome plugs develop tough oxide coatings in the damp.
    9 You can brush coat the circuit tracks with=20
polyurethane single pack varnish. You can solder through it if necessary.
    
    Because you have changed the opamp does not mea=
n=20
that the new opamp is good! When I change opamps, I usually fit a plug in=20
holder. You can wreck an opamp by overheating it during soldering.
 
    Regards,
 
    Chris Chapman