Hi Chris,

I think I will put your list of suggestions in my "great minds think 
alike" file.  I have tried all of your first 6 suggestions, albeit not 
since this latest episode started.

Regarding corrosion under solder joints, I haven't really checked 
thoroughly for that yet, but the board looks very clean.

Re. your and Roger's comments about the cover, It will take me a little 
time to build a new one.  In the mean time, I think I will try putting 
some sort of shims under the edges of it so that it is supported by the 
floor rather than the sensor frame, to see what happens.  I already have 
  some cloth wrapped around the base to keep drafts from getting under 
the cover.

One other thing that I'd be interested in hearing your thoughts on is 
the fact that the "noise" isn't really as random as I'd expect from 
something like bad solder joints and such.  If you look at the data I 
posted, especially from the LF channel, there definitely seems to be a 
dominant component with a period of around 20 seconds or so.

I've got too many irons in the fire right now to devote full time to 
debugging this, but I plan to revisit all of your suggestions.  Aside 
with messing with the cover a little, the other very easy test will be 
to disconnect the oscillator to see what happens.  I still have an old 
test data file from the last time I tried it, and it doesn't look much 
different from what I saw then with everything hooked up, or now.

Larry


> 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
>  
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