Hi Chris,

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.

But a couple of years ago when the same sort of thing happened, I did 
replace the input op amp and didn't see any obvious change.  My theory 
at the time was that since I can see problems in both outputs, whatever 
is causing it must be upstream of where the two channels diverge, and 
that doesn't leave much.  The chip is soldered to the board, so all of 
those connections got a workover as a byproduct of the change.   And, 
after changing the chip, I did a thorough cleaning of the board to get 
rid of stray solder flux.

The other possibility would be in the power supply.  I pretty much ruled 
that out by measuring the power at right at the op-amp pins on all of 
the chips individually.  Thought a decoupling cap might be getting 
flakey.  But everything looked flat to within +/- 10 mv, which is the 
resolution limit of my inexpensive DVM.

Don't know what sort of resistors are used in the board, other than they 
certainly aren't cheap composition resistors.  I'll take a closer look 
at them, and maybe I should resolder the joints in the input stage. 
But, the board looks to be very well made, and I'd be pretty surprised 
if there was a bad joint.  It looks like it was professionally built, 
probably with a wave soldering machine.

I have also tried the freeze spray idea.  It seemed like I could spray 
the board pretty much anywhere and the output would jump all over the 
place.  So, I wasn't able to draw any useful conclusions from the 
experiment.

If the problem really is something in the circuit board, I can't account 
for the fact that things were running ok right up to the exact moment 
that I disturbed it, as you can see from the data record.  I suppose it 
is possible that the large signal excursion caused by walking up to the 
sensor and making the adjustment aggravated some latent problem, but I
have a hard time persuading myself that it's likely.  I have recorded a 
quakes that drove the thing to saturation and never have seen any 
similar consequence.  I don't have to touch the board, the case or the 
cables to do a leveling adjustment, just tweak the screw with a screwdriver.

I guess my most convincing theory at the moment is that there is a 
poltergeist living in my basement.

Larry


ChrisAtUpw@....... wrote:
> In a message dated 09/02/2008, lconklin@............ writes:
> 
>     I have put a lot of effort into trying to figure out what is going
>     on, to no avail.  In one of the previous episodes, I disconnected
>     the power to the oscillator that drives the antenna plate, and
>     opened the loop for the feedback damping.  There was no significant
>     change in the output, which led me at the time to conclude that
>     there must either be something wrong with the electronics board, or
>     some sort of electrical/magnetic pick-up.   Despite a lot of 
>     diddling around, I couldn't determine a cause, and eventually, the
>     system settled down without my having done anything specific to
>     fixing it.  And,
>     neither theory fits well with this current episode, which started
>     when I mechanically disurbed the sensor a little by adjusting the
>     leveling.
> 
>       I threw together a web page that shows the onset of the problem,
>     as well as short time intervals  before and after the problem
>     started this time.  If anyone cares to take a look at it and offer
>     their thoughts (or condolences), I'd like to hear them.
> 
> Hi Larry,
>  
>     The problem seems to be with the first opamp or it's circuit.
>  
>     Clean the input connectors with fine wire wool and coat them with 
> vaseline.
>     If the resistors are NOT metal film, check them for damage / the 
> correct resistance, maybe replace the input circuit ones with metal film 
> resistors. This could simply be a faulty resistor.
>     Visually inspect the solder joints with a magnifying glass for any 
> which appear faulty.
>     You can get a solder fault called crevice corrosion, when corrosion 
> creeps in between the copper board and the solder joints. Remove one 
> solder blob on say a resistor and then scratch the tinned area with a 
> knife. If you have crevice corrosion, the solder will peel off leaving a 
> dark brown oxide film on the copper strip.
>     It could also be that the opamp is faulty. Does it unplug, or is it 
> soldered in? Can you replace it easily?
>  
>     You can buy a spray can of freezing fluid. You monitor the signal 
> output and then spray freeze the components in turn. If one is faulty, 
> you are likely to see a large change in the output signal.
>     If the board gets damp while in use, you can brush coat it with 
> single pack polyurethane varnish for protection.
>  
>     Regards,
>  
>     Chris Chapman
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