Yeah, I've thought about resoldering the entire **** board too, but it's 
not my first choice of things to try.  When I tried to bake out the 
board under an incandescent lamp that one time, I did seem to get some 
improvement, but it certainly wasn't  very impressive.

Regarding the apparent non-randomness of the noise.  I had the thought 
that dampness had gotten into one of the gain pots resulting in a larrge 
increase in the gain.  But, despite the high noise level, I am still 
able to record quakes and they produce records that are just about the 
amplitude I would expect.  I got a record of the mid-atlantic quake that 
was good enough that I was able to set the phase picks.  Even when the 
thing is running acceptably, I still notice the same 20 second or so 
hump spectrum.

I do monitor the leveling test point continuously.  I put a zero center 
meter on it so that I better center the leveling. I added a readout of 
the voltage at the test point to my data logging program.  I have a line 
from the test point connected to a spare channel on my A/D converter.

Several of your other suggestions are things I wouldn't have thought of. 
  Tapping in with headphones would be interesting to try in any event. 
I'm in a residential area not far from a busy street, and I have no 
doubt that I'm getting a lot of higher frequency noise from traffic and 
such.  I can see a definite dinural variation, quieter at night, 
noiseier during the day.  But I see no variation whatsoever for days on 
end when the thing is acting up.

Interesting.  As I have been typing this, the excess noise has 
diminished very substantially over the span of about 15 minutes,  and 
has stayed pretty low ever since.  It would be a sort of Murphy's law in 
reverse if just when I have been getting motivated to make an all out 
attack on the problem, it went away spontaneously.  Wouldn't be the 
first time.

At any rate, you've given me a lot of food for thought, for which I 
thank you.

Larry


ChrisAtUpw@....... wrote:
> In a message dated 11/02/2008, lconklin@............ writes:
> 
>     Regarding corrosion under solder joints, I haven't really checked
>     thoroughly for that yet, but the board looks very clean.
> 
> Hi Larry,
>  
>     I had a whole Sony TV with crevice corrosion. The joints looked 
> perfect, but you could peel them off a black corrosion coat on the 
> circuit strips. I had to unsolder, clean and resolder every joint on the 
> **** board.
> 
>     Re. your and Roger's comments about the cover, It will take me a little
>     time to build a new one. 
> 
>     Putting the cover on the floor isolates the seismometer from 
> pneumatic effects. I use 2" Celotex, but it isn't cheap. Bubble wrap can 
> also be very useful in reducing temperature changes and drafts ....
> 
>     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 had noted that. It looked as if you were experiencing greatly 
> increased gain, rather than just random noise. This could be resistors, 
> solder joints, diodes or the opamp.
>     Do measure the DC levels on TP. Does the Red/Green LED ever light up?
>     The NE5534 does take about 0.5 micro A to drive it - quite thirsty...
>     Try tapping the components?
>     Unsoldering C49 would isolate everything downstream of the first two 
> opamps.
>     Could there be any strong radio signals at 2 / 4 / 8 / 12 MHz? You 
> do have two radio receivers on the input... Electricity Utility time 
> switches work off radio signals on the power lines.
>     Try connecting an audio amplifier to the circuit before the 
> integrator and listening with headphones? I have solved some noise 
> problems this way. I was getting quite large random pulses which seemed 
> to be real, not instrumental. When I listened, I heard a heavy lorry 
> approaching a sunken drain grating on the corner of the local main road....
> 
>     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.
> 
>     I would expect there to be a simple fault which is sensitive to 
> humidity - since heating the board reduced the signal. Good Luck!
>  
>     Regards,
>  
>     Chris Chapman
> 
>  
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