Well, I haven't tried locking the pendulum, but in an earlier 
incarnation of this problem (in '03!) it tried a few experiments where I 
disconnected the that supplies the power to the oscillator and the 
connection to the feedback coil.  Monitored the output and found that 
with the cables connected from the detectors I had significant noise and 
with the cables disconnected still pretty much the same.  But if I 
terminated the inputs with 2K resistors, the output went to zero, save 
for a little +/- 1 count jitter.  So, at that time I concluded that the 
board probably wasn't the source of the problem.

But I find your suggestion of replacing that capacitor very intriguing, 
and I will definitely try it.  I keep a zero center meter connected to 
the leveling test point as an aid to getting good centering.  And when 
I've been having problems, I have noticed some jitter of the pointer 
around the set point.  So I'm very willing to believe that the problem 
might be in the feedback circuit.  And, it's a pretty easy experiment to 
try.  I'll let you know what happens.

As fate would have it, the thing has been running pretty well all day today.

Larry



On 6/21/2012 6:00 PM, Larry Cochrane wrote:
> Hi Larry,
>
> Have you tried locking down the pendulum so it can't move and letting 
> you system run for a while? Looking at the schematic I see two parts I 
> would replace. They are the two back-to-back 100uf (I think) 
> capacitors driving the feedback coil. I would replace them with one 
> 47uf non-polarized cap. DigiKey has these 
> http://www.digikey.com/product-detail/en/ECE-A1VN470U/P1190-ND/227631 
> in-stock.
>
> Regards,
> Larry Cochrane
> Webtronics
>
> On 6/20/2012 7:11 PM, Larry Conklin wrote:
>> Hi Roger,
>>
>> I think I remember your having posted an account of your "rug affair".
>> In fact I think that it prompted me to improve the leveling screws on my
>> system, which at the time were rather flimsy. I replaced them with
>> larger, stiffer screws and I think that it may have helped, or even
>> "solved" the problem for a while.  I'm now using fairly substantial
>> screws and the system is (and already had been) sitting on a couple of
>> 1/8 inch thick strips of aluminum that are epoxied to the basement
>> floor.  The assumption being that the plates are a more rigid surface
>> than the bare concrete would be.
>>
>> The only pivot points I can identify are the pendulum hinge itself
>> (consisting of thin metal shim material, phosphor bronze or some such).
>> But there is certainly the possibility that the contacts between the
>> feet and the plates might shuck around a little with temperature 
>> changes?
>>
>> I also have had a series of cover experiments.  Original cover was a
>> wood box that sat on the frame of the sensor.  Very bad idea. Rebuilt it
>> to fit over the sensor and sit on the floor.  Better, but probably
>> vulnerable to transmitting noise to the instrument.  Added "stuffing" to
>> fill up the unused volume and suppress air movement. Not much noticeable
>> change.  Current cover is light weight, built of insulating board and
>> sitting on the floor.  At times I've duct taped it to the floor, but at
>> the moment it's just sitting there.  Maybe putting the tape back would
>> be a good idea.
>>
>> But the overall behavior of the problem makes a lot of these
>> possibilities seem unlikely.  In one subset of episodes of the problem,
>> the noise starts abruptly and then continues indefinitely for days of
>> weeks at a time.  The data shown on my web page is typical of this
>> "class".  (And I noticed that this episode started on March 30, and is
>> still ongoing, although there were couple of periods of a week or so
>> duration since them where things were much better.)  Other episodes have
>> emerged gradually over the course of several hours, continued for days
>> or even weeks at a time and then subsided.  I'm at a loss to come up
>> with a theory that is consistent with both kinds of behavior. And none
>> of the experiments I've tried have made any clear and lasting impact on
>> the problem, although I've tried a few things that I've thought fixed
>> it, only to have it crop up six months later.
>>
>> I guess that's what makes this business interesting.
>>
>> Larry
>>
>>
>>
>> On 6/20/2012 9:41 AM, RSparks wrote:
>>> Hi Larry,
>>>
>>> My experience parallels yours with loose mounted components. I once
>>> went through a rug to concrete below and failed to make good contact
>>> with one screw on the concrete.  I was effectively floating on the rug
>>> with one of the three seismometer legs!   Very noisy!
>>>
>>> Based on this experience, I am careful to make all mechanical contacts
>>> solid connections in some way.  I even drilled holes into the concrete
>>> and mounted my seismometer solidly.  I thought there was a small
>>> improvement in the noise level after taking this somewhat extreme
>>> measure.
>>>
>>> I also avoid pivot points in favor of bending joints to avoid noise.
>>> Friction is present in our seismometers if there is a moving joint and
>>> mechanical friction is always noise.
>>>
>>> Maybe look more carefully at any mechanical connection that might be
>>> loose, maybe  not only in the leveling screws.  Look also at the
>>> mounting of the surrounding air/insulating box, making sure it is not
>>> rocking from the ever present 0.2 Hz peak noise or more likely, the
>>> thermal air currents in your room.
>>>
>>> Good luck,
>>>
>>> Roger
>>> .------ ------ ------ ------ ------ ------ ------ ------ ------
>>> ------. | Message 3 | '------ ------ ------ ------ ------ ------
>>> ------ ------ ------ ------' Subject: Haunted system misbehaving again
>>> From: Larry Conklin  Date: Tue, 19 Jun 2012
>>> 14:54:30 -0400 I have an SG sensor that has intermittently had serious
>>> noise problems, literally for years. Mechanically, it is very similar
>>> to the design that is shown on Larry's PSN web site, and it uses
>>> Larry's electronics board, which I believe he is no longer selling.
>>> When the thing is running properly I've been quite happy with it. But
>>> occasionally, for reasons I have never been able to pin down, it
>>> exhibits extreme low frequency noise, to the point where it is
>>> basically useless. The current episode began with the system running
>>> very well, but it was in need of leveling, which I've typically need
>>> to do every couple of weeks or so. Before I adjusted the leveling
>>> screw, it was quiet. After messing with it it became extremely noisy.
>>> So, you'd believe that it was some sort of mechanical problem, right?
>>> And I've had several other episodes that began the same way. This
>>> time, when I brought it upstairs on the bench, I did in fact discover
>>> that the lock nut on the leveling screw was loose, and I "fixed" the
>>> problem by re-tightening it. Ran pretty well for a week or so
>>> thereafter, but not really as quiet as it should be. Then, it
>>> spontaneously got quite noisy again, interspersed with periods where
>>> it ran ok. I have a lot of other history with seemingly mutually
>>> exclusive symptoms/probable causes, but the bottom line is I don't
>>> have a clue what is really going on. I'm tempted to believe that there
>>> is something subtle wrong with the electronics board, but in past
>>> attempts to trouble shoot it, haven't been able to find anything. If
>>> there is anyone out there who has one of Larry's SG boards that
>>> they're willing to part with, I'd be interested in buying it. If for
>>> no other reason than to rule out the board as the source of the
>>> problem. For the curious, I have a little web page that I dump drum
>>> reports up to daily. The report for today (so far) shows a
>>> particularly noisy day, but does include the quake that happened in
>>> the Aleutians this afternoon. The page also shows drum reports from
>>> the day this round of the problem started. The day started just fine,
>>> until the big transient that resulted when I went down and adjusted
>>> the leveling, at which point all hell broke loose. You can also see
>>> the consequences of two subsequent leveling adjustments later in the
>>> day. After that data was recorded, I didn't mess with the thing for
>>> maybe a week, until I finally got around to taking a closer look at
>>> it, at which point I realized that the leveling screw was loose and
>>> re-tightened it. I'd be very interested in anyone's thoughts on this
>>> problem. The link to my web page follows:
>>> http://home.roadrunner.com/~lwconklin/ Larry Conklin Liverpool, NY .-
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>>
>>
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