Hi Brett,

Well, as I understand the thing, the only purpose of the feedback 
circuit in this design is to apply damping to the pendulum.  There used 
to be a copy of the schematic for Larry's board on his web site that you 
might still be able to find if you're interested.  I have a printed copy 
that is very old, and my not reflect the design as it is implemented in 
the board that I have.

Larry

On 6/21/2012 9:46 PM, Brett Nordgren wrote:
> Larry,
>
> Your mentioning a feedback coil made me think that your S-G might be a 
> feedback design?  I hadn't thought it was, but In doing some feedback 
> instruments, I have occasionally seen one thing that could possibly 
> relate to what you are seeing.  If the loop starts oscillating at some 
> relatively high frequency, sometimes the only noticeable effect will 
> be that the instrument performance gets bad, and such oscillations can 
> possibly come and go.
>
> If you can find an oscilloscope to look at internal signals, it might 
> be able to tell you something.
>
> Regards,
> Brett
>
> At 09:21 PM 6/21/2012, you wrote:
>> 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
>
>
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