In a message dated 2007/05/30, rpratt@............. writes:

I> t is quite easy to turn the gap out without dissassembling and the wider 
> gap gets
> easier to clean.  

Hi Randy,

       After you have done this, give the iron a coat of anti rust paint. 
This prevents problems.

> My period is set at about 22 seconds so I think it must be electrical
> problem involved.  What confuses me is the background noise and walk up test
> don't seem much different than before when I could receive surface waves
> easily.  I would expect the lower freqencies of these to look like a high
> pass filtered FFT also.

       Wait till the next quake and see what the FFT looks like. Is it flat 
across the top from long periods, humpy, or what? What did old' FFTs look like? 
We have been having an earthquake 'low' so the characterisitcs may have 
changed in reality. Shallow quakes give more high frequencies.

       Check that there is no fluff  / debris anywhere and that there is full 
and free movement. I 'clean' magnets by sticking Gaffer tape on them and then 
peeling it off.

       Do you have an instrument or DVM which can measure capacitance? What 
value is it supposed to be? I would be very inclined to unsolder the coupling 
capacitor and measure it. Electrolytics do degrade with time. Do you have a 
'spare' first opamp? Their characteristics can also change / get damaged. Is the 
offset of the first opamp OK?

       Regards,
       
       Chris Chapman
   
In a me=
ssage dated 2007/05/30, rpratt@............. writes:



I
t is quite easy to turn the ga=
p out without dissassembling and the wider gap gets

easier to clean.  




Hi Randy,



       After you have done this, give the iron=
 a coat of anti rust paint. This prevents problems.



My period is set at about 22 se=
conds so I think it must be electrical

problem involved.  What confuses me is the background noise and walk up=
 test

don't seem much different than before when I could receive surface waves

easily.  I would expect the lower freqencies of these to look like a hi=
gh

pass filtered FFT also.




       Wait till the next quake and see what t=
he FFT looks like. Is it flat across the top from long periods, humpy, or wh=
at? What did old' FFTs look like? We have been having an earthquake 'low' so=
 the characterisitcs may have changed in reality. Shallow quakes give more h=
igh frequencies.



       Check that there is no fluff  / de=
bris anywhere and that there is full and free movement. I 'clean' magnets by=
 sticking Gaffer tape on them and then peeling it off.



       Do you have an instrument or DVM which=20=
can measure capacitance? What value is it supposed to be? I would be very in=
clined to unsolder the coupling capacitor and measure it. Electrolytics do d=
egrade with time. Do you have a 'spare' first opamp? Their characteristics c=
an also change / get damaged. Is the offset of the first opamp OK?



       Regards,

       

       Chris Chapman