Paul,
Good suggestion, but I don't think pop rivets had been invented when
they built this thing! :-D It's built like a piece of mil-spec
equipment. When I google for individual parts, I keep tripping over NSN
numbers. Now that I look closely at it, I realize that the case is just
the case with no electrical connection between it and the circuit other
than the BNC jacks on the front panel. The concept of 'chassis ground'
is an unknown concept. I don't think there's even one chassis ground
lug anywhere. Even when I'm poking around inside the unit, there are no
live wires that I can touch, everything is insulated. The build quality
is very high.
But now that you've got me thinking about it, I think there's something
weird about the grounding in the physics package. I've got to take
another look at that.
Thanks!
Ed
On 4/25/2012 8:59 PM, paul swed wrote:
Ed look for pop rivetted ground logs. Famous for going bad. HP even used
them.
Drill em out and put real bolts and lock washers in
On Tue, Apr 24, 2012 at 7:34 PM, Ed Palmer<[email protected]> wrote:
Paul,
On 4/24/2012 5:24 PM, paul swed wrote:
When all else fails check the grounds. Especially 40 year old screws.
Been there, done that. This unit had multiple problems with bad soldered
ground connections. I went through the unit and resoldered everything that
looked the least bit odd. I didn't find any screws that were electrically
significant. Maybe I should look again!
Ed
On Tue, Apr 24, 2012 at 6:13 PM, ed breya<[email protected]> wrote:
Ed,
Tuning the cavity should peak everything - it just maximizes the
excitation power at the microwave frequency, so you get the most output
from the Rb light wavelengths. A mechanical cavity resonator will have a
very wide (compared to the modulation frequencies you're looking for)
bandwidth, so unless something happened to it physically, it should be OK
as originally built or adjusted. However, you may want to look at the
multiplier chain and SRD bias circuit components and adjustments - those
could have drifted quite a bit over forty years, limiting the microwave
power due to being off-frequency, or having poor multiplication
efficiency.
I'm guessing that the second harmonic is indeed present, but just buried
in the noise, and the loop still can "lock" because of the further signal
processing, even though you don't see the evidence - remember it's a
lock-in amplifier capable of digging a tiny signal out of the noise. If
you
go through the multiplier and check and tweak things, you may get more
excitation power and signs that it's getting back to normal. Once you get
enough power, if the Rb cells are still good, the second harmonic signal
should show up large enough for the circuit to detect sufficient S/N
ratio
and provide a valid lock indication.
Ed Breya
Ed Palmer wrote:
Could the drift be at least partially responsible for the lack of second
harmonic? A message on the list (
<http://www.febo.com/****pipermail/time-nuts/2006-****April/020562.html<http://www.febo.com/**pipermail/time-nuts/2006-**April/020562.html>)
said that you could peak the second harmonic by adjusting the cavity tuning.
If the cell and the cavity are out of sync would that kill the second
harmonic? How close to they have to be? If this thing has a cavity
tuning adjustment I haven't found it.
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