On 14/02/11 04:26, Bill Hawkins wrote:
Group,
Jim Garland on the boatanch...@theporch.com list asked about crystals:
"A 22.5MHz crystal (HC-5 case) in my homebrew receiver, built about forty
years ago, no longer oscillates. It seems to be purely an age-related
problem.
It is in a standard solid state circuit which bandswitches six crystals, and
the other five work just fine. I wonder what causes a crystal to stop
working, and whether it is possible to repair them? I've "repaired" dead
100kHz calibrator crystals, and hamband crystals in FT-243 cases, by
cleaning off the brass pressure plates, but am not sure if one can do this
on thin high crystals. As I recall, the metal electrodes are evaporated onto
the sides of the element. 73, Jim W8ZR"
Any help appreciated.
On the any help level:
Aging usually makes crystals go adrift in frequency. Other parameters
may also shift, especially if the DC connectivity is lost due to
oxidation or so. I think it is likely that this is what happend and all
of a sudden the loop conditions does not support oscillation (i.e. gain
> 1 when phase shift is 0 degrees modulus 360 degrees). There might be
hacks to be added to get the loop oscillating again and there might be
hacks to the crystal assembly, but swapping the crystal is probably a
good remedy in this case.
Cheers,
Magnus
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