Art -
Yes they do. How important it is depends on several factors.
Certainly the quality of the original manufacturing process has a large effect.
First, MOST crystal filters are below 10 MHz until just recently. Many, like the Drake twins, are
at 5.6 MHz, and since aging is generally a percentage factor, the lower the crystal frequency, the
fewer 'Hz' shift will occur. I don't know if this is true, but logic tells me that six or eight
crystals all made by the same manufacturer, on essentially the same frequency, will 'probably age at
about the same rate and roughly the same number of Hz. So I would expect (hope?) that the filter
bandwidth would just shift slightly higher in frequency over time. Obviously, the bandwidth of the
filter also determines how much shift can be tolerated.
All that aside, the early 'soupcan' filters used in the TR-3/4 seem to have a pretty high 'failure'
rate, not from aging of the crystals, but from drastic changes in one or more crystals causing large
chunks of dropout. The LSB filter in the T-4X/B/C is the only one of the later hermetically sealed
filters that I have heard about failing. There don't seem to be THAT many, but seems like it always
the LSB. I wonder the the LSB filter line just had a bad day? :-) The 'good' ones MAY just be
slowly drifting, hopefully all at the same rate, and just shifting the Carrier Oscillator frequency
will take care of it!
I'm certainly not an expert on crystals, and hopefully we have someone here who is and can debunk
all my theories and give us the 'right' answers!!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Arthur Delibert wrote:
...what about crystal filters? We're all using crystal filters in our rigs
that are as old as the crystals that we're talking about replacing. Do the
crystals in the filters also drift over time, shifting the passband (or
distorting it, if the crystals in it don't all drift at the same rate)? Is the
drift just too small to worry about in the case of a filter?
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