[Scan-DC] Xtals

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On 9/15/2013 6:38:52 PM, Doug Kitchener (oldsdoug at yahoo.com) wrote: 
> Do crystals go bad, as in going off-frequency?  

Absolutely. The cheap scanner crystals of the 60's, 70's and 80's started
drifting almost immediately. It was most noticeable at UHF where the error
was multiplied. The Regency TMU-8 and others had an AFC circuit to try to
pull the crystal back on frequency. It's the pot that stands upright with
the blue adjustable part. You would adjust the pot when components in the
AGC circuit itself changed value. There's a test point where you can measure
the voltage that indicates how far off frequency the crystal is. I also used
a frequency counter at the receive frequency minus the 10.7 IF or 10.8 in
Bearcats. The AFC circuit did the job for a month or two. It was probably
designed to deal with drift caused by temperature changes and not cheap
crystals that moved. I had a very large supply of crystals with extras for 3
surrounding states. I would check the crystals about once a month and would
end up exchanging about five to ten a month at the local dealer. I replaced
the MuRata ceramic filter in each of my Regencys with a narrower commercial
version so I probably noticed the drift a whole lot more than those using
the standard filter. I had about 8 Regency's mobile at one point. BTW the
narrower filters made a major difference in the amount of interference
received especially in a high RF environment like a downtown area or densely
populated mountain top. 

The drift of the VHF crystals wasn't very noticeable and the lowband
crystals not at all. I bought a few commercial grade scanner crystals and
those did not drift outside of the published tolerance.