Heterodyne Crystal Failures Mode?

[Bob and Bettina Groh]

Good observation, Jim, and astute thinking. As a long-time (40+ years) 
as an electrical engineer (many of those in the RF field), I would also 
have a skeptical attitude that 'bad crystals' are to blame. They could 
be bad but it is also possible that the oscillator circuits (due to 
aging of components and tubes) have simply fallen off in gain. A another 
contributing factor might be an increase in the series resistance of the 
crystals.

So if I were faced with this problem, I would start off by measuring the 
characteristics of the crystal (series resistance, shunt capacity and 
equivalent series L and C). If you don't get anything then it's bad and 
should be replaced. If the series R is a bit high then I would take a 
look at the oscillator circuit and see if I couldn't tweak it's design 
to to handle the increased R.

Hard to say where one might go past this point but I think the start is 
to measure the 'bad' crystals first. Then decide what to try next. If 
anyone has some 'bad' crystals, I would be glad to try to get some 
measurements on them (contact me off list).

I have a story here that bears on 'bad' crystals (one of many stories, 
by the way). Way back when, at a large electronics firm (you can guess 
which one - it was located in St. Joseph, Michigan) which had just 
started building computers, the first boards for a new computer had a 
'crystal' problem (including mine which was one of the very first) which 
lead to a temperature related failure (the computer stopped working 
after 30 minutes - not a desirable attribute!!). I wandered down to the 
production area and talked with a technician on the line. He said 
'yeah....we are getting a lot of those - bad crystals' and he pointed to 
large pile on his work bench. They would just replace the crystals until 
the board worked!  So I took a scope home and trouble shot the problem 
myself - it was not bad crystals at all but rather a weak oscillator!  I 
came up with a fix, took it to the Computer Department the next day and 
the change was made on the line in rush.

So it always helps to apply a little engineering and science to problems.

73 de WA2CKY
Bob Groh
420 NE Sunview Circle
Blue Springs, MO 64014-2045
(H) 816-228-6402


 


Jim Flanagan wrote:
> Hey guys..
> I noticed that the Het Osc crystals seem to fail fairly often in the 
> HW101. This appears particularly true
> for the upper 10 meter band crystals.  Has there ever been a technical 
> evaluation as to
> why this seems to be the case?  I was curious if anyone has attempted 
> to remove the outer case
> of these bad crystals and resolve the problem.  Of course, if the 
> actual quartz material has
> been broken due to shock,etc, then that is one thing.
>
> Any thoughts?
>
> Jim
> WB5KYE
>
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