[Heathkit] Re: Heathkit Digest, Vol 42, Issue 12

[Bob W5EUQ]

FYI:
In case you want to check crystals, you can use various methods, however 
if you have an RF signal generator and an oscilloscope, then it's pretty 
easy.  First try this method with some known quality crystals to get an 
idea of what to look for on the scope.  I've noticed that some good 
crystals check with the scope trace level bouncing when going through 
resonance.  On weaker crystals, the trace is more stable and doesn't 
bounce.  With bad crystals, you will be lucky to get RF through it at all. 

Some history: We used to use crystals to set up oscillators in 2" video 
tape machines in the 60's, 70's and 80's.  For example, if we had a 6.2 
Mhz oscillator, we bought a precision crystal for that frequency and put 
the crystal in series with the test point on the tape machine and the 
input to the oscilloscope.  We then tuned the oscillator in the tape 
machine for peak level of the trace on the scope. 

To test crystals:
Run an RF signal through the crystal to the scope.  In other words, put 
the crystal in series with the RF feed to the scope.  Move the generator 
around the frequency of the crystal and peak the trace on the scope with 
the signal gen.  If this frequency is near that of the crystal then that 
is a good thing.

 Also, look at the level without the crystal and compare the levels.  A 
bad crystal will attenuate the RF.  A good one will have the same RF 
level as the generator without the crystal and probably even higher due 
to the crystal oscillating during the test.  (Depending on the 
excitation from the generator, or level you are using.)

I've found that with a good generator, one that is very accurate, I can 
tell pretty darn close what the frequency of the crystal is.   Also, 
after checking several crystals, good and bad and mediocre, you can get 
an idea of how they rate.  Weaker crystals need more excitation to 
oscillate.  Some just won't oscillate though.  Could be they have been 
damaged.
Some Heathkit crystals out of SB and HW units check good, but weak.  
I've found that they need more excitation to oscillate.  Instead of 
throwing them away, sometimes one can salvage them by tweaking the 
circuit they are in.  Many times the problem is exacerbated by the fact 
that the caps and resistors have all changed a little too much and need 
replacing.  Good crystals will oscillate in these circuits, but mediocre 
or weaker ones may not.  So a new crystal  will typically work unless 
the circuit has real problems of course .  However, give the new crystal 
in this weak circuit a few more years and it too may stop oscillating.  
The components need to be changed out, dirt cleaned out and solder 
joints improved.

Just thought I'd share some crystal info.  I'm sure many others have 
test setups and other ways to test them.  Now I'm gonna sit back and 
watch the threads.  They can really get interesting and we can learn a 
lot from them.  HI.

Have a great weekend... that is after that darn Friday the 13th.... 
maybe we should skip it. 
Bob W5EUQ

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