[ARC5] ARC-5 Crystal Question

[WA5CAB at cs.com]

The calibration crystals are in holders DC-8-A or ARC #7785.  The 
photograph of the O-4/ARC-5 Test Oscillator in the AN/ARC-5 Maintenance Manual shows 
it with two of these holders installed.  The instructions later in the 
manual say to use any ARC #77785 holder with crystal in the range of 1 to 10 MC.  


This has nothing to do with the Command Set calibration crystals but if, 
while testing FT-243 crystals (in a GDO or otherwise) you find a totally dead 
one, and it does not appear to have ever been opened since new, the odds 
approach 1:1 that the failure problem is that one or both of the soldering legs 
on the brass contact plates have broken.  Repair is preferably by 
replacement of the broken plate, as they almost always break at the point where the 
leg becomes the plate.  And a solder-joint bump at that point is not 
tolerable.  If you have a lot of other FT-243 holders on "uninteresting" 
frequencies, you can repair by salvage.  However, there are several different shape 
plates as almost every contractor made theirs a little differently.  And due to 
the varying shapes of the rest of the contacting parts (spring, rubber 
gasket, etc.) they don't all substitute successfully.  So the best solution 
(time wise) if you have one is to open another holder made by the same 
contractor as the one that you want to repair.

In a message dated 8/13/2010 10:40:01 AM Central Daylight Time, 
dennis.monticelli at gmail.com writes: 
> It's just as Henry says; You connect it to directly to the coil socket 
> with
> short leads.  Any GDO relies upon an external inductor which is all a
> crystal is electrically when operating in its parallel resonance mode 
> (less
> the DC path which the GDO doesn't care about).  Of course the crystal
> represents a lot more inductance than the coils and likewise the tuning
> capacitance of the GDO represents more shunt capacitance than the crystal 
> is
> likely to see in a real circuit.  But it still oscillates readily and you
> can spin the tuning dial while watching the meter to maximize the reading.
> You won't find a peak per se, but usually the oscillation gets stronger at
> one end of the dial or the other.
> 
> The post-Millen GDO's started using smaller coil sets whose pin dia and
> spacing match that of an FT-243 so it's particularly convenient to use one
> of those units to check crystals.  I have built small adaptors that adapt
> other crystal holders to the FT-243 both for use in equipment and for use
> with my GDO.  The smaller transistorized GDO's are particularly useful to
> have in your bag at the hamfest, flea market, or surplus store. I picked 
> up
> just such a unit recently very cheaply because it was missing all its
> coils.  It won't "GDO" but it makes a dandy portable crystal checker.  In
> fact, that's what I used to check the crystal in my ARC-5.
> 
> BTW, if you leave the sensitivity control at one setting, you can test a
> whole batch of crystals at one time for relative activity.  I do this to
> identify sluggish crystals.  The more active the crystal, the higher the
> meter reading.
> 
> Dennis AE6C
> 
> 

Robert & Susan Downs - Houston
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MVPA 9480