[ARC5] Re: [Milsurplus] ARC-5 Transmitter Crystal Summary

[Mike Hanz]

C Whitaker wrote:

> Good point, which I forgot to mention, was that the
> crystals in the magic eye circuitry didn't have to
> oscillate on their own as a good crystal is supposed
> to be able to do.


That's an astute observation, Clete.  When I mentioned the duds I was 
finding, I was checking them in the O-4 oscillator and watching for a 
waveform on the Tek scope with the probe placed near the little stub 
antenna.  Though it's a very low level (28 volts) on the plate of the 
oscillator tube, a marginal crystal might not have tested "good" but 
capable of being a marker.  Time to break out the big crystal tester, I 
suppose.

> Maybe rejects and faults could be
> used.


I would hope not...otherwise you would need specially marked crystals 
for the spot tune oscillator, and lesser performers for the command 
transmitters.  Still, it could happen...  :-)

Another data point - I opened up one of the "do it yourself" DC-8s and 
put a photo of the innards with dimensions at 
http://members.cox.net/aaf-radio-4/DC-8-H_crystal_holder.jpg

I might take apart one of the dud crystals someday and see what the 
failure mode turns out to be.  Has anyone tried this?

73,
Mike

> There was no "standard", but there was some pattern
> to the crystal frequencies.    So the three patterns
> I see are frequencies near the frequency band, crystals
> on the operating frequency, and crystals that were in
> between high and low frequencies on a transmitter.



That's an excellent point, Clete, but I was under the perhaps mistaken 
impression that John's question went more to what the transmitters were 
delivered from the factory with during the war, and not so much with 
USAF field practice after 1948.  The ARC-5 doesn't seem to have been 
acquired for, nor used in AAF aircraft prior to the end of the war in 
any quantity, unless it was an occasional "swap" with a Navy receiver 
unit at a collocated base to see how they worked - the ARC-5 *did* have 
a better AVC than the 274N (or 274-N if you prefer... :-) ).  So I would 
guess that the Navy requirements likely drove the crystal frequency 
delivered with the transmitters more than any AAF influence.  After the 
sets arrived in theater, all bets were off, of course.  The crystal 
freqs I listed were based on either NIB transmitters or preponderance of 
crystal frequency over a bunch of like transmitters, but there's always 
room for error and exceptions.
Just to confuse the issue, I have an original box of six crystals for 
the O-4/ARC-5 spot tune oscillator, and they are:

2609kHz
3265kHz
4435kHz
5505kHz
6835kHz
8870kHz

which largely (but not completely) replicate the ones in the 
transmitters and lends credence to the idea of them being delivered with 
more than one "standard" frequency over time.  If anyone has an NIB 
transmitter that they are sure is original, I'd appreciate a note so I 
can update the list.

One other crystal issue I thought I might mention as long as we're 
talking about this subject - how many of you have found bad crystals?  
I've noticed a fair percentage of them not working, but perhaps my 
sample is too small to be significant.

73,
Mike