[GreenKeys] Fwd: The Flying White House 1947

[Duncan Brown]

Nick & all,

An interesting mystery!

If the ART-13 is running AFSK, it would still need the blower on the 
finals and a switch to switch the audio input from mic to AFSK source.  
The PL-259 connector is strange.  But the AFSK audio input would 
probably want to be shielded just to keep our noise & RF.  A SO-239 
takes up very little space behind the panel, so maybe it was chosen for 
the AFSK audio input in the shallow add-on box?

The problem with FSKing a wide range transmitter is that the FSK 
sensitivity changes a lot as you change frequencies and bands.  It can 
be designed into a transmitter, but complicated as an add-on to an 
existing design.  From the early days of RTTY into the 1960s, the 
standard method of generating FSK was to take the output of the TX's VFO 
to an external "modulator", mix it with a fixed frequency reactance 
modulator and then mix it back to the original frequency and send it 
back to the TX.  This outboard "modulator" also had to control the 
amount of shift for different frequencies and for TX bands where 
multiplication was used, which required a band switch or shift control.

The last of the Army's true-FSK radios (designed in the 1950s when RTTY 
was well established), the AN/GRC-46 (T-195 TX) and the AN/GRC-26D 
(T-368 TX) operated this way.  The MD-203 in the GRC-46 had a 10 
position band switch, and the MD-239 in the GRC-26D had a 8 position 
band switch, both to cover 1.5-20 Mc.

So I don't think the ART-13 was running FSK  unless they were operating 
on only one frequency and/or didn't care too much what shift they were 
running. Or, there was an external FSK Modulator used.  But it would 
have to be close to the transmitters (so the modulator's band can be 
changed along with the TX's).  Also there should be a second coax 
leaving the TX going to the modulator.

have fun,

Duncan


On 16-Feb-12 11:09, Nick England wrote:
> I'm still doubtful that this ART-13 is running AM with AFSK. Looking
> back at the original photo (Thanks for posting a great mystery photo,
> Don!!!), I see there is a switch lever for the add-on box with the
> co-ax connection on the right hand side - the lever positions are
> marked NORMAL (up) and TTY (down). Along with the "CAUTION: USE BLOWER
> ON RTTY SERVICE" label, that sure looks like exactly what you'd have
> for an ART-13 modified for FSK service.
>
> The AN/SGC-1 was an AFSK TU used extensively on UHF, but I haven't
> come across any evidence of use on HF. The Navy had FSK adapters for
> their older HF CW/AM transmitters - some were just a simple reactance
> tube modulator, others quite sophisticated replacement master
> oscillators.
> http://www.navy-radio.com/rtty-keyer.htm
>
> Later, AFSK became pretty standard on SSB of course, but that's
> another case altogether.
>
> On another note, the gadget beneath the receivers appears to be a
> diversity RTTY converter, judging from the markings on the Meter B
> switch. That doesn't square with the AGA-1 description either.
>
> cheers,
> Nick K4NYW
> www.navy-radio.com
>
>
> On Wed, Feb 15, 2012 at 10:43 PM,<WA5CAB at cs.com>  wrote:
>> OK.  I had a TT-30/AGA-1 once upon a time.  Still have the manual but I
>> think that George has machine now.
>>
>> The TT-31 must therefore have been the functional equivalent of the
>> TT-40/SGC-1.  My guess is that the black panel with two meters above and to
>> the left of the operator could be the TT-31.  Running the T-47 for extended
>> periods in AM with high modulation levels would account for the addition of
>> the blower..  However, an encrypted AFSK system doesn't account for the
>> coaxial cable going into the right end of the transmitter.  As the audio
>> would have been on the secure side of the system, there would be no need to
>> run it shielded
>>
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