[GreenKeys] FREDERICKS (Plantronics) Model 1280 RTTY Terminal -Modem

Thu Feb 27 18:42:41 EST 2020

Is there any chance this unit is for F6 demodulation? This is DFSK, or two simultaneous AFSK pairs 
on the same carrier frequency. NOT a form of multiplex. I was told that it was used commonly 'at the 
Embassy level' for safe communications. Designated as, "Dual Frequency Shift Keying", in the Mode 
Lists of the FCC at one time. With better, faster encryption and other computer based communications, 
F6 may be totally passé at this time. 

Hank, W6SKC, was working on an F6 unit at the time of his passing. I visited him at his office/shack 
in the 1990s and he had a prototype on his desk. It shows in a photo I took that day. It is quite similar 
to the DOVETRON units of those days except for having TWO of the LED Cross Tuning Displays on 
the panel. Since his passing, and the untimely passing of his son, who took over the business, such as it 
was, there is no trace of any of the unsold DOVETRON units or any spare parts etc. After Hank's 
passing, I had a conversation with one daughter who 'knew nothing' about what her brother had in mind 
for the business after he took over. 

Guessing that most or all of it went into some kind of dumpster and is long gone now. Or sits in some 
warehouse, unidentified and gathering dust. 

- - - - 

If there is any interest in the photo I will post it on here. I shows Hank at his desk, and the F6 prototype 
a bit behind him. 

For now, 

Ralph - W8ROI 

- - - - - - - - 

From: "David I. Emery" <die at dieconsulting.com> 
To: "Jim Haynes" <jhhaynes at earthlink.net> 
Cc: "boatanchors mailman.qth.net" <boatanchors at mailman.qth.net>, greenkeys at mailman.qth.net, milsurplus at mailman.qth.net 
Sent: Thursday, February 27, 2020 5:18:17 PM 
Subject: Re: [GreenKeys] FREDERICKS (Plantronics) Model 1280 RTTY Terminal -Modem 

On Thu, Feb 27, 2020 at 01:37:31PM -0600, Jim Haynes wrote: 
> It's an interesting machine, but not very useful in my opinion. You can 
> choose the mark and space frequencies, but the tuning indicator doesn't 
> tell you which way to go if you are off. Apparently intended for copying 
> a single channel of a multi channel frequency division multiplex signal. 

Clearly intended for use in situations (SIGINT/COMINT and 
various monitoring of communications systems) where there are other 
means in use for determining tone frequencies - such as audio spectrum 
analyzers - or actual hard information about the tones in use in a 
system from documentation and/or knowledge of the gear in use. 

By the 70s or early 80s when these things were designed and in 
use most systems of interest were implemented using VF audio channels 
with little or no inherent frequency shift error - examples are ISB 
signals from the stable HF receivers and synthesized transmitters 
typical of that era (and later) (such as WJ, Racal, Harris and even 
R-1051 HF receivers) and any number of FDM-SSB carrier systems on 
satellite, wireline and point to point LOS microwave radio circuits. 
Most of these VFT systems of course did pack many TTY signals into one 
VF circuit - though the 1280 also has provisions for wider bandwidth 
mark and space filter operation with up to 600 (or more) baud higher 
speed wider bandwidth FSK tone pairs in addition to slow narrow shift 
lower speed stuff. VFT tone pack signals with mixtures of narrow slow 
and wider faster FSK tone pairs in one voicegrade audio channel were 
extremely common in commercial use in the late 60s through 80s on 
wireline, satellite and microwave networks. Many of these were 
eventually replaced by voice grade modems and stat mux gear... which 
could stuff many more circuits into one 3.2 KHz voice channel. 

Even the remaining traditional single channel FSK signals on HF 
back then were quite likely to be generated from AFSK tones input to a 
SSB modulator in a synthesized transmitter, or by mixing a LF VCO which 
did the FSK with a synthesizer that generated the basic carrier 
frequency. And most receivers used (past the R-390A era) were also 
synthesized and very stable and accurate. Thus tweaking the tuning 
continuously was mostly a thing of the past back then. 

These boxes are ALSO explicitly intended for computer control in 
monitoring systems... and probably were used that way more than in any 
manual mode. 

However as far as available tuning indicators go, the units DO 
have the ability to output the recovered mark and space tones at the 
(audio) IF frequency which could of course be used with a XY scope of 
some sort to display the classic RTTY crossed ellipse tuning indicator 
pattern. This is not very obvious from the documentation in the 
manual, but very obvious if you study the schematics and experiment. 
The relevant mark and space tones at the audio IF ARE brought out to 
terminals on the rear connectors. 

> The 1280 has three board slots, one for the microprocessor and the other 
> two can be a transmitter and a receiver, or two transmitters or two 
> receivers. 

The two receiver version can do either space (multiple receiver) 
or frequency (same thing on different tones or another RF channel) diversity 
reception and the 1280 can do AM limiter's or limiter type detection. 

-- 
Dave Emery N1PRE/AE, die at dieconsulting.com DIE Consulting, Weston, Mass 02493 
"An empty zombie mind with a forlorn barely readable weatherbeaten 
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either." 

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