[GreenKeys] Filterless TU

[Duncan Brown]

Jim, Ralph,

It may have been designed for use on AFSK VHF, where  you would have a 
relatively clean signal with no interference.  I used it on HF to 
receive commercial transmissions (AP & UPI) that did not generally have 
QRM. They also used 850 hz shift.  I would imagine that at 170hz shift 
in a busy hamband, it would not be very good. But it was just supposed 
to be a basic unit.

Duncan
K2OEQ

On 9/16/2019 16:29, Ralph Mowery wrote:
> Looking at the schematic, I just wonder if it is made to work with a narrow filter in the receiver and the older 870 hz shift.  The receiver would be set so that only the space tone would pass through the receiver filters and really make it like on and off keying.  There is a circuit in the 1966 ARRL handbook that shows a one tube circuit that does this.  The mention setting the BFO to zero beat with the mark frequency so only the space frequency makes it to the converter.
>
> A local ham told me he built that converter many years ago and it did work on good signals.
>
> Ralph KU4PT
>
>
>
>
> -----Original Message-----
> From: greenkeys-bounces at mailman.qth.net [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Duncan Brown
> Sent: Monday, September 16, 2019 3:34 PM
> To: Kenneth Gartland; Green Keys
> Subject: Re: [GreenKeys] Filterless TU
>
> Ken - It is not "limiter-less" - it is "filter-less"
>
> Tom - I found the schematic (right where it was supposed to be -
> amazing!) of the "Filter-less TU" - it looks like the same, or very similar, to the one Jack built. Schematic enclosed.
>
> It uses two 12AT7s (one as a limiter and one as the DC amplifier), a
> 12BH7 as loop keyer and an OA2 regulator.  Mine does not have a bias or speed control.  I don't know where I found the design for this unit. I built it in 1969, while I was in the Army.  Possibly  it came from QST - will have to look it up.  Mine did not have an audio transformer and was built on a very small chassis. the power transformer took up about half the chassis.  Due to having an external loop resistor, a short burnt up the transformer. (Note to loop supply builders - keep at least part of the loop current limiting resister inside the loop supply to protect the
> transformer!)
>
>    I kept it around for about four moves, thinking that I would rebuild it, but never did. On the last move, a few months ago, it went to metal recycling.
>
>
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