[Milsurplus] Free - running osc at 7 MHz

[Kenneth G. Gordon]

On 22 Aug 2019 at 21:17, Hubert Miller wrote:

> Ken, I don't want to look for the manual I have for the loop set,

Never mind: Howard Holden pointed me to the schematic and I have it here now.

> SCR-131/ 161, but it used a Colpitts, I assume, because the
> inductance, the loop, was not tapped.

Actually, that is not correct: it uses a Hartley circuit with the "cathode" "tapped" across the 
"plate/antenna" inductor. The tap occurs by simply connecting part of L-3 to one side of the 
filaments and another part of L-3 to the other side of the filaments, both of which are series 
connected to the "antenna" inductor L-4, which DOES have a tap to the antenna.

L-3 is called the "Transmitting Reactor", but the way it is connected, L-3 and L-4 are really a 
single coil, with the antenna tuning circuit across only L-4.

There is a 2 - 35 pfd tuning-capacitor C-2 connected directly from grid to plate to, obviously, 
adjust the feedback necessary to sustain oscillations, because feedback is not really 
controlled by the "tap" of L-3.

In a normal (later model) Hartley transmitter, the tuning caps would be paralleled with the 
entire coil, L-3/L-4, but in this case since the tuning components are only paralleled with 
L-4, feedback is lacking. Thus the need for C-2.

It is a very interesting early circuit.

The system also appears to require a "counterpoise" wire.

The circuit is connected in such a way that the plate voltage never appears across the coil.

I am reviewing the schematic now and am not quite finished with it, but will give you more 
details later.

Also, the send/rx switch shorts the receiver antenna to ground when in TX and does some 
other interesting things which I have not yet winkled out.

I think you could change the frequency by shorting out some turns on the antenna coil for 
the transmitter. I haven't yet looked at the receiver circuit.

BTW, I am with Howard on this one: this is MY kind of transmitter!!! Snazzy! :-)

I love these ancient circuits.

Ken W7EKB
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