[Milsurplus] Free - running osc at 7 MHz

[howard holden]

Hue what is the size/windings of the inductor? According to the Chesson list on Radionerds, the -156 covered 2-3 Mc, not 5 Mc but who knows?  Any idea of the contract date? The use of the VT25 suggests it's after 1930, but the design is certainly pre 1930.  C1  (350 pF) is across L3 and L4 by virtue of the .01 uF cap, so it is   fairly high C Hartley, and the variable across C1 would tune the frequency. (Along with the antenna characteristics!)

On 8/22/2019 3:52 PM, Kenneth G. Gordon wrote:
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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