[ARC5] SCR-274N Update

[Mike Morrow]

Mark wrote:

> Both the 40 and 80M transmitters have a slight chirp which doesn’t bother
> me in the least.
>
> The stability and tone on the 80M transmitter are fine, but the 40M
> transmitter drifts and the tone is about T8, maybe T7.  The drift is
> noticeable as I sometimes have to slightly retune during a transmission.

By "drift" you indicate that the frequency of the transmitter is changing
during keying, and also that it does not return to the frequency to which
it was initially set after keying stops.  One would expect the initial
frequency setting to return on key-up, save for heating effects that
develop during the transmit session.

> The power supply for both transmitters is the same.  It’s a hefty 28V power
> transformer with a full wave bridge rectifier and a 38000 uF cap (with
> bleeder) as the filter.  The HV/OSC/Screen supply is completely separate.

Your set web page and what you write there indicates that you are not
yet using the original modulator and dynamotor.  The original design
uses modulator relay K-52 to key **all** high voltages going to the
transmitter rack, where only the transmitter selected by the control box
responds because the control box select circuit has energized only
the desired transmitter's K-53 and K-54 select relays.  It is commendable
that your installation uses those relays properly and has an external HV
keying relay standing in until you get your BC-456-* working!

Some try to just key the desired transmitter's K-53 and K-54, but that's
a bad idea because it (1) is not the way the system was designed to function,
and (2) relay K-53's two sets of contacts do not always make up in the proper
sequence to minimize chirp...the PA cathode ground may complete through one
set of contacts before the MO HV is applied by the other set of contacts.
That guarantees lost of chirp.

I suspect the likelihood that any of these command sets were ever used
in CW mode in military service is vanishingly small.  In the AN/ARC-5
system, none of the transmitter control boxes (C-29, C-30, C-30A) have a
key, and the mode select switch is covered with a screwed-down cover on
the most common transmitter control box, the C-30A.  The most common
receiver control boxes and panels (C-27, C-38*, and C-125/ARC-5) do not
even *allow* selection of CW on the receivers.  (Only the C-24 local
control adapter and the C-26/ARC-5 single tunable receiver control box
allow CW selection.)

As for longevity, I saw the R-23*/ARC-5 beacon band receiver still flying
on TS-2A twin-engine training aircraft at NAS Corpus Christie in mid-1972
(alongside an AN/ARC-2 or -2A HF set).  I doubt too that many full SCR-274-N
sets were ever found on B-29s in WWII, except for the beacon band BC-453-B.
Most seem to have used the new (and excellent) AN/ARC-3 VHF command set.

Mike / KK5F