[ARC5] Army/Navy transmitters from a NON COLLECTOR viewpoint, the ham version...

[Rich Post]

Hi Peter,

You're in good company here actually.  The PS-23 should be fine, even as
is.  I used a stock Heath HP-20 which despite the data from Heath is nearly
the same as the PS-23 and HP-23 series. The HP-20 was specified at voltage
under load (but measured at 730V key up and 664 key down on the nominal
600V side). The  PS-23/ HP-23 data were specified at no or very light
load.  For feeding the SCR-274 and ARC-5, I used a pair of high voltage
N-channel Mosfets on the nominal 300 volt side (366 no load!)  fed by
zeners on the gates and mounted on a simple heat sink for independent
regulated 200 and 270 volts for the oscillator and the screens, all on top
of the chassis under the cover inside the HP-20 with one new screw hole in
the chassis. The high-voltage Mosfets and heat sink were recycled from
computer power supplies.

The Command sets I used  had already been lightly modified by hams of
yesteryear.  Since the selector-relays had been removed as recommended in
early ham publications, I built a simple outboard QSK box consisting of a
pair of small 12 volt relays in series to emulate that same capability that
the selector relay would have provided, the one DPST relay turning on the
oscillator power and also energizing the coil of the second relay, an SPST
which powered the cathode connection for the final.  Worked great.  Both
simple conversions were tested on the air and approved by a CW ham expert
which I am not and had his endorsement. No problem with relay lag even at
higher speeds and no chirp.   And despite the notion of needing a series
cap for 50 ohm dummy load or coax, a parallel cap worked quite well, as
others have noted.  I originally used a variable cap to find that sweet
spot that still gave me full use of the original tuning and loading
controls, measured the cap value, and then mounted a fixed silver mica.
In a pinch, just use your tuner.  Make very sure that the loading inductor
is clean and working well by watching a VOM on low ohms for continuity
while turning the roller inductor.

I have not tried to plate modulate one of these but like you see no reason
why that would not work, keeping in mind the need to reduce the power to
stay within the tube capabilities and tying the screens by way of the usual
resistor to the plates for combination plate-screen modulation as
recommended in old ARRL Handbooks in order to achieve clean full
modulation.  Keep regulation on the oscillator but obviously not  the
screens.  I'll bet that a simple cathode modulator could also be made to
work. The results of my powering these for CW is in the December 2015 and
January 2016 issues of The Spectrum Monitor.  For the purists, I also have
a pair of untouched SCR-274 in an FT-226A rack that will remain un-hacked.

73 de Rich KB8TAD

On Sun, Feb 9, 2020 at 12:55 PM Peter Bertini <radioconnection at gmail.com>
wrote:

> I'm probably in the wrong group... but...
>
> Beyond the bewildering confusion deciphering between ARA/ATA, CBY/CCT,
> SCR-274, BC-xxx, T-xx/ARC-5 variants... I am looking for a common
> denominator..
> Assuming one  is NOT interested in the correct accessories, cables,
> modulators, dynamotors, etc...
>
> And for the sole purpose of putting any variant on the air with minimal
> modifications to transmitter itself... don't want
> shelf queens, I want these to be usable and on the air.
>
> 1: Can either variant be either screen or plate modulated? I don't see
> anything that would preclude doing so..
>
> 2: Any suggestions on how a beat-up FT-226-A tray might accommodate either
> style transmitter? I am thinking having each
> rear connector wired for a particular transmitter pin out, but how to
> avoid accidentally putting the wrong version in the wrong
> slot? One side Army, one side Navy...
>
> 3: For power... a modified Heathkit PS-23 junker... get rid of the voltage
> doubler nonsense and use full wave bridges on the
> high and low voltage windings. Series the two to provide around 125 vdc
> for the RX, and around 500 vdc for the TX.
>
> 4: Add a half-wave voltage doubler on the 12.6 winding for DC filament
> voltage for the transmitter filaments and relays.
>
> 5 Any benefit to regulating the 200 VDC for the 1626 oscillator? I suspect
> it would not hurt.
>
> 6: transmitters will be modified following AB5S's suggestions regarding
> relay adjustments, etc
>
> Flame away...
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: https://www.qsl.net/donate.html
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20200210/1cdc51e3/attachment.html>