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

Tue Feb 11 00:04:34 EST 2020

 Thanks for the info Martyn,

What impedance tap did you use on the UM1 transformer for your setup?   By
coincidence I am resurrecting a World Radio Labs UM-1 Universal Modulator
on the bench at the moment.  Its highest power output from a pair of 6L6GBs
at class AB2 is 45 watts.  For that AB2 connection, it requires an outboard
22.5 V battery but a simple half-wave tripler and 22.5 V zener off the
filament line should be able to provide for that need.

73, Rich KB8TAD

On Mon, Feb 10, 2020 at 11:10 PM Martyn Google <martyn.seay at gmail.com>
> wrote:
>
>> For the record, I have successfully run a ‘simplified’ (previously
>> hacked) ARC-5 tx on 80 mt AM for years. The circuit is as original, MOPA,
>> plate modulated by 2+2 807’s ( p-p/parallel) &  UM1 transformer. There’s
>> 700v (via Modulator) on plates, a screen dropper resistor giving 300v (also
>> modulated) and regulated 225v on the oscillator. It produces 60-70 watts
>> output, the AM is reasonable with a bit more LSB than USB which you don’t
>> notice listening to it. Has been heard in Australia and all over New
>> Zealand.
>> I now have a full set of SCN274‘s returned to original, (with a lot of
>> help from friends on this group!) 2 tx, 3 rx, screen modulator and control
>> boxes, operate on 24vdc they work well on 80 and 40.
>> As already mentioned, once the ARC-5 bug has bit, you are hooked!
>> 73 to all,
>> Martyn ZL3CK
>> Sent from my iPhone
>>
>> On 11/02/2020, at 4:14 PM, Rich Post <kb8tad at gmail.com> wrote:
>>
>> 
>> 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...
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