[Milsurplus] Off topic, missing pages

[Jim Whartenby]

JackGood catch, forgot about the servo amplifier and wasn't really aware of the dual IF bandwidth.
The only difference I found between the 618S and the ARC-38 was a few wiring changes around K1501 which was for the benefit of the automatic antenna tuner, IIRC.  Perhaps for reduced power while the antenna tuner solves for the impedance match?  It has been a while since I looked at both schematics so particulars are a bit fuzzy but the differences were very minor.
If you look at the 618S-1 service manual schematic around pdf page 367, you will see the three connectors for the SMO, 500kc generator and servo amplifier have nothing connected to them.  Then look at pdf page 206 for the main chassis layout and if you squint just right you will see on the bottom left that J318 is for the SMO servo, J317 is for the 500kc oscillator and P302 is for the SMO.  Your scrapped chassis should have those connectors clearly marked as to function.RegardsJim
P.S. Welcome to the ARC-21 club.  Unlike you, all I need is the time since I have everything else you mention! I wonder why people argue over the 10% of their differences and ignore the 90% they agree on?

      From: Jack Antonio <scr287 at att.net>
 To: milsurplus at mailman.qth.net 
 Sent: Friday, July 27, 2018 8:15 AM
 Subject: Re: [Milsurplus] Off topic, missing pages
   
On 7/26/2018 1:48 AM, Jim Whartenby wrote:
>
>
> It seems that all 618S main chassis contain connectors and wiring that 
> mate with the SMO and a 500 kc oscillator module needed to make the 
> SMO function.  All of this is hidden under the 144 crystal L.O. module 
> and is the basis of the Navy ARC-38.

Also needed would be a servo amplifier module, to drive the frequency 
correction trimmer on the SMO assembly.
It is slightly different from the servo amp module used for the tuner 
table and PA.

I stripped a naked 618S chassis for the connectors to make test 
harnesses for my ARC-38, and it certainly
appears that the wiring was there to support the SMO and its associated 
modules.  Also the wiring was
there to support the selection of bandwidth in the IF amplifier module 
for the ARC-38.

(And if anyone needs a completely stripped 618S-1 chassis,  they are 
welcome to it).

However, I am not going to go out on a limb and say that a module swap 
is all that would be needed to
make a 618S into an RT-311/ARC-38. It *might* work, it certainly appears 
it *should* work, but there
may be other considerations.  I  have both an ARC-38 and a 618S-1, but 
don't really feel the need
to test the theory.
>
> I believe that there are only a few members of this fine list who 
> would have an interested in this bit of Cold War Radio History.
> Even fewer when it comes to the earlier ARC-21 by RCA.

If I had the room, time, and a strong back, I'd  tackle an ARC-21 project.

Jack Antonio
WA7DIA

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