[Collins] Re: Differences Between the 51J and the R-388

[Gerald]

On Wed, 2005-08-10 at 19:45 -0400, BSugarberg wrote:
>  From "Un-crossing the R-388 Cross Modulation" by Chuck Teeters,
> W4MEW.  Electric Radio, Issue #81, January, 1996:
> 
> "A valid complaint however is the amount of cross modulation
> in the receiver. The cross modulation is the result of the
> direct connection of the antenna to the RF amplifier grid.
> Despite the coax connector, the input is high impedance."
> 
> The author then tells about testing the 51J at Fort Monmouth
> at the start of the Korean War:  "A shock mounted rack was
> installed in the AN/GRC-26 with 2 51Js, and everything
> tested out fine except sensitivity.  The low impedance of
> the receiver connected by 12 feet of RG-11 to a 6 foot whip
> didn't cut it.  W2VQR, the test director, said treat it like
> an automobile BC receiver, and try a hi-Z input.  It worked
> great with a direct connection of the short antenna to the
> 6AK5 grid."
> 
> "After some negotiations with Collins, we had a JANitized
> 51J with hi-Z input, antenna trimmer, and IF output."
> 
> "Col. John F. Rider's publication branch glossed over the
> high impedance input, in their rewrite of the Collins IB
> into TM-11-854."
> 
> "You need to restore the R-388 to a 50 ohm input.  The
> R-388 eliminated the primary windings on the RF coils.
> It is easy to put new windings on the RF grid coils, and
> rewire the band switch."   The author now tells you how to
> do that.
> 
> "For about 2 hours work, you have given your R-388 a
> normal input that will match the coax most of us use,
> and will have eliminated any strong local RF from
> getting a free ride into your receiver."
> 
> 73, Bruce WA8TNC
> 
> 
> 
My 51J-3 or re-paneled R-388 sure has that high impedance input and it
makes the RF stage squirrelly on the 3 and 4 MHz bands. Don't want to
peak the antenna trimmer with a wire antenna nearby.

As far as differences between military and commercial manuals, those I
expect. There were probably several versions of each manual as the 51J
family was made for quite a few years and used heavily in house as test
equipment in laboratories and production line test setups.

I know similarly know there are several different manuals for each S-
line receiver. The one that came with my new 75S-3B in the spring of
1964 was so badly written that it often didn't make sense. The next
version was far better. I wrote it. Traded the tech writer the revisions
for a copy of the revised manual. Gee, I worked cheaply those days. I
had to rewrite lots of his text for the 821A-1 manual too but the
company paid for my time to do that because he just didn't understand
circuits but he thought he did.

So I have confidence there were repeated manual revisions, not always
accompanied by a change of receiver circuits, controls, connections or
performance. Likely some of the receiver changes weren't accompanied
immediately by manual revisions to add complications for the users.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer